Sheep Health & Production

Introduction | Metabolisable energy | Metabolisable protein | Energy requirements of the animal | Dry sheep equivalents, DSEs | Pasture Production | Feed intake of grazing animals | Grazing Management | Diagnosis of dietary deficiencies of energy and protein | Concentrate feeding | Feeding and management of sheep and cattle during drought | Recommended reading

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Introduction

For the last 20 years in Australia, the energy requirements of ruminant animals and the energy value of feeds have been expressed using a system based on metabolisable energy (ME). The system now favoured for describing protein nutrition was introduced in 1992. This system is based on metabolisable protein (MP).

ME and MP values are estimated through feeding experiments and in laboratory analyses. ME values for many ruminant feeds are known and published in tables. MP values are not yet widely available but will become increasingly available as the system is more widely used. At present, MP values must be inferred from crude protein (CP) values and estimates of rumen degradability. Both ME and MP values are usually cited per kg of dry matter (DM).

Feed requirements for body maintenance, growth, pregnancy and lactation can be expressed directly in terms of ME once it is known how efficiently feed ME is used for these purposes. The efficiency of use depends on the ME value of the feed; feeds with a lower ME value being used less efficiently.

Metabolisable energy

To understand what ME is, consider first the total energy produced by the burning (oxidation) of a feedstuff. The gross energy (GE) produced is the total heat produced when the feed is combusted in oxygen in an adiabatic calorimeter. Most feeds eaten by ruminants have GE values largely determined by carbohydrates like cellulose, of about 17.6 MJ/kg, but influenced upwards by varying amounts of protein (which has a GE of about 24 MJ/kg) and fat (about 36 MJ/kg). For a wide range of pastures, conserved forages and grains which have varying proportions of carbohydrate, fat and protein, the total GE usually falls between 17 and 20 MJ/kg with an average of about 18.8 MJ/kg.

Not all of the gross energy of the feed is available to the animal through its digestive processes. Some is lost in faeces, some in urine and some in gases lost by eructation. Metabolisable energy is the portion of feed energy which can be utilised after losses in faeces, urine and combustible gases, mainly methane.

ME = GE - FE - UE - M_E (1)

For a given feed source, the ME declines as the level of feeding increases, due to variation in the amounts of energy lost in faeces, urine and methane. ME, therefore, is defined as the value measured at maintenance level of feeding. ME is usually quoted per kg DM and MJ of ME per kg DM is expressed in shorthand as M/D or ’M over D’. The ME contents of a variety of commonly available feedstuffs are shown in Table 6.2.

The sum of energy losses in urine and methane are usually around 19% of GE but sometimes as low as 12%. Faecal losses vary with the quality (digestibility) of the feed, ranging from 20% to 65% of GE.

Figure 6.1 The partition of feed energy during ruminant digestion (from APC 1990)

Digestibility

The most important single measure of the energy value of a feed is its digestibility determined in vivo or in vitro. The figure for digestibility usually quoted is dry matter digestibility or DMD and it is defined as

DMD = (Feed DM - Faeces DM)/Feed DM (2)

While ME values are now usually quoted, digestibility is still frequently used, particularly in reference to pastures. DMD can be approximately converted to ME using the expression

M/D (MJ/kg) = 0.156 x DMD% - 0.535 (3)

Metabolisability

The metabolisability (q_m) of the GE of a feed at maintenance is defined as

q_m = ME/GE (4)

This term is being used increasingly to describe the nutritional value of feed in place of M/D and digestibility, particularly for ration formulation.

Efficiency of use of ME

ME is used with varying degrees of efficiency for purposes of maintenance and production. In other words, it is converted to net energy (NE), where it is stored as growth of muscle, bone, fat, wool, conceptus, or excreted as milk, or used for exercise or other forms of work energy, with varying degrees of efficiency. That portion of ME which is not used for maintenance or production is lost as heat; some heat is lost in ruminal fermentation, most is produced during the use of ME at tissue level. Thus

NE = ME - heat

and, if we use the letter k to represent the efficiency of conversion of ME to NE,

NE = k x ME

and

heat = (1 - k) x ME

The efficiency with which ME is converted to NE also varies with the quality (specifically, the metabolisability of the diet). Standard values for k are

for maintenance: k_m = 0.35 x q_m + 0.503 (5)

for lactation: k_l = 0.35 x q_m+ 0.420 (6)

for growth: k_g = 0.78 x q_m + 0.006 (7)

For some forms of production, k is independent of q_m;

for growth of conceptus: k_c = .133 (8)

utilisation of mobilised body tissue for lactation

k_t = 0.840 (9)

The ME used to maintain an animal results in no weight gain, no milk production and no work, except that necessary for basal metabolism. Thus, in the case of ME used for maintenance,

heat = ME_m

For other forms of production, such as growth,

heat = (1 - kg) x ME

Energy released by heat is not necessarily ’lost’ to the animal. In ambient temperatures below thermoneutrality, the heat produced by the ’inefficient’ use of ME helps maintain homeothermy. In fact, if heat produced by functions associated with maintenance, grazing, growth and/or lactation is insufficient to maintain body temperature, additional ME must be expended specifically to produce heat.

Fermentable metabolisable energy

It is necessary to estimate fermentable metabolisable energy (FME) of a feed to use the Metabolisable Protein system proposed by AFRC (1993).

FME = ME - ME_fat - ME_ferm (10)

ME_fat is the ME in dietary fat and oils. Whilst highly digestible in the ruminant digestive tract, they cannot supply molecules of energy-yielding ATP to the rumen microbes. Most of the commonly used sheep feeds have some fat content; the FME of oats, for example, is about 1.4 MJ less than its ME. ME_ferm is the ME of fermentation acids, mostly lactic, acetic, propionic and butyric acids, in partially fermented forages, such as silage, and brewery and distillery by-products. These acids are themselves products of fermentation and cannot yield more energy in the anaerobic conditions in the rumen, although they are absorbed and metabolised.

Metabolisable protein

The microbial population in the rumen degrades a proportion of the total crude protein intake to simpler nitrogenous molecules, mostly ammonia, and uses these as substrates for the synthesis of microbial cell protein and other cellular constituents. The rumen degraded protein is termed RDP. The balance of the protein passes out of the rumen undegraded (UDP). Usually about 80% of dietary protein in roughage feeds and grains is degraded in the rumen. In protein supplements, such as meat meal, only 30% to 40% is degraded().

Ruminants whose diet is deficient in RDP can be effectively supplemented with non-protein nitrogen (NPN) sources such as urea. Since urea is broken down quickly by rumen microbes to ammonia, it can adequately supplement dietary crude protein in providing effective rumen degradable protein (ERDP). ERDP is defined as a measure of the total N supply that is actually captured and utilised by the rumen microbes. It can include non-protein sources of rumen ammonia but does make allowance for some loss of soluble quickly degradable protein (QDP) which escapes capture by rumen microbes. To prevent ammonia toxicity urea must be fed with care. ARC (1980) (page 168) recommend an upper limit of urea of 0.5g/kg bodyweight because animals could not effectively use more than this amount. The use of NPN in ruminant diets is further discussed by APC (1990) (page 122).

An additional source of nitrogen (N) for the micro-organisms is endogenous material entering the rumen in forms which include proteins in saliva, sloughed epithelial cells and urea excreted in saliva and across the rumen wall. As well as N, the ruminal microbes require other nutrients for their survival, reproduction and growth, particularly sulphur, and cobalt for vitamin B₁₂. The main factor, however, which usually limits the synthesis of microbial protein is the amount of energy available in the rumen. Rumen microbes require an energy source for protein and cellular synthesis. When N and other essential nutrients are not limiting, about 8g of microbial protein are synthesised from rumen ammonia for every MJ of dietary ME consumed, and about 8g of RDP are necessary to produce the ammonia for the synthesis of 8g of microbial protein. Production of microbial protein may, therefore, be limited by the availability of either energy or RDP.

Energy supply for microbial protein synthesis

The major supply of energy for the rumen microbes comes from the fermentation, by the microbes, of dietary carbohydrates. The major by-products of the fermentation, in addition to heat and methane, are the volatile fatty acids (VFAs) acetic, propionic and butyric, which themselves provide the ruminant ’host’ with around two thirds of the total amount of ME they gain from their diets. Only those nutrients which yield ATP on fermentation provide the fuel to drive the rumen microbes’ synthetic processes. The yield of microbial crude protein flowing from the rumen to the stomach and intestines is, therefore, related to the fermentable metabolisable energy (FME). At maintenance, about 9g of microbial protein is produced per MJ of dietary FME. Diets high in silage may be low in FME and may produce less MCP than predicted by its ME value. Animals on silage diets may respond to protein supplements, particularly those supplements high in UDP.

Rumen retention time and outflow rate

The time for which ingesta is retained in the rumen is highly correlated with the level of feeding (L); as L increases, the rumen empties faster. Ruminal outflow rates vary from about 2% to 8% of total contents per hour. Outflow rates for three levels of feeding are shown in Table 6.1.

Retention time has an effect on the degradability of dietary crude protein. Dietary protein which is slowly degraded (SDP) will be less degraded and produce more UDP as retention time falls. For example, soybean meal at feeding level 1 is predicted to have 50g UDP and 360g RDP but, at level 3, 160g UDP and 250g RDP.

Effect of feeding level

Feeding level (L) has an effect on a number of nutrient parameters, including digestibility, metabolisability and the degradability of dietary crude protein, as described above. It also has an effect on the amount of microbial crude protein (MCP) produced per MJ of dietary FME (Table 6.1). The outflow of MCP per MJ of FME increases with L, principally because there is less recycling of microbial protein in the rumen as the outflow rate increases (assuming supply of N is not limiting).

Table 6.1 : Effect of level of feeding on microbial protein synthesis (from AFRC 1993)

Nitrogen supply for rumen protein synthesis

If dietary crude protein is insufficient or insufficiently degradable, then the synthesis of microbial protein will be limited by RDP supply. Recycling of urea to the rumen will help replace some of the deficiency of dietary RDP but ultimately, feed intake will decline.

If dietary RDP is available in excess quantities, FME limits MCP. RDP is wasted, blood ammonia and urea levels rise. Ideally, for greatest efficiency of nutrient use,

ERDP/FME = 9, 10 or 11 for feeding levels of 1,2 or 3

The components of metabolisable protein (MP)

MP is the total digestible true protein available to the animal for metabolism after digestion and absorption of the feed in the animal’s digestive tract. MP has 2 components.

1. Digestible microbial true protein (MTP); produced by the proteogenic activities of the rumen microbes. About 25% of MCP is present as nucleic acids which cannot be used by the ruminant. Of the remaining 75%, 85% is digestible in the intestine. Thus

DMTP = 0.75 x 0.85 x MCP = 0.6375 MCP (11)

2. Digestible UDP (DUP); from zero to 90% of UDP is digestible post-rumen, depending on the feed, its composition and pre-treatment. DUP is usually 60% to 80% of UDP.

Figure 6.2 Ruminant digestion of crude protein (CP) described by the Metabolisable Protein system (from AFRC 1993)

Table 6.2 : Composition of some common forages (per kg DM) (from AFRC 1993 Appendix 1 and various NSW sources where indicated)

ME : metabolisable energy
FME : fermentable metabolisable energy
EE : ether extract, an estimate of fat content
CP : crude protein

ERDP: effective rumen degradable dietary protein with the potential to be captured by rumen microbes at rumen digesta outflow rates of 2%, 5% or 8% per hour (feeding levels 1, 2 or 3).
DUP: digestible undegraded rumen protein

Energy requirements of the animal

Energy requirements for maintenance

In nutritional parlance, the term ’maintenance’ can have different interpretations depending on the particular circumstances in which it is applied. Strictly, the maintenance requirement is the energy required to maintain a fasted, penned, adult animal at zero weight change under thermoneutral conditions. For relevance in situations where sheep are run at pasture, energy allowances for zero weight change must include expenditure on grazing activities and temperature regulation.

Basal energy requirements vary with liveweight. Heavier animals, however, are able to meet basal metabolic functions with greater efficiency than light ones, at least partly because the demand for energy for those functions is related to surface area rather than volume of the animal. Energy for maintenance requirements, therefore, has been found to increase linearly with metabolic liveweight, which is liveweight raised to a power less than 1, usually 0.75 (W^.75).

Young animals have higher requirements for energy, even after scaling for metabolic liveweight. This effect declines at a decreasing rate with age, an effect which can be described with an exponential function. NE requirements for sheep at maintenance can be estimated from the expression

NE_m = 0.26 xW^.75 x e^-.03A       (12)

where A = age in years. The term e^-.03A evaluates to 0.99 for sheep of 3 months of age to 0.84 for sheep of 6 years.

NE requirements do not vary with M/D or metabolisability, but ME requirements do, as inferred by the equation defining km on page . Consequently, ME requirements can be described by equation 13 (Figure 6.3)

ME_m = (0.26 x W^.75 x e^-.03A) / k_m       (13)

Figure 6.3 Relationship between ME_m and W (equation 13) for confined animals (assuming M/D = 11 & k_m = 0.72). Animal require less energy for maintenance as they age, regardless of liveweight.

Prolonged low level feeding

Sheep fed under drought conditions are usually allowed to fall to low bodyweights (35kg for medium frame strains of Merino) and maintained at those weights, usually with rations of predominantly cereal grain. ME intake for maintenance under conditions of confinement and mild temperatures for adult 35kg sheep may be as low as 4.5 MJ ME per day with a wheat plus roughage ration (M/D of 11). This estimate, based on feeding trials, is lower than that predicted from equation 13 (illustrated in Figure 6.3) indicating a probable increase in efficiency of conversion of ME to NE in sheep on prolonged low energy diets. Publications commonly recommend 2.7 to 3.0kg of wheat per week for 35kg adult sheep; about 5MJ ME per day. (See also Table 6.14.)

Increase in maintenance with high level feeding

There is evidence also that the inescapable non-productive energy expenditure, the ’maintenance’ requirement, does vary directly with feed intake. In other words, as feed intake increases above maintenance levels and the animal stores or secretes NE in some form of product, the portion of energy intake assigned to maintenance increases. The reason for this probably includes an increase in the size and metabolic activity of a number of organs and tissues with changes in the rates and energy costs of blood flow, protein turnover, sodium and potassium ion transport and other essential processes. Consequently, APC (1990) advise that the term 0.09 x ME_I (ME intake) be added to the maintenance component of above-maintenance rations.

Energy for grazing

Compared to pen-fed or confined sheep, grazing sheep expend additional energy walking, climbing and, possibly, additional energy eating if feed availability at pasture is less than that in a pen. (It usually is.) A 50kg sheep, for example, which walks 5 kilometres per day, climbs 500 metres and eats for 4 hours longer at pasture than would occur in a pen, will require an additional 2 MJ ME per day above maintenance for those activities. As a general rule, sheep in good grazing conditions (high stocking densities) may require an additional 20% to 40% above maintenance for grazing; in extensive, hilly country that may increase to 50%.

Energy for temperature regulation

At temperatures below thermoneutrality, animals must increase heat production to maintain body temperature. The highest ambient temperature at which this occurs is termed the lower critical temperature (LCT). The LCT, and the rate at which heat must be generated to maintain homeothermy at temperatures below it, depend particularly on the animal’s insulation which, in turn, is strongly influenced by the depth of wool (Table 6.3).

Table 6.3 : Lower critical temperature in dry still air(from APC 1990)

Maximum attainable heat production is termed summit metabolism. The summit metabolism of adult sheep is about 2.16 MJ/kgW^.75 which is about 8 x maintenance in thermoneutral conditions. This rate of heat production can only be sustained for a few hours. Half summit metabolism can be sustained for several days.

As described earlier, heat production occurs as an unavoidable consequence of maintenance and productive functions such as growth or lactation. The inefficient conversion of ME to NE produces heat so, consequently, animals being fed for higher levels of production, such as lactating ewes, will unavoidably produce more heat than those fed at lower levels, such as adult wethers at maintenance. One would expect, therefore, that the LCT of the latter would be higher than lactating ewes.

Wool provides much insulation, as indicated in Table 6.3. The sheep which are most likely to require the diversion of energy into heat production and suffer the risk of death from hypothermia are lambs and any class of sheep off-shears. Sheep undergo a dramatic change in their ability to withstand cold temperatures at shearing. Loss of insulating fleece is compounded by a usual decline in feed intake immediately after shearing. This decline in intake has two causes; one a direct effect of shearing which has been observed even in penned sheep and, two, a reduction as a consequence of enforced deprivation during yarding and shedding for shearing which leads to a decline in rumen microbial population. Both effects are reversed after a few days and feed intake returns to higher levels than those before shearing. In the immediate post-shearing period, however, the sheep may be extremely susceptible to chilling. The provision of shelter and, if conditions are severe, the provision of external heat in a shed, may be a more appropriate action than the provision of extra feed to cold-stressed sheep off-shears.

As an example, a fully fed adult wether off-shears has an LCT of 18”C (Table 6.3). In a temperature of 0”C, heat loss is 13MJ per day, 5MJ per day greater than that produced by ’maintenance’ heat production. The same increase in heat loss would be achieved by a wind of 12 km/hr (5 km/hr at sheep height) and a temperature of 10”C. 5MJ represents a 60% increase in energy requirements above maintenance at thermoneutral temperatures. Severe conditions, such as freezing conditions and 25 km/hr winds (at sheep height) increase heat loss to 24 MJ per day. (The nomograms necessary to calculate energy requirements for homeothermy are published in APC (1990).)

Summary of energy requirements for maintenance in grazing conditions

The general equation for maintenance requirements for sheep, an extension of that shown on page , is

ME_m = (0.26 x W^.75 x e^-.03A)/k_m + 0.09 x MEI + E_graze/k_m + E_cold (14)

For adult sheep of 40 to 50 kg in thermoneutral conditions, ME_m is generally in the range of 7 to 8 MJ per day but this value can be significantly increased by low quality feed(lowering km), extensive grazing conditions, or cold stress.

Energy requirements for productive functions

Gestation

The rate of energy storage in the foetus, placenta and uterus increases exponentially throughout pregnancy and, by term, the ME required to match the energy demands of a single pregnancy approaches that required for maternal maintenance alone; for a twin pregnancy, ME requirements exceed 2 x maintenance.

Figure 6.4 illustrates the rapid increase in energy requirements which occurs in the last 30 days of gestation. For a single pregnancy in a 50kg ewe, the ME required to satisfy the energy demands of the foetus and associated maternal structures is around 5.6MJ, on top of around 8MJ required for ewe maintenance. (The term 0.09 x ME_I is not included in calculating energy requirements for gestation.)

Figure 6.4 : Daily energy requirements to maintain a 50kg pregnant ewe, allowing an additional 2MJ above maintenance for grazing, with a single foetus weighing 4kg at term, on high quality (M/D = 11) pasture (from APC 1990).

Total energy requirements for ewes in late pregnancy are seldom met by ingested feed, not least because the physical limitation on feed intake during pregnancy makes it unlikely that ewes can consume sufficient feed unless it is of high quality (high metabolisability). When the deficit between the demands of the pregnancy and the supply of dietary energy is small, ewes can manage to meet the requirements from bodily reserves. When this compensatory capacity is overloaded, however, serious consequences including pregnancy toxaemia become likely.

Energy requirements for liveweight gain

The NE stored per kg of liveweight gain varies with the age and sex of the animal and with the rate of gain because the proportion of fat, protein and water in the gain varies. In young animals, a lower proportion of the gain is fat and a higher proportion is protein and water than in older animals. At higher rates of gain, more of the gain is fat. The NE stored for each kg of liveweight gain varies approximately from 12MJ at 10kg liveweight, to 20MJ in a half-grown sheep, to 26.5MJ in a mature animal (APC 1990, p 45).

The efficiency of conversion of ME to NE for liveweight gain (equation 7) varies from 0.3 for feeds of low quality (q_m = 0.38, M/D = 7) to 0.51 for feeds of high quality (q_m = 0.65, M/D =12). Thus, for a half grown animal, 1kg of liveweight gain will require 39MJ ME on a high quality diet (3.25kg of feed DM) to 53MJ ME on a low quality diet (7.5kg of feed DM), above maintenance requirements.

The ability to predict liveweight gain is important in the preparation of rations for meat sheep being grown for sale, in the prediction of rates of gain at pasture and in the supplementation of sheep on inadequate pasture. In grazing sheep, particularly wool-producing breeds, liveweight gain is an opportunistic response to good feed conditions rather than a result of a deliberate attempt to increase liveweight. While not formally predicted by wool producers, liveweight gains in wool-producing flocks are still essential components of normal sheep management - to enable the growth of young animals, the recovery of lost liveweight of ewes post-weaning and the development of body reserves to provide nutrition when feed conditions deteriorate.

Energy requirements for milk production

The NE of ewes’ milk varies with the fat content and day of lactation but is around 5MJ/kg. The efficiency of conversion of ME to NE_l (equation 6) varies from 0.55 (q_m = 0.38, M/D = 7) to 0.6 (q_m = 0.65, M/D = 12). The ME required for 1kg of milk, therefore varies usually in the range from 8 to 9MJ.

The milk production of sheep varies with breed, age, number of lambs, feed quality and availability, and stage of lactation. Merino ewes given adequate pasture at commercial stocking rates will usually produce, on average, 1kg of milk for the first 4 to 6 weeks of lactation and sustain lamb liveweight gains of 200 to 300g/day.

Energy requirements for wool production

The energy required for wool growth is relatively small and usually ignored when formulating feed rations. Wool growth occurs in wool breeds at maintenance and even sub-maintenance feeding levels so can be considered effectively as a part of maintenance.

What is of interest, however, is the wool growth response to additional levels of feeding, particularly supplementary feeding, as the high value of wool may, at times, justify additional expense on supplements. APC (1990) report that 0.5g to 0.9g of clean wool is grown per MJ of dietary ME in non-lactating, non-pregnant sheep. Thus, an additional kilogram of DM of supplementary grain (M/D = 11) will result in an additional 5g to 10g of clean wool().

Dry sheep equivalents, DSEs

As we will read in the following section of this chapter, pasture provides the major source of nutrition for grazing sheep in Australia. The relationship between the variable provision of energy and protein from pastures and the requirements of the animals grazing the pastures is very complex. Producers, however, through experience and intuition, develop the ability to predict the performance of animals at pasture and know approximately how many animals can be nourished per hectare of each of their paddocks. To do so, they frequently use a system of describing the nutritional requirements of their flocks and herds relative to a standard ’animal’. In Australia, the system most widely adopted uses a wether (a ’dry sheep’) as the standard and animals are rated as ’dry sheep equivalents’. So, instead of calculating the requirements of animals in MJ of energy and attempting to predict the energy availability of the pastures in the same units, it is much simpler to describe an animal’s requirements in terms of DSEs and a pasture’s ability to provide nourishment as a ’carrying capacity’ of so many DSEs per hectare.

A dry sheep equivalent is defined thus :

The DSE of a wether is generally considered to be 1.0. More correctly, this should be adjusted for mature size, so that the DSE of a fine wool wether is 1.0, a medium wool wether (mature weight 50 kg) 1.08, a South Australian strain wether (mature weight 55 kg) 1.16. As DSE values are commonly used for within-flock comparisons, the variation between strains is often ignored.

The DSE of a ewe is commonly quoted, somewhat uselessly, as the relative requirements of a ewe at peak lactation and maintained at constant liveweight. A much more useful definition recognises that lactating ewes lose liveweight in lactation as a normal event and, provided it is not excessive, maintain adequate health and productivity.

The DSE value of a ewe varies throughout the year, depending on the stage of the reproductive cycle. It varies also with the mature size and, if we are concerned with the average DSE of a ewe flock, the reproductive rate.

An estimate of the minimum levels of feed required to allow for acceptable levels of production and health are shown in Tables 6.4 and 6.5. These values allow for weight loss of the ewe in early lactation and a slight weight gain in late lactation.

Confusion often arises in discussions about the ’annual’ DSE value of a ewe. For example, a producer wishes to compute the number of ewes which will replace a mob of 400 wethers in one paddock, to remain in the one paddock all year. What DSE value should be attributed to a ewe?

Table 6.4 : Requirements of a flock of 1000 ewes and 800 lambs, relative to a flok of 1000 wethers[1]

At some times of the year the DSE value of a ewe is less than 1.0 (ewes are smaller than wethers) and sometimes the DSE value approaches or exceeds 2.0. Should we use average values, or peak values? In real life, the matter of concern to the producer is that the level of nutrition for the ewe flock at the most feed limiting time of the year should be similar, relative to their requirements, to that which had been provided to the wethers. Consequently, the value we should use is the DSE value for the ewes in the most feed limiting month. It is this value which determines how many ewes should be stocked in the paddock for the entire year.

If the ewes are lambing in June, the relative requirements of ewes in the feed-limiting months of July and August will be 1.73 and 1.84 (see Table 6.4), relative to wethers. Thus the paddock should not carry more than 400 / 1.84 = 219 June lambing Merino ewes.

Table 6.5 DSE values of a range of grazing livestock

This definition of a ewe DSE is the only appropriate definition for a producer who wants to know how many ewes can be carried in a particular area all year round and how that number can be varied with time of lambing. The feed limiting months could be in autumn or in winter, depending on the district, the time of lambing, the timing of stock sales and purchases, and may coincide with late pregnancy or a post-weaning period.

In any event, this approach is only ever used for predicting the most likely outcomes of planned strategies, or for attempting to explain past events such as disease outbreaks associated with undernutrition. Most times we are asked to deal with the actual health and production responses of flocks of sheep to current feed availability. Graziers must try to match feed requirements to pasture availability well in advance - they cannot readily ’put and take’ stock on their property at short notice. Often veterinarians are asked to advise graziers who have stock which are underfed because seasonal pasture growth has been worse than expected. The notions of a DSE and feed limiting periods help us to understand what we observe and to plan for increased efficiency of production in the future. Table 6.5 shows DSE values for a range of sheep and beef cattle.

Pasture Production

Seasonal Patterns and Species

Figure 6.5 Pasture production varies from district to district but follows a similar pattern in general. Low temperatures and, in some districts, lack of moisture, limit winter growth. Lack of effective rainfall (precipitation less evaporation) limits pasture growth in late summer and early autumn. Pasture growth rates are highest in spring.

Pasture growth is determined mainly by rainfall and soil temperature. Rain falls mainly in the summer in northern Australia and winter in southern Australia. Moving north from Victoria and SA, summer rainfall increases, so that on the northern Tablelands there is normally sufficient rain for substantial pasture growth in the summer. Pasture growth is strongly seasonal and the average pattern of pasture growth varies from district to district. In general, pasture growth is limited in winter, by low temperatures and also by lack of moisture in the summer rainfall zones. In southern Australia, pasture growth peaks in spring with a less marked peak in late autumn-early winter. In northern NSW, pasture production peaks in late spring-early summer (Figure 6.5).

In South Australia and Western Australia there is a consistent and distinct dry season in the summer and early autumn. In the low rainfall zone and parts of the medium rainfall zone the dry period is so long (ò5 months) that perennial plants will not survive. For this reason annual plants form the basis of pastures. These germinate in the autumn and are productive for 6-9 months after which the only pasture available is comprised of mature dessicated residues. Annual pastures are also widely planted in central NSW and northern Victoria. In the wetter areas of the medium rainfall zone and in the high rainfall zone, as well as annual species pastures include perennial grasses such as phalaris, perennial rye grass and cocksfoot. These plants are dormant during the dry summer-autumn period but will not die out provided the length of the dry period is not excessive (as a guide, less than 5 months). In some areas there is sufficient rainfall during the summer to allow useful growth from perennial species, such as lucerne.

The main pasture legume grown in southern Australia is subterranean clover, of which there are several varieties to suit areas with annual rainfall from 400 to 800 mm. The varieties differ in their time of flowering. Early flowering varieties are best adapted to low rainfall areas because they can set viable seed before the growth season ends. Sub-clover has a unique survival mechanism whereby the plant buries its seed in the spring. These seeds have a hard coat which repels moisture and thus prevents germination during brief periods of rain which may occur during the summer, particularly in the north. In drier areas, annual medics such as snail and barrel medic are grown.

Where there is good summer rainfall or irrigation, white and red clovers are grown. White clover is perennial and red clover is a short-lived summer-growing perennial. Where there are deep, well-drained soils which are neutral to alkaline, lucerne, which is a perennial, is grown. The very deep rooting system of lucerne allows it to survive very dry conditions whilst, with irrigation, it can be very productive (>25 tonnes DM/ha).

In the ley-farming zone, also known as the wheat-sheep zone, annual grasses grow with the annual legumes. These grasses include annual ryegrass, barley grass and silver grass. On the Tablelands perennial grasses grow with the annual and perennial legumes. In all areas, grasses extend the growing season and increase pasture production potential beyond that achievable by legumes alone. Native grasses, such as Microlena (weeping ricegrass) and Danthonia (white top or wallaby grass) can be as nutritious as introduced grasses, with better persistence and drought tolerance but with lower levels of productivity in winter.

easonal variation in pasture quality

In general, pasture plants are highest in energy when they are green, actively growing and before flowering and seeding occurs. As they mature and then, in some cases, die, the digestibility and ME level decline. The decline in quality is rapid over the period of flowering and seeding. Pasture quality and the corresponding growth stages are shown in Table 6.6. Note that these are average energy values about which there is variation due to differences between plant species and between leaves and stems.

Table 6.6 Pasture nutritional value declines as plants mature

Consequences of seasonal variation in pasture production and pasture quality

Not only does pasture growth and pasture quality vary throughout the year but the requirements of grazing flocks and herds varies, according to variations in their size (as animals are bought and sold) and their reproductive state. In a general sense, it is best to match times of increased requirements of the grazing animals to increased availability of pasture.

In many flocks, the reproductive cycle of sheep is managed so that ewes lamb at the start of or just before the main season of pasture growth. This coincides with the increase in energy demand by the ewe flock as the ewes lactate and the lambs grow and graze. This is one important factor influencing the choice of time of lambing, discussed in Chapter 3.

If good quality pasture is produced for 5-6 months from lambing, it is possible to maintain growth rates of 250-300 g/day and ’finish’ lambs at about 40 kg liveweight in the one season. This is important for prime lamb production but of less importance in Merino production. (Merino lambs rarely exceed 30 kg at 4 months of age.) In most areas, the pasture availability and/or quality declines as shown in Table 6.6 and at least some lambs which are being raised for meat production may have to be carried over to the next pasture growing season. Merino lambs in self-replacing flocks are usually carried over the dry season and the provision of supplementary feed or special purpose fodder crops is normal.

Assessment of pasture mass

Herbage mass is measured in terms of kg/ha of either total or green dry matter. The dry matter content of herbage varies with stage of growth:

Table 6.7 Digestibility of plant material varies with its stage of growth

Animal performance is determined by intake of DM and its nutrient content and for this reason, herbage availability is expressed in terms of DM. The nutrient content of green herbage is usually much higher than that of dead herbage (see Table 6.6). Thus availability of green DM gives a better prediction of animal performance than availability of total DM. However this requires sorting pasture samples into green and dead material or estimating the proportion of pasture which is green. Herbage mass can be estimated directly by cutting sample quadrats or indirectly with a plate meter, an electronic probe or by eye assessment.

Feed intake of grazing animals

Sheep and beef cattle in Australia satisfy the majority of their nutritional requirements from pasture. The successful and profitable management of sheep and cattle at pasture depends on the provision of sufficient nutrients to allow a defined or expected level of production, but to avoid providing more pasture than is required and thereby wasting opportunities to produce more meat or wool by adding more animals.

To understand the complex inter-relationships between the growth of pastures and the performance of animals grazing them, it is useful to explore some of the relationships one by one. The first set of relationships we will discuss are those which relate the amount of pasture on offer to the amount of pasture which animals consume.

Feed intake and quantity of pasture on offer

Within certain limits, the more pasture is offered to animals, the more they will eat. The amount of pasture ’on offer’ is termed ’pasture availability’, and is measured in kilograms or tonnes of pasture (expressed in terms of dry matter (DM)) per hectare. Generally, the denser and taller the pasture, the greater the pasture availability. When pasture availability is low, because it is too short or because the plant density is too low, animals cannot physically prehend and ingest as much feed in the time they can spend grazing as they can when pasture availability is high. To some extent, animals can extend their grazing time to compensate for low pasture availability but the compensation is not complete. Even when availability is very low, animals will not spend more than 11 or 12 hours per day foraging.

The general relationship between feed intake and pasture availability is shown in Figure 6.6. It is clear that the pasture intake of sheep is strongly influenced by availability below 700kg/ha but increases only slowly over 900kg/ha. While the general shape of this relationship remains fairly constant, the actual values are, however, strongly influenced by a number of factors. Figure 6.6 illustrates the case for an adult, non-lactating ewe of 55kg grazing pasture of four different digestibilities, ranging from very low to very high and with about 30% of the pasture present as clover.

Three of the most important factors which modify this relationship are

• the size of the sheep; in general, the bigger the animal, the greater the intake
• the reproductive status of the sheep; during lactation, ewes ingest more pasture at all levels of availability. Potential intake peaks at about 60% above that of a dry sheep 30 days after lambing, and declines to 20% above at 80 days post-partum. Ewes with twins have even higher intakes than ewes with singles, peaking at 90% above the intake of a dry ewe 30 days post-partum
• the quality of the pasture; sheep eat less pasture when the digestibility is lower. This last factor is probably more important than low pasture availability as a contributing factor to poor animal nutrition leading to requests for veterinary advice, and is discussed in more detail below

There are a number of other factors which also influence intake but less strongly than these three factors. These include the age and condition of the sheep and the proportion of leguminous plants in the pasture.

Feed intake and pasture quality

Pasture quality, in the nutritional sense rather than the agronomic sense, refers to the amount of metabolizable energy (ME) which the grazing animal can release from each kilogram of pasture dry matter. The level of ME is closely and directly related to digestibility. The higher the quality of the pasture, the more digestible it is and the higher the ME level.

High quality pasture approaches 80% in digestibility and exceeds 11MJ ME /kg pasture DM. Dead, dry grass may be as low as 40% digestible and produce only 5 to 6 MJ ME/kg DM. This large variation in digestibility has large and important effects on feed intake because low digestibility depresses energy intake in two ways; first, less pasture is eaten and second, each kilogram of pasture eaten produces less ME (Figures 6.7 and 6.8).

It seems paradoxical that less feed is eaten when its energy content is lower. One might think that animals would attempt to increase the intake of low energy feeds in order to maintain the total energy content of the diet. The factor that limits the intake of low energy feeds, however, is the rate at which feed is broken down and leaves the rumen. Feeds of lower digestibility take longer to break down into particles small enough to leave the rumen, compared to feeds of higher digestibility.

So, clearly, intake of MJ of energy falls even more rapidly with declining availability and declining digestibility than does intake of pasture dry matter. Figure 6.7 illustrates the intake of energy for the same range of digestibilities and availabilities as in Figure 6.6

Considering that adult dry sheep require about 8MJ ME per day to maintain body weight, it is clear that diets of moderately low quality will not provide sufficient energy for weight maintenance, no matter how much is provided. Figure 6.8 illustrates the variation in energy intake with different feed quality and different levels of pasture availability for different classes of sheep.

Figure 6.6 Pasture intake increases with pasture availability but is significantly affected by the digestibility (D) of the pasture. The example illustrated is for a —dry’ adult sheep of 50 kg.

Figure 6.7 Energy intake increases with pasture availability but is very strongly affected by the digestibility of the pasture. This figure illustrates the predicted energy intake of a 50 kg ’dry’ adult sheep.

Figure 6.8 Intake of metabolisable energy (ME) varies with pasture availability, pasture quality, and the age, size and reproductive status of the sheep. Below certain levels of availability and/or pasture quality animals cannot eat enough pasture to maintain weight.

Top - For feed of any given quality (M/D = 9.6 in this case), intake increases with increasing pasture avail-ability. This illustration predicts that an adult, dry (non-reproductive) sheep of 50 kg will not ingest sufficient pasture of that quality for maintenance of liveweight (requiring about 9 MJ of ME per day) unless there is more than 600 kg DM/ha available. Even when feed of this quality (moderate only) is at high availability, the sheep will not increase liveweight very fast, because it cannot ingest much more energy than is required for maintenance.

Bottom — For feed of any given availability (2 t/ha in this case), adult dry sheep require diets with more than 9 MJ/kg in order to ingest sufficient energy for liveweight maintenance. With feeds of high quality (> 11 M/D), sheep can ingest substantially more than required for maintenance, provided availability is satisfactory, and will therefore gain liveweight.

Feed intake and animal species

Most feeds have been evaluated for sheep and the nutritional values applied directly to cattle. The assumption that the feeds are digested similarly between sheep and cattle is generally true with minor exceptions. When DMD is about 0.66 or higher, values for DMD obtained with sheep tend to overestimate the value for cattle. Conversely, cattle digest low DMD diets better than sheep do, possibly because the feed remains longer in the rumen. Goats also appear to digest fibrous feeds better than sheep.

Feed intake and pasture edibility

Sheep prefer leaf to stem and green to dead material. Legumes have a higher edibility than grasses. Edibility is a plant property which affects quantitative intake by animals. With grasses there is a strong positive relationship between M/D and edibility. With legumes, edibility is an average of 17% higher than that of grasses of similar M/D. This is due to the more rapid rate of particle comminution (reduction in size) in the rumen and consequent faster outflow rate from the rumen. Legumes usually have a higher content of protein at an equivalent growth stage, and a greater proportion of this protein escapes breakdown in the rumen, due to the faster outflow rate. Thus legumes have very desirable nutritional properties as well as the capacity to fix atmospheric nitrogen in the soil. Some varieties of legumes have undesirably high contents of oestrogens which have adverse effects on fertility (Chapter 7). Legumes can also cause bloat under some circumstances. On balance, legumes are highly desirable components of pastures.

Grazing Management

The object of grazing management is to convert pasture into animal product and to optimise the level of inputs to maximise the profit of the grazing enterprise. Producers use agronomists and other specialised advisers to assist with the management of pastures, but veterinarians need to have a clear understanding of the relationship between pasture production and the nutritional state and the productivity of the grazing flocks and herds.

We have discussed already how livestock have different requirements for feed energy depending on their age, size and reproductive state, and how pasture availability and pasture quality determine the amount of energy livestock can harvest from pastures. How, then, should pastures be managed to best meet the needs of the grazing livestock?

Most sheep and cattle farms have relatively stable flock and herd numbers for periods of months at a time. Graziers determine how many animals they wish to have present for the rest of the year or some other time period and buy or sell stock to achieve that number. Certainly, adjustments can be made in response to unexpected seasonal events, like drought or above average rainfall, but these adjustments are generally minor or involve low sale prices (in droughts) or high purchase costs (in good seasons).

Thus, in any one year, the pastures present on a farm are the result of past management, seasonal effects like rainfall and soil temperatures and current grazing management. In sheep and beef cattle production, little can be done profitably to change the productivity of pastures in the short-term. Unless stock are moved off-farm or very high levels of supplementation are used, the only way in which a grazier can change the amount of herbage on offer in any one paddock is to move stock to or from another paddock. A decrease in stocking rate on one pasture will lead to an increase on another, and vice versa. So, within a season, the amount of pasture on offer is a consequence of seasonal effects, which are beyond the producer’s control, and stocking rate, which is under the producer’s control.

Because feed intake is determined by pasture availability and pasture quality, it is possible to predict the performance (weight gain or loss, level of wool or milk production, et cetera) of a group of grazing animals from an assessment of the pasture on offer to them. Table 6.8 shows the expected level of production for sheep of different classes from high quality pasture for a range of pasture availabilities.

Grazing management decisions, therefore, should be an attempt to balance the amount of pasture available to each class of stock to achieve the best combination of animal production levels from each class of stock. For example, it would not be rational to graze adult wethers on pastures with an availability of 800 kg DM/ha when, on the same farm, lactating ewes are grazing pastures of 600 kg/ha (see Table 6.7). The wethers would be gaining nearly 0.5 kg of liveweight per week while the ewes, particularly the twin rearing ewes, would be losing weight rapidly and reducing their lactations to levels which threaten the survival of their lambs. It would seem more rational to ensure that pasture availability is more fairly distributed - perhaps 400 kg/ha for wethers and 800 kg/ha for ewes. The grazier will achieve this by changing the stocking rate of the pastures.

Table 6.8 Expected levels of animal production from different levels of herbage mass

It should be noted that the production levels shown in Table 6.8 are given for highly digestible green pasture. The predictions are based on the expected level of performance of fine wool Merino sheep grazing pastures with a green component of high digestibility (73%) with a 15% legume content and 500 kg/ha of dead herbage. Predictions for lactation are for 25 days after lambing. When the pasture is of lower digestibility, lower levels of productivity will be achieved.

Even under set-stocking regimes, pasture availability does not stay constant but varies with the pasture growth rate. Thus any attempt to equate the level of nutrition of a group of grazing animals to the pasture availability must include consideration of seasonal changes which may have influenced, or may in the future influence pasture growth.

For example, at the end of winter, ewes lambing on pastures with herbage masses of 600 kg/ha may be under nutritional stress because they are ingesting substantially less than they require for maintenance of bodyweight (Table 6.8). Provided that their condition is moderately good and that the expected seasonal improvement in pasture growth occurs (Figure 6.5), the short period of undernutrition will not have any permanent deleterious effects.

There is no ’ideal’ level of pasture for grazing animal production and attempts to match pasture availability to a particular desired level of production will be thwarted by seasonal variations in pasture growth, unless a ’put-and-take’ system of moving animals is practised. Such a system may achieve the desired result on one pasture but presumably other pastures on the farm are being penalised simultaneously.

Similarly, there is no ’ideal’ condition score or growth rate for any particular class of stock at any particular time. If a producer seeks to achieve desired condition scores or growth rates in one group of animals at pasture, it will usually be at the expense of another group.

In profitable grazing systems, animals will fluctuate in liveweight and condition score throughout the year. At times they will be fed above maintenance, at other times below maintenance. No two years will be exactly the same and the condition scores or pasture availabilities which are associated with good grazing management in one year may be inappropriate in another year. In some years it may be necessary to use supplementary feed to avoid financial losses from undernutrition.

The objective of grazing management should always be to share pasture and other resources between the classes of grazing animals to maximise the enterprise profits. The way in which these resources are combined will be different from year to year, month to month and from farm to farm.

Stocking rate

Stocking rate (SR) is a major determinant of profit, as discussed in Chapter 3. It also has a major effect on pasture composition and the persistence of productive, edible pasture plants. If stocking rate is too low, productivity is too low and the enterprise is unprofitable. If the SR is too high, there may be insufficient nutrients for all of the grazing animals to produce efficiently or even to survive. In addition there may be undesirable changes in pasture composition and exposure of soil to erosion by wind and water. Stocking rate is often defined in terms of DSE/ha.

The reason that SR has such a profound influence on productivity and profitability is because it has a large influence on how much pasture each animal consumes and the proportion of the total mass of pasture which grows that is consumed, rather than left uneaten and decaying.

As SR increases, there are more animals present per unit area to eat the pasture. As pasture availability (on set stocked pastures) is the result of a balance between pasture growth and pasture consumed and lost from trampling and decay, it follows that pasture availability will be lower at higher stocking rates. It has already been shown that pasture intake per head declines with lower pasture availability (Figure 6.6). Consequently, we would expect that as SR increases, pasture availability will decline and pasture intake per head will decline.

This process in itself does not lead to any increase in grazing efficiency - indeed one might expect the opposite because more nutrients are being used for maintenance of body mass at high stocking rates than low stocking rates. In addition, pasture growth rates usually decline as herbage mass declines below an optimum level.

Increases in grazing efficiency do occur, however, as SR increases from low levels. The reason is that a higher proportion of the total pasture grown is consumed at high SR than at low SR. At low SRs, for example, perhaps only 20% of the pasture grown might be consumed. The rest of the pasture will be trampled or left ungrazed to die and decay. At high SRs, as much as 80% of the pasture grown will be consumed.

Thus, as SR increases from low levels, pasture availability declines, feed intake per head declines but a higher proportion of the available pasture is consumed and the total mass of pasture consumed increases. Animal production responses to stocking rate are illustrated in Figure 6.9.

Figure 6.9 Relationship between SR and production parameters

For sheep, an increase in stocking rate has a negative effect on wool production per head and a positive effect on wool production per hectare at low stocking rates, then a negative effect on wool production per hectare at high stocking rates. For all grazing animals, the same relationship applies to liveweight gain, and number of offspring conceived, reared and weaned. Fibre diameter of wool is reduced as SR rises, which increases the value of the wool up to a point, above which under-nutrition causes a break in the staple and consequent loss of value.

Figure 6.10 illustrates the general relationship between the main economic measures of farm production, based on Figure 6.9. Generally, gross income per hectare increases with production per hectare. (This will not be completely true if the quality of the product declines as SR increases, but is approximately true in most cases.) Variable costs rise linearly with SR because both variable costs and SR are directly related to the number of head per hectare. (In fact, variable costs will often increase exponentially, mainly because of increasing levels of supplementary feed which are required at higher SR.) The gross margin is the difference between gross income and variable costs and it is curvilinear like gross income, but reaches a maximum at a lower SR than the maximum gross income. The maximum gross margin occurs at a point which is referred to in simple economic theory as the ’optimum stocking rate’. It is the point at which profit is maximised, rather than production. In real life, the optimum SR is often considered to be slightly lower than that predicted in this way, for reasons associated with risk.

Figure 6.10 Relationship between SR and economic parameters

Optimal stocking rates for districts are strongly influenced by rainfall, rainfall pattern and soil type. For farms within the same district, optimal stocking rates are influenced by soil fertility (fertilizer application history or innate fertility) and pasture species. As a guide to the highest stocking rates which might be considered optimal, the following table indicates predicted optimal stocking rates and ’usual practice’ stocking rates for a range of average annual rainfall categories.

Table 6.9 Optimum and usual stocking rates vary with the expected rainfall for a farm

Grazing systems

Continuous grazing is the simplest form of grazing management which minimises management inputs. Pasture is rarely spelled from grazing and animals concentrate on preferred pasture species. If stocking rate is too high these species may be eliminated.

Set stocking can be used to describe continuous grazing, although the term is often used to refer to relatively short grazing periods when stock are not moved, eg for lambing.

Rotational grazing involves regular movement of stock between paddocks. It is widely practised in the belief that it improves both pasture and animal production. Experimental proof for this is lacking. Most experiments have shown little or no improvement in animal production. However, some pasture species require regular resting if they are to survive, in particular lucerne and kangaroo grass. Experiments on the southern Tablelands indicate that it is possible to maintain stands of lucerne with resting periods of 55 days and grazing periods of 5 days, with 20 ewes/ha; this would require 12 paddocks to operate this system. Other proven reasons for rotational grazing are:

• to avoid pugging damage following irrigation
• to avoid nitrate poisoning by grazing too soon following application
• of nitrogen fertiliser (not usually done in wool or beef production)
• to concentrate stock on undesirable species (eg barley grass) at flowering time to prevent seeding
• to concentrate stock on desirable species which are already present at high density and which may be starting to produce flowering stems; hard grazing will remove these stems and maintain the sward in a vegetative state

Benefits claimed for rotational grazing are:

• pasture may be rationed so that more is available when it is most needed; (there is no firm evidence for this; pasture which is saved deteriorates and the pasture which is grazed harder may be adversely affected)
• growth of pasture may be increased; (there is no evidence for this)
• adverse effects of excreta on intake may be reduced; (this is unlikely)
• transmission of parasites may be reduced (this is virtually always untrue)

Cell grazing is a form of rotational grazing which involves having a large number of paddocks or cells (40 or more), and grazing at very high stocking rates, with frequent movement of the stock. Sheep are rotated rapidly around the many cells during periods of rapid growth and more slowly during periods of slow growth. Thus sheep are prevented from grazing newly-emerged shoots. Heavy concentration of stock for short period ensures that grazing is uniform, although severe. Thus grazing selection is largely prevented. This form of grazing management was first developed in Zimbabwe by Alan Savory and was designed to simulate the grazing activity of large herds of wild ungulates moving across savannas.

Currently, a firm of agricultural consultants is promoting the technique through courses which are being well received by farmers. There is, as yet, no experimental proof that the technique is superior to conventional forms of grazing management. NSW Agriculture has laid down experiments to compare the systems and results will be available in the next few years.

Major benefits which are likely with cell grazing are those arising from closer monitoring of stock and pastures, due to the frequency of moving stock. This would allow problems with stock and pastures to be identified and corrected sooner. Also it would develop the observational skills of the grazier due to the frequency with which management decisions have to be made on stock movement.

Stocking density is the term used to describe the number of stock per unit area on a grazing area at any one time. Both stocking rate and stocking density may be expressed in terms of stock numbers, but are better expressed as numbers of a particular class of stock eg wethers, or DSE/ha. Stocking rate is the same as stocking density with continuous grazing. With rotational or cell grazing, stocking density is much higher than stocking rate, eg with rotational grazing around 20 paddocks, the stocking density is 20 x the stocking rate.

Subdivision increases the flexibility of grazing management and therefore the potential pasture production. Basic subdivision is usually determined by the availability of water, differences in soil and pasture types, and differences in slope and aspect. Further subdivision is determined by convenient mob size. Electric fencing has made subdivision much less expensive.

Pasture budgeting

Pasture budgeting is a planning exercise which compares the supply of pasture to the animal requirements. The supply is determined by the initial availability plus the expected growth rate, less the loss through decay and depradation of insects. The demand is determined by the animal requirements (Table 6.10). Initial availability is estimated using the techniques described earlier. Data on estimated growth rates can be obtained from district agronomists. Techniques for more accurate prediction of pasture growth rates on particular properties and paddocks are likely to be developed in the near future. They will be based on instruments which measure available moisture and nutrient concentrations.

Paddock recording

Monitoring the carrying capacity of paddocks is a valuable aid to improving the efficiency of grazing management. This is done by estimating herbage availability when stock enter and leave a paddock, and recording the number of grazing days, number of stock and their DSE. Estimation of condition score when stock enter and leave a paddock provides additional valuable information. Collation and analysis of this type of data helps to identify superior paddocks and plan stock movements and stocking rate.

Diagnosis of dietary deficiencies of energy and protein

Estimates of ME intake by grazing sheep are shown in Figure 6.8. These may be compared with ME requirements in order to identify deficiencies which can lead to undernutrition. The principles for calculating ME requirements are revised on pages 1 to 12 of this chapter. ME requirements listed in Table 6.10 are based on a combination of information from APC (1990) and AFRC (1993). The former is an Australian publication which deals more comprehensively with principles of nutrition in a user unfriendly manner. The latter is a UK publication which is more practically-based but has heavy emphasis on housed animals eating silage-based rations.

Energy deficiency

ME requirements in Table 6.10 may be compared with expected ME intakes in Figure 6.8. The following are major points which should be noted:

• Poor quality pasture (M/D 7.0 and lower), such as cereal stubble or mature pasture, is inadequate to provide maintenance requirements for weaners or adult sheep, at any level of herbage availability
• Medium and poorer quality pastures are inadequate, at any level of herbage availability, for weaners to grow at 100 g/day. In order to sustain this level of growth or better pastures of good quality (M/D 10.0) or better are required
• ME requirements increase rapidly in late pregnancy, being 63% higher than maintenance for ewes with singles and more than double for ewes with twins. This large difference justifies segregation of ewes when significant numbers of twins are expected. Following pregnancy diagnosis, ewes carrying twins can be given preferred access to better pastures, or fed supplements

• The effect of pasture quality on energy intake is potentially most serious for ewes in late pregnancy and for lactating ewes and their lambs. Unless ewes in late pregnancy are offered diets exceeding 9.5 M/D (or 10 for twin-bearing ewes), they will mobilise body tissues to provide energy for foetal growth. With information from Figure 6.4 and Figure 6.7, it can be seen that ewes cannot eat enough of a medium or lower energy diet to meet all of their requirements for maintenance, no matter how high the pasture availability is.

  The penalty of requiring ewes to mobilise body tissues is not necessarily high. During late pregnancy, ewes in good condition in mid-pregnancy may receive up to 15% below their energy requirements for maintenance without undue penalty to lamb birth weight. Ewes in moderate or good condition at lambing may receive 20% less and lose " a condition score in the first weeks of lactation without penalising milk production. Severe undernutrition may result in pregnancy toxaemia, low birth weight lambs, delayed onset of lactation, poor maternal instinct and reduced milk output leading to reduced lamb survival and reduced growth rate in survivors. Wool production, wool quality and possibly future reproductive performance will also be reduced[2]

• At least 1000 kg/ha of herbage DM of medium or better quality is necessary to prevent severe restrictions to the nutrition of both lactating ewes and their lambs. Lamb growth rates can be approximated as 20% of daily milk yield. Twin lambs need about 1 kg of milk to survive (a growth rate of about 100 g/day) and single lambs need about half that amount. At 600 kg/ha twin lambs are unlikely to survive and the growth of single lambs will be severely reduced. Below this level, it is likely that a significant proportion of the lambs in a flock will die at 2 to 3 weeks of age as the lactation of the dams falls to very low levels or stops completely. The condition score at lambing of the ewes influences their ability to sustain lactation in the face of low energy intake. The lower the condition score, the higher the ewe’s energy intake must be to avoid lamb losses
• Crossbred ewes with high lactation potential can only produce to their potential when they are grazed on abundant high quality pasture and they lamb in good body condition

Protein deficiency

Protein requirements in terms of dietary concentrations may be compared with concentrations in feeds (Table 6.2) for an approximate assessment of adequacy. For more detailed assessment and calculation of supplementary feed requirements, it is necessary to calculate intakes of metabolisable protein (MP) and compare with MP requirements. The concentrations of MP in feeds are the sum of RP and UP for the appropriate rumen digesta outflow rate (see Table 6.1 for the relationship between feeding level and rumen outflow rate).

For example, the MP intake by lactating ewes grazing grass at the late vegetative stage (Table 6.2) would be:

92 + 40 = 132 g/kg DM intake.

In order to calculate MP intake it is necessary to predict DM intake (DMI), dividing ME intake by the appropriate M/D concentration in the diet.

Intake prediction - For lactating ewes with singles grazing pasture with M/D of 9 at 1 tonne DM/ha, DMI is:

13.3/9 = 1.5 kg/day

Protein intake - MP content of early flowering grass (M/D 9.5) is 64 + 40 = 104 g/kg DM when eaten by lactating animals. Therefore MP intake is:

1.5 x 104 = 152 g/day

This may be compared with requirements, from which it appears that protein intake is likely to be sufficient for single-rearing Merinos but not for crossbred sheep in early lactation.

Table 6.10 ME requirements for grazing sheep on pasture of medium quality (M/D 9.0)* maintenance requirement increased by 20% to allow for energy cost of grazing[3]

Concentrate feeding

Veterinarians in sheep practice need a strong working knowledge of the nutritional basis of supplementary feeding of sheep because undernutrition is an important and common predisposing factor in a number of clinical conditions. By far the most important of these are pregnancy toxaemia of ewes, a disease which is almost entirely based on nutritional deprivation, and lamb pre-weaning lamb mortality, which is a complex of disorders in which nutrition plays a large part. Such conditions arise when the available pasture supplies insufficient energy and protein to maintain even minimal levels of health and production.

When the nutrient deficiency is minor and expected to be of short duration, it is customary for sheep to be supplemented at pasture with high energy, high protein ’concentrate’ feeds like cereal grains (wheat, barley, oats and maize) or legume grains (lupins, peas, beans). Roughages, like hay, and milling by-products (such as brewers’ grain and pea hulls) are occasionally used, depending on local availability.

’Drought’ refers to a severe and prolonged insufficiency of pasture. Under such conditions, generally the entire flock needs hand-feeding for periods of weeks or months and concentrate feeds form the basis of most drought rations.

Climatic conditions vary extensively from year to year but it is only the most extreme of the ’dry’ seasons which are considered droughts. The rest are broadly considered to fall in the broad range classified as ’normal’ seasons. Within the range of normal seasons the requirement for supplementary feeding varies widely. In some years, no animals require hand-feeding; in other years, most or all mobs on the farm require supplementation. There is also variation between farms in the frequency of hand-feeding. This may be a result of particularly favourable and reliable weather patterns, the use of fodder crops or crop residues, the choice of lambing season or stocking rate or other management procedures.

Supplementation at pasture and substitution

The need for supplementation at pasture occurs in response to one of two sets of circumstances. The more common circumstance is that of low pasture quality. As pasture senesces, nutritional value declines as the digestibility declines. The ME value (M/D) and, usually, crude protein (CP) value both decline sharply. Low digestibility limits intake so, paradoxically, as grazing animals need to eat more to maintain ME intake, they in fact eat less. This form of nutritional deprivation is not very responsive to reductions in stocking rate. While low stocking rates may allow some higher level of pasture selectivity, particularly of scarce plants which are still vegetative, the basic problem is one of pasture quality rather than availability. Supplementation due to low pasture quality is characteristic of summer grazing in southern Australia, and winter grazing in northern New South Wales. The object of supplementation in these circumstances is to raise the average nutritional value of the diet and to increase intake.

This example illustrates an important phenomenon associated with supplementary feeding of grazing animals at pasture - that of substitution. When animals are offered supplements they substitute part of their roughage diet (the pasture) for the supplement. The substitution rate is the amount of roughage intake depression divided by the weight of supplement eaten. In the above example, the substitution rate has been 120/200 = 0.6 or 60%. Substitution contributes to the inefficiency of supplementation with low quality forages (such as hay with an M/D of 8) of low quality pasture (with an M/D of 6, for example). A high substitution rate means that each kg of hay may increase the energy content of the diet by only 4MJ, not the 8MJ for which one might hope.

The less common circumstance requiring supplementation is low pasture availability. The availability of the pasture (its height and density) also limits intake and ruminants have only a limited ability to increase grazing time when availability is low. This form of pasture insufficiency occurs frequently in the winter in southern Australia, when soil temperatures limit pasture growth. Pasture is green, of reasonable nutritive value per kg DM, but is short. Producers may be able to adjust the stocking rate to overcome the deficiency but generally the period of low pasture growth is short and is often followed by a period of exuberant pasture growth - so permanent changes of stocking rate which affect the number of sheep present in spring are counter productive. Supplementation is frequently used to ’bridge’ the nutritional deficit of the winter ’feed trough’. The object of supplementation is simply to increase the total nutrients ingested rather than improve diet quality. Compared to the above example where feed quality was low, the effect of substitution may be less in this case, particularly where pasture availability is very low and the supplement is of high M/D.

The two circumstances of low quality and low availability of pasture can also occur together, particularly in autumn in winter rainfall areas. By the end of a long summer dry period the availability of the low quality senescent pasture falls and supplementation may become necessary until autumn rains arrive.

When hand-feeding is required on farms, it rarely involves all groups of sheep on the property at the same time. Generally, weaners or breeding ewes are the first to need supplementation, adult wethers the last.

Weaners

Weaner sheep frequently require hand-feeding because they have a limited ability to maintain health and productivity if they are losing liveweight for sustained periods[6]. High quality hay can be used to maintain liveweight but concentrates are generally more efficient and, consequently, are used more frequently. The amount of additional energy required by weaners which have access to some pasture is generally low - of the order of 1 to 3 megajoules of metabolisable energy (MJ of ME) daily. 100 to 300 grams of grain will supply this amount of ME - a feeding rate of 5 to 15 grams of grain per kg liveweight. Skill and experience is required in estimating the contribution of pastures to total dietary needs.

Ewes in late pregnancy

The dramatic increase in the energy requirements of ewes in late pregnancy frequently creates a need for supplementation and often at high rates. Roughage feeds rarely have sufficient energy density to usefully increase energy intake. Ewes grazing low quality senescent pastures may well require an additional 5MJ of dietary ME to prevent excessive loss of body reserves and lower the risk of pregnancy toxaemia. Ewes carrying twin foetuses may require a further 2MJ daily. Consequently, supplementation rates of 600 to 800 grams of grain per head per day (12 to 16 grams per kg liveweight) are occasionally required.

Lactating ewes

The nutritional requirements for ewes in early lactation are quantitatively similar to ewes in late pregnancy but they have additional requirements for protein and long fibre to sustain satisfactory lactations. The voluntary food intake (VFI) of lactating ewes is significantly higher than that of pregnant ewes, a fact which allows lactating ewes to perform relatively better than pregnant ewes on deficient pastures, particularly pastures of low digestibility.

Wethers, dry ewes and hoggets

These classes of sheep need hand-feeding less often because they generally have a better ability to tolerate periods of sustained weight loss. Nevertheless, in some seasons on some farms, supplementation is necessary but rates required are usually lower than those suggested for breeding ewes or weaners. Rates of concentrate feeding of 150 to 300 grams per head per day (4 to 8 grams per kg liveweight) are common.

Recommendations for preventive action

Cases of undernutrition of breeding ewes and weaner sheep frequently require veterinary intervention and advice to producers on the use of concentrate feeding. The advice given should include:

• a list of suitable feedstuffs which will supply the deficient nutrients
• an estimate of the amount required, as ’kg per head per day’ or ’kg per head per week
• instructions on the safest way to introduce the ration to avoid grain poisoning
• recommendations on the best frequency of feeding

Introducing concentrate feeds

Sheep are fed a variety of supplements when pastures are inadequate to meet their needs. The normal practice is for grain to be fed in a trail along the ground. Occasionally troughs are used, but for short-term feeding this is uncommon. Large grains, like lupins, can also be fed by broadcasting - a process which effectively produces an artificial stubble. When cereal grains or other high starch foodstuffs are used a number of precautions should be taken to avoid grain poisoning (see Chapter 16).

1. Sheep should be ’trained’ to eat the ration. Training can involve daily feeding for a few days and driving sheep to the trail or trough and holding them quietly near the feed. Grain feeding of ewes before their lambs are weaned will facilitate the future training of the lambs. Training is complete when the majority of sheep run to the trail when feed is delivered and immediately start eating. It is recommended that feeding rates do not exceed 10g/kg/feed during the training period; so, for a 50kg ewe, 500g per feed is recommended as a maximum
2. Once training is complete the frequency of feeding can be reduced. Initially, the level of feed should not exceed 5g/kg/day, but the feed can be given every 3 days at three times the daily rate (15g/kg/feed)
3. After two feeds at three day intervals, the rate of feeding can be increased, if nutritionally necessary, to 30g/kg/feed gradually over a 2 week period, if desired. Some mild cases of grain poisoning may still occur, even if the increase is made slowly
4. High levels of supplementation (over 45g/kg/feed) may be necessary to adequately nourish ewes in late pregnancy or low weight weaners in poor seasons. A low incidence of grain overload is almost unavoidable in such cases. To minimize losses:

• introduction should be gradual - over 3 to 4 weeks
• care should be taken not to disturb the routine. Changes from oats to barley or wheat or from barley to wheat should be accompanied by a reduction in feeding rates for several feeds
• some roughage should be offered if pasture roughage is in low supply
• some cereal grain can be replaced by lupin grain which has a much lower level of fermentable carbohydrate[5]

Computer Models

GrazFeed

GrazFeed is a computer model developed by CSIRO[6]. It can be used to predict the performance of grazing stock and either the response to nominated amounts of supplement or the amounts of supplement to achieve nominated levels of production. It considers the following factors:

Pasture: mass, % green, % clover

Animals: species, breed, age, liveweight, physiological state, wool depth, condition score

Climate: temperature, rainfall, wind speed

Topography:

Supplements: nutrient content in terms of M/D, crude protein and protein degradability

In order to use the model you need to have mastered the skills of assessing pasture in quantitative and qualitative terms, as described in these notes. Also you need to be able to assess condition score, which involves palpation of the spine and ribs; visual assessment of sheep is usually totally inaccurate because the fleece masks the body outline.

Feeding and management of sheep and cattle during drought

Droughts are an unfortunate but inevitable occurrence which cause deficiencies of both feed and water in most agricultural areas of Australia at irregular intervals. The effects of drought can be exacerbated by over-stocking, poor livestock management and poor planning of farm resources for storage and handling of animal feeds. It is advisable for graziers to have a planned strategy to invoke whenever drought threatens. In practice the onset of a drought usually is not apparent at the time. When rainfall is below average, and pasture production is reduced, supplementary feeding may be necessary, and this can drift into total hand feeding. Supplementary feeding may be practiced as a normal part of the annual cycle of feed supply. However, before contemplating supplementary feeding which is not part of a normal annual cycle, a drought management strategy should be invoked as follows:-

Initial appraisal

Numbers of animals, their condition, and market value should be determined according to the following categories:

• suckling lambs and calves
• weaners
• lactating ewes and cows
• pregnant ewes and cows
• dry ewes and cows
• wethers and steers
• rams and bulls

Feed resources available on the property should be determined in terms of quantity, quality, market value and negotiability:

• pasture
• hay and silage
• fodder shrubs and trees
• grains and other concentrates

Pasture can be used to take other people’s animals on agistment. Pit silage is the least negotiable and has to be used on site. Round bale silage can be transported, although with only about 50% DM, the cost of moving the dry matter is high. Hay and concentrates can be sold off the property.

Feed available off the property should be determined in terms of quantity, quality and cost delivered or agistment cost. Additional considerations include machinery and feeding facilities available and the labour available for feed preparation and hand feeding.

Planning decisions

Alternative procedures for animal management are:

• sell and buy back after the drought
• send away on agistment
• feed at the production level and sell in prime condition
• feed at the maintenance level
• feed for survival only

The choice between these alternatives will be determined by feed resources and costs, and the availability of labour, machinery and feeding facilities. The choice is likely to differ between animal categories. The obvious animals to sell are culls from all categories and non-breeding stock which are in good enough condition to sell for meat. The most valuable animals to retain are those which have been selected as the nucleus of the breeding flock or herd. If no particular selection has been practised, it may be more profitable to sell all breeding stock and replace them at a later date.

When animals to be retained have been selected, feed resources should be allocated preferentially to those animals which are most susceptible to drought. Animals which are pregnant, lactating or suckling are the most likely to die during drought. Young and aged stock are more susceptible than mature stock. Mature wethers and steers are most likely to survive. Nutrient requirements of lactating stock can be greatly reduced by weaning lambs and calves early. This can be done anytime after 5 weeks of age, providing that the weaners are given a good quality ration. A decision to join breeding stock should only be made after carefully considering the escalating cost of feeding such animals during pregnancy and lactation.

The choice to send animals away on agistment should be made only after personal inspection of the property. Due allowance should be made for the costs of stock transport, and regular inspection visits.

The choice to feed at a production level should be made on the basis of feed costs and the quality margin between store and prime animals, which is usually substantial during a drought. The attraction of feeding at production level is that the duration and cost of feeding, as well as the likely returns, can be determined with reasonable accuracy.

In contrast, the choice to feed at a maintenance or survival level is an open-ended decision in which the total costs of feeding are unknown. The decision should be based on a study of the probability of rainfall at particular times in the future and the calculation the costs of feeding are expected to be less than the replacement cost of equivalent livestock at the end of the drought.

Feeding for maintenance or survival

When animals have a body condition score greater than 2 (scale 1-5), it is unnecessarily expensive to feed them a maintenance diet. By feeding them at 50% of maintenance for a period, they can be allowed to lose weight and so reduce their maintenance requirements. This usually happens at pasture at the start of the drought. A guide to critical survival liveweights is given in Table 6.11.

The energy made available to the animal from tissue mobilisation is about 84% of the gross energy of tissue mobilised. The gross energy varies with age and fatness, ranging from 8 MJ/kg liveweight in animals which are young and lean to 34 MJ/kg in animals which are mature and fat.

Feed requirements (in kg DM/day) for maintenance are calculated as:

(Fasting heat production x 1.1) x 1/k x 1/(M/D)

The fasting heat production of sheep is 250 kJ/W^0.75 and that of cattle is 350 kJ/W^0.75. The fasting heat production is increased by 10% to allow for the energy cost of walking. If animals are allowed to graze under conditions where pasture availability is very sparse, this energy allowance for exercise could increase to 70% of fasting heat production. For this reason it is preferable to confine animals in small paddocks during drought feeding.

Table 6.11 Critical liveweights for survival by sheep and cattle; the liveweights are after an overnight fast, and in the case of sheep, minus fleece weight; from NSW Department of Agriculture

Nomograms have been devised for rapid determination of feed requirements for the maintenance of sheep and cattle. Amounts of feed DM are converted to an as fed basis by multiplying by 100/DM%. With pregnant animals, the feed allowance is 1.5 x maintenance during the last 6-8 weeks of pregnancy. With lactating animals, the feed allowance is 1.6 x maintenance for cows and 2.5 x maintenance for ewes.

When animals are fed at a low level of intake, heat arising from the diet is much less than in well fed animals, so that their critical temperature is much higher than normal. Under cold conditions, the actual critical temperature increases with wind speed and decreases with pelage insulation. In sheep on a maintenance diet with only 1 cm depth of fleece, the critical temperature is 30”C when the wind speed is 10 m/sec.

When the environmental temperature is less than the critical temperature, animals generate heat to maintain homeothermy by breaking down body tissues. This can be averted by increasing the maintenance allowance as shown in Table 6.12:

Table 6.12 Effects of wool depth on maintenance requirements of sheep under normal winter conditions on the southern tablelands (0-10”C, wind speed 2 m/sec.)

Wet conditions exacerbate cold by increasing heat loss due to the latent heat of evaporation of water. Clearly, shorn sheep in poor condition are especially susceptible to the effects of cold. In practice, the feeding allowance should be increased 2-3 weeks before adverse conditions are expected.

Selection of feedstuffs

Under drought conditions, the major limiting nutrient is energy. Feeds should be selected primarily on the basis of lowest cost per unit of metabolisable energy at the point of consumption. Due to the low density of roughages, costs of transportation are higher per unit weight than with grains; also their energy concentration is lower than that of grains, so that grains are invariably the cheapest source of metabolisable energy. Another problem with roughages is that within a particular type, such as pasture hay, the nutrient content and palatability varies substantially, according to the stage of cutting, presence of weeds, drying conditions and possible mould development. Thus there is normally a greater risk in buying good quality roughages than in buying good quality grains. Even with cereal grains, it has been found that their energy content, if grown under drought conditions, can be very low; eg 8 M/D.

Advantages of hay and silage are that they are much less likely to cause digestive disturbances than cereal grains and also acetate production is much greater on roughage than on grain diets. For these reasons, roughages are necessary:

• during the period of adaptation to all-grain feeding
• for feeding to sick animals, including those suffering from scouring or anorexia due to grain poisoning
• for feeding to lactating stock to ensure that milk fat levels are not depressed due to excess propionate production

Depression of milk fat content has been shown to cause starvation in lambs. Of the main roughages, hay and silage, silage is cheaper to store with little deterioration when stored in pits. Silage in pits is a particularly useful drought reserve as it can be left for many years. In contrast, hay is much more expensive to store when adequately protected from weather damage, and is more susceptible to deterioration. A guide to the nutrient contents of potential feeds for use in drought feeding is given in Table 6.2.

The values given in Table 6.2 are only a general indication of likely nutrient contents which can differ substantially between sources of feeds. During droughts many unconventional feeds are used and a guide to the nutrient content of many of these is given in ’Funny Feeds Summary’ printed by NSW Agriculture Feeds Evaluation Service.

The risk of grain poisoning should be considered when selecting feedstuffs. See Chapter 16 for a detailed discussion of grain poisoning.

Protein grains and meals can be valuable in improving the efficiency of use of cereal grains and roughages, by producing a better balance of nutrients at the tissue level, ie increasing the proportion of amino acids and glucose (from amino acids). This is particularly necessary in very young stock, as well as in pregnant and lactating stock, as indicated by their higher protein requirements in Table 6.13.

Table 6.13 Minimum protein and energy concentrations for drought feeding (per kg DM)

Lupins contain very little starch so that there is much less likelihood of grain poisoning with them; they are also very palatable and can be useful in adapting animals to grain rations. Whole cottonseeds are sometimes available at low cost. They can be fed up to 50% of a grain mixture (with higher levels the fat concentration inhibits rumen fermentation).

Mineral and vitamin supplements

Cereal grains are deficient in calcium and carotene. Calcium deficiency is most likely to affect lactating animals and weaners, and vitamin A deficiency is most likely to affect weaners and rams. Ground limestone can be mixed with the grain (15 g/kg) to provide Ca, and vitamin A may be given as a drench or intramuscularly. With pregnant animals, the limestone should be omitted from the ration for two or three periods of two days during the last three weeks of pregnancy to stimulate Ca absorption and reduce the chances of hypocalcaemia.

Grain from some sources has a low sodium content, in which case addition of salt is recommended at the rate of 5 g/kg for maintenance feeding and 15 g/kg for production feeding; the higher level stimulates intake by increasing palatability.

Feeds such as straws, leaves from certain edible trees and molasses require supplementation if animals are to survive. Of