VEIN : Sheep Health & Production : Chapter 3. The farm as a production system

Sheep producers do not manage their flocks so that the health of the animals is maximised. Nor do they provide such high levels of nutrition that the individual productivity of each animal is as high as its genetic potential will allow. They do not choose a lambing time which will maximise lamb survival or maximise lambing rate; they do not choose a shearing date simply because it will minimise the risk of off-shears mortalities. Their skill as managers is to combine resources and to compromise the operation of parts of the farm's production capacity so as to achieve the best overall result. They know from experience that, if they seek to maximise the nutrition of their stock, they will have too few sheep on the farm or a very high supplementary feed bill, and they will make no profit. Producers know that virtually all of the major production strategies that they employ interact with one another. The whole farm, its climate, pastures and livestock, are part of a system and the system is composed of parts which are highly interrelated. It is not possible to disturb one part of the system without disturbing other parts. 'Improving' one part, say, the survival of lambs by lambing the ewes in October instead of August, may lead to an increase in weaner mortality in autumn and a decline in the productivity and profitability of the whole system.

Science-trained advisers need a systems approach

A scientist's education is usually broken up into disciplines, such as nutrition, agronomy, animal health and others. Consequently, the tendency for scientists when advising producers is to emphasise their own discipline area and neglect others. The challenge we have as veterinarians servicing the needs of sheep producers is to know the extent to which our field of expertise influences the productivity of the whole system. This does not require that we be expert in all fields. It does require, however, that we are prepared to take a systems approach to formulating advice for producer clients.

What defines a good production system?

The success of a production system is usually measured in economic terms. This does not always perfectly predict the producer's objectives but it usually is close. The financial success of the system cannot be described by simply recording the success of parts of the system, such as the value of lambs sold, or the total income from lines of fleece wool. The financial success must be measured in terms of the whole system and with consideration of the costs involved in producing all income. For reasons discussed in the previous section, gross margins are often preferred for measuring the productivity of farming systems. Because farm area is usually the most limiting resource for graziers, gross margins are usually quoted as dollars of gross margin per hectare (GM/ha). The objective of the farm system is, therefore, to maximise the gross margin per hectare. To achieve this, the components of the farm system will be optimised, rather than maximised. For example, in one particular flock, the GM/ha may be maximised when the weaning rate of lambs is 85%. If money is spent on raising weaning rate, GM/ha may fall. Thus for that particular system, weaning rate is optimal at 85%.

Land area (hectares) is not always the most limiting resource. Sometimes labour or capital funds are the most limiting. In such cases, a farmer may wish to maximise GM/labour unit, or GM per $100 of capital. In Australia now, it is uncommon for labour to be limiting so GM/ha and GM/$100 of capital are the more common indices of farming success. GM per animal is not an index of success of the system. Maximal values of GM/sheep or GM/dse do not usually coincide with maximal values of GM/ha.

What are the boundaries of a farm system?

It is true that a farm is not a full system in itself but, in fact, part of a bigger system. For example, an increase in meat production from one farm has an effect on the price received by all producers. The effect is usually immeasurably small when only one farm is considered so it is often valid to consider that a farm system stops at the farm boundary. Regional policy makers cannot simplify farm systems to this degree when fixing price schedules. Thus milk production from dairy farms under restrictive licensing arrangements may operate with a quite different set of farm objectives from those of wool production based on a wool price determined on a deregulated world market. When working with individual wool producers it is usually an acceptable simplification to limit the system to one farm.

Computers and farm systems

All agricultural systems are complex and it is natural that computers are being increasingly applied to farm systems. Computer models in many cases replace simpler models of the system which are drawn on paper or even exist only in the mind of the farm operator. Computer models are unlikely to be perfectly accurate but they should be at least as good as any model we can devise without computer assistance and much easier to use. In many cases, computer models have a useful educational role for advisers, alerting them to possible consequences of new farm management strategies which had been ignored or had not been quantified. Examples of sheep farm models include GRASSGRO, from CSIRO Division of Plant Industry.

Designing a sheep production system

Once the decision has been made to operate a sheep production enterprise, there are a number of key decisions which have to be made. These include:

Although these key decisions can be seen as something of a hierarchy in that the first two are long-term strategies which cannot be easily changed in one or two years, all of them are inter-related and cannot be considered in isolation. For example, to compare prime lamb production to Merino wool production, it is necessary to compare them on an equivalent stocking rate basis and to ensure that the optimal implementation of each form of production has been considered. A prime lamb production operation may be more profitable than a Merino sheep enterprise which sells all the wether lambs at weaning, but less profitable than one which keeps all wethers to 4 years of age.

The first two decision areas listed above are often reviewed by producers and their advisers. Veterinarians who develop a special interest in sheep production are likely to be involved in decision making about production objectives and genotype selection. Veterinarians who are in general rural practice, rather than consultancy practice, are more likely to be providing advice to their clients on health, reproduction and nutritional management which has an effect on the latter 4 areas; flock structure, stocking rate, time of lambing and time of shearing. It is important to understand something of the relationship that these 4 factors have with whole farm production before making recommendations to improve particular aspects of flock performance.

Flock structure

The flock structure or flock composition refers to the age and sex profile of the flock - ie, the relative numbers of sheep of each age and sex. The flock owner determines the flock composition on the basis of economic and management considerations and implements the plan by adopting appropriate buying and selling policies. The composition is also influenced by reproductive and mortality rates. It is usual for flock owners to adopt a broad aim for a particular flock structure but then to make minor adjustments in response to market fluctuations and year to year variation in reproductive rates. Optimisation of flock structure is one of the more powerful strategies which producers have to maximise profitability.

Determination of the best flock structure is strongly influenced by the production objective. For example, prime lamb producers with crossbred ewes and terminal sires have restricted options. Variations in the sex balance of flocks is of most interest in self-replacing flocks.

When planning, flock owners will attempt to predict the profitability of alternative flock structures. To do this correctly, it is necessary to ensure the total size of each proposed flock remains the same and this is usually achieved by comparing flock structures with the same total dry sheep equivalents.

Figure 3.1 illustrates the composition of five different flocks, all of the same size in terms of grazing intensity, or dry sheep equivalents. Note that, if wethers are retained, fewer ewes can be run and that the larger size and prolificacy of crossbred ewes restrict their numbers, compared to Merino ewes.

Optimum age to sell wethers

Merino wool-producing flocks are broadly of two types; all-wether flocks or self-replacing flocks. All-wether flocks are often preferred because of their resilience under tough conditions and their low labour inputs. They suffer the disadvantage that the genotype available to buy may not be as productive as the genotype which could be maintained in a self-replacing flock. Additionally, there is a constant risk of introducing diseases like Johne's disease, footrot and lice.

In self-replacing wool-producing flocks, wethers are often retained in the flock to 3, 4, 5 or greater years of age solely as wool producers. In some flocks, however they are sold off-shears as hoggets (12 years of age) or even as weaners. In the place of wethers, more breeding ewes are maintained. The relative merits of any particular structure are largely determined by the following characteristics:

Figure 3.1 The comparative composition of some 6000 dse flocks, from left to right, (top); all wether, self-replacing with maximum wether numbers, self-replacing selling wether hoggets, (bottom) self-replacing selling wether lambs, prime lamb flock. Note that a 6000 dse all-wether flock contains about twice as many adult animals as a 6000 dse prime lamb flock.

The optimum size of a wether flock varies with wool type

The adoption of a particular flock structure by a producer is, therefore, strongly influenced by the price of wool (of which wether flocks produce more) relative to the price of sale sheep, and the rate of depreciation of wether sheep with age. Consequently, fine wool producers tend to maintain large wether flocks because their wool is relatively valuable but their sale value, because fine wool wethers are of small stature, is often relatively low. Influenced by the same market conditions, strong wool producers have tended to sell their wethers at young ages. These habits are also related to climatic factors in particular areas - some districts have climates which favour lamb growth and therefore establish a reputation for producing well grown young sheep.

Optimum age to sell ewes

Ewes can be retained for more or less than 4 lambings - although in many flocks reproductive rates dictate that 3 lambings are too few to maintain breeder numbers. It is likely to be economically wise to keep ewes for 5 or more lambings when the return from livestock sales is high (thus favouring the higher reproductive ability of older ewes) and the return from wool is low (thus discounting the deterioration in fleece value as ewes age). Prime lamb flocks are an example of flocks where ewes have low fleece values and high initial purchase cost, but which produce valuable lambs for sale. It is usual in such flocks to keep ewes to 6 or more years of age.

The 'best' decision will depend on the particular economic environment in which each flock operates. If the economic relativities between wool and livestock values change and the producer expects the new economic environment to persist, it is rational to review the flock structure. Vizard (1990)[2], in a computerised linear programming exercise, found that higher wool prices favoured retaining more wethers, spring lambing favoured selling wethers at younger ages than in an autumn lambing flock and that, although there is no single flock structure which can be recommended for all flocks, there are some structures which perform well under a wide range of economic conditions.

Table 3.1 : Age and sex related variables which affect optimality of flock structures

Character[3]	Sheep of either sex	Breeding ewes
Fleece weight	Fleece weights in ewes and wethers peak between 3 & 4 years of age then decline. Wethers produce more wool than dry ewes but this is related to body size (skin area) and, consequently, feed intake.	Pregnancy and, more particularly, lactation depress fleece weights. Lactation for a single Mo lamb depresses fleece weight by 12% to 15%.
Fibre diameter	In all sheep, FD increases with age so fleece values tend to decline from 2 or 3 years onwards.	Pregnancy and lactation depress FD, which increases wool value per kg provided tensile strength is not also reduced significantly.
Reproduc-tive rate		Prolificacy and rearing ability increase with age peaking at 5 to 7 years.
Purchase price	Purchase price is highest for well grown weaners or, more usually, hoggets (1½years). Purchase prices usually decline beyond that age.
Sale price	Sale price follows the same pattern as purchase price. The rate of decline with age varies with market demand.	Sale price of older ewes is variable - sometimes there is a significant drop between 4½ and 5½ years.
Mortality rates	High in weaners and in sheep over 5 to 6 years of age. Older wethers have high mortalities on phyto-oestrogenic pastures.	Similar to wethers but added stress of reproduction increases mortality rates in ewes over 5 years of age.

Stocking rate

The stocking rate of a sheep farm is the number of animals run per unit area. It is usually expressed in terms of sheep per hectare or dse per hectare. Generally, discussions about changes in stocking rate assume that no change in inputs has occurred which would alter the pasture productivity of each hectare of land. Thus, changing sheep per hectare is equivalent to changing sheep per tonne of dry matter grown. Occasionally, producers talk of increasing stocking rate following pasture improvements - such as additional fertilizer or a change to more productive pasture species. If the additional inputs result in extra pasture production, the higher stocking rate (SR) may not represent a change in sheep per tonne of dry matter grown. Our discussions here, however, are concerned solely with changes in SR where it is the only variable changed. Thus, unless increasing SR itself increases pasture production (it usually does not), increasing SR means that more sheep have to share each tonne of dry matter grown.

At stocking rates which are commonly in the range adopted by commercial wool producers, the effect of increasing stocking rates is to reduce the pasture availability. Unless feed availability is very high (1500 to 1800 kg of pasture dry matter per hectare), daily intake of pasture is a function of availability*. Thus, in most seasons of the year, increasing SR reduces daily pasture intake of each grazing animal. Increases in the total amount of pasture consumed per hectare (because a higher proportion of the pasture is consumed, not because more is grown) may, however, lead to substantial improvements in the efficiency of conversion of pasture to animal product.

For the veterinarian involved in rural practice, the effect of SR on farm profitability is generally a peripheral issue. Nevertheless, it is important for the practitioner to be aware of the profound influence of SR on profitability because some of the most common sheep diseases involve nutrition - and nutrition in grazing animal management is almost entirely a function of stocking rate. For example, the clinical expression of worm parasites, pregnancy toxaemia, weaner ill-thrift and uncomplicated under-nutrition all have some relationship with pasture availability and, hence, stocking rate. The astute practitioner, realising the association but also realising the importance of SR to the client's income, will not advise a reduction in SR to limit the effects of disease but attempt to institute control measures which allow the maintenance of profitably high SR.

For the veterinarian who is a sheep specialist or who provides a consultancy service to sheep producers, analysis of the sensitivity of farm profit to changes in the current SR is an essential activity on every client's farm. Some key review papers are listed in the footnotes throughout this chapter.

Effects of SR on soils and pastures

At high SR, lower levels of soil cover can lead to an increased risk of soil and nutrient loss through wind and the scouring effect of running water. In many of the low rainfall areas of Australia, experience indicates that stocking rates have been excessive and damage to the soil-pasture ecosystem has been severe. In medium and high rainfall areas, the deleterious effects of high stocking rates may be avoided by taking steps to prevent erosion by means other than reduced stocking rates. Nevertheless, it may be necessary to substantially destock some pastures in some years - particularly in years when pasture growth has been much lower than expected[4].

SR also has an effect on the amount of pasture present, the proportion of particular pasture species which make up the pasture sward and, usually, an inverse relationship with total annual pasture productivity.

Effects on animal health

Increased stocking rates lead to lower liveweights at most times of the year and the consequences of this for ewes depend largely on when the low liveweights occur in relation to the annual reproductive cycle. For example, if under-nutrition on short autumn pastures coincides with late pregnancy, pregnancy toxaemia may be expected at a higher incidence at high SR than occurred at a low SR. If, however, the same increase in SR has reduced the liveweight of the ewes at conception and, subsequently, reduced their conception rates, the expected outbreak of pregnancy toxaemia may not occur. Probably the most consistent effect of higher SR on breeding flocks is a deterioration in lamb survival, particularly of twin lambs[5].

For all stock and, particularly, for weaners higher stocking rates generally lead to increased need of supplementation, for two reasons. First, liveweights are often lower at the start of prolonged 'feed trough' periods, so supplementation needs to start earlier and be continued for longer. Second, the profundity of pasture shortfalls is greater at high SR so the level of supplementation may need to be greater. Diseases associated with undernutrition and with grain feeding are expected to occur more commonly at high SR.

The association between SR and helminthosis is less clear. Whereas an increase in pasture contamination is expected at higher SR and sheep are expected to be more susceptible to parasites when underfed, shorter pastures in fact may lead to higher mortality rates of free-living nematode parasite larvae. This effect may be more useful with parasites which are particularly susceptible to environmental exposure, like Haemonchus contortus, than resistant parasites, such as Nematodirus[6][7]. One could conclude from the work of Brown et al[8] that a good nematode control program is still sufficient to maintain productivity at high SR but the penalty of an inadequate program is increased.

Effects on animal productivity

As already stated, increasing SR leads to reductions in pasture intake per sheep at most times of the year and, consequently, lower levels of individual animal productivity. Thus, at higher SR, sheep grow lighter fleeces of lower fibre diameter with shorter staple length and lower staple strength. Additionally, sheep have lower liveweights, at least for part of the year, and ewes have lower reproductive rates; survival rates may decline and the requirement for supplementary feed may be increased. Sheep which have been run at higher stocking rates will often have a lower average sale price.

Despite these negative effects on individual productivity, a change from a low to a medium stocking rate generally leads to increases in wool produced per hectare, number of lambs produced per hectare and number of sale animals available per hectare. The relationship between production and SR is curvilinear; ie, total production rises to a maximum with increasing SR then declines. (See Chapter 6 for a more detailed description of the relationship between production and SR.)

Most variable costs rise, on a per hectare basis, linearly with SR. The major exception is the cost of supplementary feed which usually rises on a per head basis with increasing SR. Most other variable costs rise on a per hectare basis but remain constant on a per head basis (shearing costs, for example).

Time of lambing

The time of lambing chosen by producers varies from district to district but the most common choices are the 5 months between and including May and September. April and May lambing is usually referred to as autumn lambing; June and July lambing as winter lambing; August, September and October lambing as spring lambing (despite the fact that August is late winter).

Autumn lambing is favoured in districts where the normal summer-autumn dry period is prolonged and in flocks where lambs are sold at the end of the growing season in their first year of life for a price based on their liveweight. Spring lambing is favoured in districts with prolonged spring and summer pasture growth which facilitates the growth of weaners and in flocks for which the main production objective is wool, rather than young sale sheep. Winter lambing is something of a compromise - often favoured by flock owners whose principal objective is wool production but who operate in districts with prolonged summer-autumn dry periods. It is usually only considered viable in areas with mild climates in winter which are therefore not severely adverse to lamb survival.

In many wool growing districts, late winter is one of the most critical times for maintaining the nutritional level of sheep because the climatic conditions place extra nutrient demands on sheep and the pastures are often short and growing slowly, if at all. More ewes can be carried on the farm all-year-round if their peak demands for reproduction (lactation) are delayed until spring when feed surpluses start to appear. In some districts this is possible but in other districts the rapid deterioration of pasture quality in summer makes the management of spring born weaners very difficult. Consequently, it is common for producers to lamb in late winter and sacrifice some carrying capacity (ie, they run fewer ewes) for the sake of easier weaner management. There are a number of important factors which influence the merit of different times of lambing. Consider, for example, the factors involved in selecting a best time to lamb on a farm in the winter rainfall zone of southern NSW.

All of these effects, and a few others[10][11] must be considered when selecting a time of lambing. The stocking rate effect is a major one, because greater numbers of ewes produce more wool as well as more lambs. It is often necessary to at least partially compromise other production parameters, including conception rate and lamb survival rate to allow higher stocking rates, but stocking rates are rarely maximised (by lambing late in spring) because of subsequent difficulties in lamb marketing (for prime lambs) or in weaner management over summer and autumn (for Merino and wool producing breeds).

In most self-replacing Merino flocks, lambs are not sold but are kept over summer. A subsequent section of the notes will discuss weaner management but it is relevant to mention the subject here. In general, the later is lambing in the year, the lighter are the lambs at the end of the growing season and the more lambs will be below critical liveweights and at risk of death or debilitating intercurrent disease.

There is the opportunity on many properties to improve the quality of weaner management to the extent that lambing can be postponed - from early winter to late winter or from late winter to early spring. The benefits which arise from such a change in management come from the ability to run more ewes but still achieve similar levels of productivity and health. Effectively, by coinciding the winter feed 'trough' with an earlier, and less nutritionally demanding, phase of the reproductive cycle, the dry sheep equivalent of each ewe in winter can be lowered and the number of ewes raised to compensate. Consequently, more ewes can be run per hectare, more ewes can be shorn and, provided RR does not fall, more lambs can be weaned per hectare. But, as always, there are some drawbacks. The three major economic ones are:

Plant (1981) lists 13 factors which need to be considered when producers select a time for lambing. Morley (1983) discusses 5 factors in more detail, particularly the relationship between optimum stocking rate and time of lambing.

Time of shearing

Many producers choose to shear in spring or early summer for some of the reasons listed above but the opportunity to shear later may increase the utilization of spring feed surpluses by maintaining higher sheep numbers, and growing more harvestable wool, for a longer period from the animals destined for sale as cull or cast-for-age. An additional advantage of later shearing is that it may increase the attractiveness of later lambing by maintaining fleece weight and fleece length on the lambs.

In winter rainfall areas, shearing can logically be postponed until late summer or early autumn so the sheep are sold before pasture residues become low. In summer rainfall zones, shearing can be postponed into early autumn or early winter. Unfortunately, in some districts, the presence of annual grasses with damaging awns makes summer or autumn shearing impossible. On some properties, changes to pasture management may reduce the prevalence of grass seeds and shearing times can be adjusted to take full advantage of pasture growth.

[1] White DH, McConchie BJ, Curnow BC and Ternouth AH (1980) A comparison of levels of production and profit from grazing Merino ewes and wethers at various stocking rates in northern Victoria Aust J Exp Agric Anim Husb 20 p 296

[2] Vizard AL (1990) Optimum structure of Merino and Corriedale flocks Aust Adv Vet Sci 1990 p 70

* Pasture intake is most strongly influenced by pasture availability and pasture quality. The relationship will be discussed further in the section on nutrition

[4] Daniel G (1991) Stocking rate & soil erosion Proc Aust Sheep Vet Soc, Darling Harbour, 1991 p 19

[5] Abbott KA (1991) The effect of stocking rate on farm profitability Proc Aust Sheep Vet Soc, Darling Harbour 1991 p 1

[6] Morley FHW (1983) Stocking rates In Sheep Production and Preventive Medicine University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 67, p 95

[7] Abbott KA (1988) Stocking rate and flock structure In Sheep Health and Production University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 110, p 399

[8] Brown TH, Ford GE, Miller DV & Beveridge I (1985) Effect of anthelmintic dosing and stocking rate on the productivity of weaner sheep in a Mediterranean climate environment Aust J Agric Res 36 p 845

* Many of the aspects of time of joining which affect reproductive rates are discussed further in Chapter 7. These two sections should be read conjointly.

[9] Foot JZ and Vizard AL (1993) Current sheep management: South Australia and Victoria In Proceedings of a National Workshop on Management for Wool Quality in Mediterranean Environments, Perth, Department of Agriculture, Western Australia, Wool Research and Development Corporation, p 67

[10] Plant JW (1981) Infertility in the ewe In Sheep University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 58, p 680

[11] Morley FHW (1983) Date of joining In Sheep Production and Preventive Medicine University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 67, p 83

Sheep Health & Production

Chapter 3. The farm as a production system

What is a production system?

Producers take a whole-farm view