Sheep Health & Production

Introduction | Factors determining the value of a wool clip | Recommended reading | General references

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Introduction

Importance of wool to the sheep's farm gross income

In self-replacing Merino flocks, income from wool usually makes up 80% to 95% of total gross income, and the remaining 5% to 20% is derived from the sale of cast-for-age adult sheep and cull hoggets. The actual proportion derived from wool varies with the flock composition, the relative value of wool and sheep, and the reproductive success of the flock ewes. To some extent, the emphasis on wool income relative to income from sheep sales is determined by the flock owner but some districts are better suited to breeding and growing young sheep, while others are better suited to an emphasis on wool production.

At the other end of the spectrum, prime-lamb flocks based on crossbred ewes mated to terminal sires typically receive 40% to 50% of their gross income from wool sales, and the balance from sales of prime lambs and cast for age adults. If, however, the cost of purchased replacement ewes is deducted from the sheep sale income, wool income becomes a higher proportion of total income relative to sheep trading profit. So, even in specialist lamb producing flocks, wool is a very important source of income.

How is wool harvested and sold from sheep farms?

Frequency of shearing

In virtually all Australian sheep flocks, sheep are shorn once annually. More frequent shearing leads to increased harvesting (shearing) costs and a reduction in the price received for wool on a per kilogram basis, because of low staple length.

Time of shearing

The time (month) of shearing is a critical decision for sheep producers because it has ramifications for so many other management strategies. Time of shearing is the second most important decision, after time of lambing, in determining the flock's management calendar. The decision has implications for wool quality attributes (staple strength, vegetable matter) and on the methods which can be properly used for flystrike control.

Age of lambs when first shorn

The time of shearing must be determined with time of lambing in mind, for several reasons. First, ewes should not lamb with ten months wool or more because they will have an increased risk of becoming cast during lambing and because shearing will clash with the early part of lactation. Second, the age, and therefore wool length, of the lambs at their first shearing has a large effect on the value of each kilogram of wool they produce. The age at which lambs are shorn cannot be decided in isolation because, at their second shearing, they are usually shorn with the adult flock.

Clip preparation in the woolshed

In Australia, sheep are shorn in woolsheds by professional shearers who shear between 100 and 200 sheep per day. Shearers generally work in teams of two to twelve, depending on how many shearing stands are in the woolshed ­ clearly larger flocks tend to have more stands in their shed. Typically, shearing occurs for two to four weeks, with 5% to 10% of the flock being shorn on each working day.

The fleece is removed from the sheep and thrown onto a skirting table, where the sweaty fribs are removed from the points and edges, corresponding to the belly, crutch, neck and mid-leg regions of the fleece, in a process called skirting. The fleece is then rolled into a loose ball and placed on the classer's table. Wool classers[*] are trained and licensed professionals whose job it is to examine a sample of the fleece, assess (subjectively) a number of characteristics of the fleece and allocate it physically to the appropriate wool bin. A wool presser removes wool from the bins and, using a hydraulic or (rarely now) a hand press, presses the wool into bales of approximately 195 kg. The presser brands the wool under direction of the wool classer (Table 4.1).

Table 4.1 A typical adult Merino sheep flock wool clip consists of a number of lines of wool, identified by a brand on each bale. The example below shows how a 60 bale clip might be prepared for sale.

The auction system for selling wool

The shorn wool clip is transported from the farm to a woolstore where it is prepared for sale by auction. At the woolstore, lines of wool are tested by core sampling for fibre diameter and yield and, if additional measurement is requested, for staple length, staple strength and position of break. These attributes are then made available to potential buyers when the wool is catalogued for sale at auction. Wool can be sold in as little as two weeks after it arrives in the woolstore, or the producer may prefer to delay the sale of some or all of the clip.

Alternatives to the auction system

Increasingly, producers are searching for alternatives to the traditional methods of clip preparation and auction sale in attempts to reduce production costs or, more importantly, achieve a higher average price for the clip.

Factors determining the value of a wool clip

Key attributes determining price

Fleece weight and yield

Assuming all wool quality characters are held constant, the more wool produced by a wool grower, the more money is banked. The amount of wool produced from a property is determined by the amount of wool produced per head of sheep (the fleece weight) and the number of sheep shorn. There is an important interaction between fleece weight and stocking rate, discussed elsewhere, but it should be noted that sheep are not, under commercial conditions, fed at such high levels as to maximise their fleece weights.

Wool sold at the woolstores consists of about 30% grease and other non-wool constituents. One of the pre-sale tests applied to samples of lines of wool is to estimate the yield (specifically the Schlumberger dry combing yield) of clean wool. The presence of grease in wool has virtually no bearing on its price when calculated on a clean basis; in other words, buyers calculate their offers to purchase wool on a cents per kg clean basis, then multiply that price by the yield stated for the line in the sale catalogue before making their offer on a greasy basis.

Consequently, the first key determinant of the value of a wool clip is its clean weight, or, alternatively, its greasy weight and its yield.

Fibre diameter

The most important determinant of price per kg of clean wool is the mean fibre diameter. Approximately 60% to 80% of the variation in price in any one wool sale can be accounted for by variation in fibre diameter (Figure 4.4). Fibre diameter is measured in micrometres (µm), frequently abbreviated to micron or µ. The finer the wool, the higher the price. In industry parlance, wools below 19.5µm are fine, wools between 20.5µm and 22.5µm are medium and wools over 23.5µm are broad or, confusingly, strong. Wools in the intermediate categories are called fine-medium and medium-broad.

The relationship between price and fibre diameter is not linear; price differences between each one micrometer category become larger as fibre diameter declines (Figure 4.1).

Figure 4.1 The category indicator prices for wool sold at auction in Australia, mid-December 2000. Source, Australian Wool Exchange Market Reporting Service

For some discussions, the relationship between price and fibre diameter is expressed as micron premium, which is the percentage increase in price for a 1 µm fall in fibre diameter. The micron premium of 19µm wool over 20µm wool in Figure 4.1 is 65%, while the micron premium of 20µm over 21µm is 26%. For broader wools, the micron premium is very low. The extreme nature of the premium being paid for wools under 19.5µm is a relatively new development in the market but, over the past decade, the 19µm premium has averaged 22%.

Staple length

The staple length (SL) of wool in a 12-month grown fleece varies with the breed, strain, the genetic characteristics of the individual sheep, its age, nutritional state and reproductive activities since the last shearing. Typically, in Merinos, adult fine wool sheep grow 85mm to 95mm of wool each year, while strong wool sheep grow 95mm to 105mm. Short wool has always been penalised at wool sales, relative to normal length wools but, over the past decade, wool buyers have increasingly penalised long wools as well. Figure 4.2 illustrates how these discounts are applied, on average, in the current market, and how fine wools are particularly subject to penalties. Wools below 65mm in staple length are heavily discounted because they are more likely to be directed into a lower value sector of the manufacturing industry ­ into woollen yarn manufacture, rather than into worsted goods. For worsted manufacture, wools are combed to produce a wool top, so raw wools which are processed this way are said to be combing wools or of combing length.

Long SL wool is discounted because many spinners believe that wool top made from long SL wool is less suited to their needs in terms of both spinning performance and product quality. The penalty for longer length would not occur if buyers were free to blend short and longer wools to meet a given specification. However many mills also put limits on the range of fibre lengths in the top. Research has shown that any effects of length variability are small but most mills are not yet convinced.

Figure 4.2 Both short and long staple lengths are penalised, with the heaviest penalties applied to finer wools. Source, The Wool Press, November 1999 and May 1999

Staple strength

Staple strength (SS) (also called tensile strength) is measured as the force, in newtons, required to break a staple of wool of defined mass and length, or linear density. Linear density is measured in units of tex, where one kilotex equals 10g/m. SS, therefore, is reported in units of Nktex^-1. Samples of wool are collected from lines of wool held in the woolstore before sale and submitted for testing. Staple strength is measured in wool testing laboratories with one of two types of machines which, basically, clamp each end of a staple of wool of known length and mass, and pull it until it breaks.

In these machines, the staple of wool is held at the tip (the end on the outer part of the fleece) and the base (the end cut at shearing). The point at which it breaks is called the position of break (POB).Because in any one sample a large number of staples is tested, the POB for a line of wool is reported as the percentage breaking in the tip, middle or base part of the staple.

Both the SS and the POB are important in wool manufacturing. In particular, wools with low SS and a high percentage of breaks in the middle are not desirable. Consequently, discounts are applied to wools of low SS. Discounts are particularly high in fine wools and the threshold at which they commence is also higher (Figure 4.3). In recent seasons, the point at which discounts commence is about 36 Nktex^-1 in fine wools and 31 Nktex^-1 in broad wools. This threshold has increased in recent years. The discount varies depending on demand but, generally, a decrease of 10 Nktex^-1 reduces the value of the wool about the same as an increase of 1µm in FD.

The mean SS of the Australian fleece-wool clip is typically around 35 Nktex^-1, so much of it receives some penalty every year. Wools of high tensile strength are called sound. Wools of low tensile strength are called tender. A wool break results from the development of a point in the staple of such low staple strength that the staple breaks spontaneously and the outer part of the fleece is shed.

Figure 4.3 Wools with staple strengths below about 35Nktex^-1 are subject to discounts. Source, CRC for Premium Quality Wool

Vegetable matter

As sheep graze they come in contact with bushes, weeds, trees and other forms of plant material. If the material is smooth it will generally fall out of the fleece but spiny or hairy burrs, seeds or stalks usually become entangled in the wool. This material has to be removed from wool in the manufacturing process and different forms of plant material vary in the ease with which they can be removed. The vegetable matter (VM) content of the core sample of wool is reported as a percentage, on a dry weight basis, and type, whether it be burr, shive or hardhead. Both the amount and type of vegetable fault in a line of wool affect its value. VM levels up to 1% usually have no or little effect on the price of wool at auction and levels between 1% and 2% have only a small impact. VM above 2%, however, will attract significant discounts which become greater as the VM level increases.

Figure 4.4 By attributing the variation in price received at auction to wools of different type to variation in key attributes of the wool, generally those reported by testing the core samples, it is possible to determine the major source of price variation. Fibre diameter is clearly the most important. Note that, in this figure, the price refers to that for clean wool, so yield is not mentioned. Source, CRC for Premium Quality Wool.

Colour

Colours can arise in fleeces from inherent pigmentation, staining from urine or chemicals applied to sheep, diseases like fleece rot or in response to skin irritation from lice infestations. Putting these attributes aside, there is still variation in the colour of fleeces which can be attributed to either greasy wool colour, which will therefore be removed when the wool is scoured, and colour in the clean wool. These latter two forms of variation in the colour of wool are a minor source of variation in price.

Style

Wool style is appraised subjectively on a grab sample of wool removed from each line of wool at the woolstore. While style is highly correlated with yield, staple length and greasy wool colour, it also takes into account a number of other features of wool relating to its general appearance. These include crimp frequency and definition, tippyness, dust penetration and weathering (IWS, 1997. Pricemaker quarterly).

The price paid for different style grades from MF1 (Choice) to MF7 (Inferior Topmaking) varies with FD. For wool between 19.5 and 20.5µm there is no extra benefit, in price received, for improving the style of wool beyond MF4 (best topmakers). However for 19 µm and finer wool, any extra improvement in style will generate increased returns.

Factors which affect key wool attributes

Staple length

Within a breed of sheep, staple length is largely determined by the length of the inter-shearing interval. Provided there has been twelve months between shearing events, Merinos and Merino crossbred sheep will produce staples about 80mm to 100mm long. Low staple length is generally only a serious issue for producers, in terms of any effect on the wool price, when lambs are shorn with less than nine or ten months wool and when shearing times are changed so that adult sheep are shorn with only ten months growth of wool, or less.

Staple length is also affected by nutrition. Sheep fed high levels of nutrition have higher fleece weights than sheep fed on a low plane and the increase in weight arises principally from an increase in the diameter of wool fibres and an increase in the length of fibres, rather than a change in the number of fibres growing. Other activities which compete with wool growth for nutrients, particularly late pregnancy and lactation, will reduce fleece weight by reducing mean fibre diameter and staple length.

Similarly, genetic selection for increased fleece weight generally results in increases in staple length. In genetic selection programs, fibre diameter is generally controlled by direct measurement so the only biological avenues for increasing fleece weight are to increase fibre numbers (selecting bigger sheep with more skin area, or sheep with more densely packed wool follicles) or staple length.

Staple strength

The diameter of a wool fibre varies as it grows in response to changes in the levels of nutrients available to the wool follicle. The variation in nutrients may be simply a result of seasonal variation in feed on offer to the sheep, or it may be complicated by competition from reproductive demands, in the case of ewes. When wool is tested for staple strength by grasping the ends of a staple and pulling, it breaks at the point where the diameter of the fibres is lowest. If there is a big variation in the diameter of the fibres of a wool staple along their length, the reported SS will be low.

Yet, if sheep are fed a constant low level of nutrition such that the variation in diameter along a staple is relatively constant, the reported SS will be high. In fact, the intrinsic strength of the wool fibre material does not change at times of low nutrition. The fibres become thinner, but the wool proteins and structures which compose the fibres retain the same strength.

The explanation for the apparent anomaly lies in the way SS is measured. Remember that SS is measured in newtons per kilotex, where kilotex is a measure of linear density and is akin to volume. Fibres can achieve a given volume by being even in diameter or by being thick in some parts and thin in others. If they are the latter, the strength of the fibres will be determined by the strength of the thinnest point. As that point will be thinner than any point in the even-thickness fibre of the same volume, it will break with a lower force. In reporting the SS, the applied force to break the fibres is divided by the linear density, which in this case is the same for both fibres, so the fibres with the most variable diameters will have the lowest reported SS.

To illustrate this in another way, any one fibre with an average thickness of 20µm but a thin point of 16µm, will require the same force to break it as a fibre which is an even 16µm along its whole length. The SS of each fibre, however, is different, because the 20µm fibre has a higher linear density and, therefore, a lower SS in terms of Nktex^-1.

Figure 4.5 As the intrinsic strength of wool protein does not vary, a wool fibre and therefore a staple made up of fibres with a similar profile, will be weakest where it is thinnest. If, however, parts of the fibre are much thicker, the fibre will have a much larger linear density, measured in tex, without being any stronger. The reported staple strength divides the force required to break a staple by the linear density (effectively the volume) of the staple. Thus, staples made of fibres which are even in diameter will have higher staple strengths than those which vary along the fibre length. Source, CRC for Premium Quality Wool

It appears, therefore, that the ways for sheep producers to control staple strength is to reduce the variation in diameter of the fibres the sheep are growing. This may require taking steps to

• reduce the severity of the most severe nutritional check applied to the sheep during the year. This will have the effect of increasing the minimum fibre diameter.
• reduce the amount of feed offered to the sheep during the time of the year when feed is most available, usually the spring time

Figure 4.6 Effects of pregnancy and lactation on mean FD along the staple of a ewe's fleece.

The understanding of staple strength described here is relatively new and management strategies to use these approaches have not yet been fully tested. In Western Australia, where low SS is a major problem for sheep producers, some early results have indicated that the restriction of feed offered to young sheep in spring will lead to increases in SS although the coincident reduction in fibre diameter has a greater beneficial effect on wool price than did the improvement in SS.

Vegetable matter

The amount of vegetable matter in fleeces and, therefore, in lines of wool is generally a consequence of the timing of shearing in relation to the late-spring/early-summer appearance of grass seeds and burrs. If sheep are shorn just before this time, they pick up much less vegetable material, and have longer to lose what they do pick up before the next shearing. If sheep are shorn soon after this time, such as in summer, there may be very heavy levels of vegetable fault in the fleece. The seriousness of the issue depends on the type of pasture and weeds present on the farm and the grazing system used. In higher rainfall areas and on improved pastures with good weed control, summer or early autumn shearing may not necessarily lead to levels of VM in fleece wools above 2%, but the pieces may be quite high in VM.

Grass seeds can cause problems beyond those associated with wool price. Grass seeds of some plants can penetrate the skin and eyes of sheep and lead to mortalities or losses of weight gain and wool productivity.

Recommended reading

Miller BG The Husbandry of cattle, sheep and other ruminants Handbook for Faculty of Veterinary Science, First Year, Animal Husbandry (V101), The University of Sydney.

Australian Wool Innovation Ltd

General references

Australian Woolclassing revised ed (1990) Raw Wool Services, Australian Wool Corporation.

Pattinson RD (1981) What characters determine the clean price of wool at auction? Wool Tech Sheep Breed 29 p 93.

Williams AJ (1991) Wool growth In Australian Sheep and Wool Handbook ed DJ Cottle, Inkata Press, Melbourne, p 224.

Teasdale D (1991) Wool preparation, marketing and processing In Australian Sheep and Wool Handbook ed DJ Cottle, Inkata Press, Melbourne, p 311

[*] Wool classers are not the same as sheep classers. Sheep classers class sheep, usually when they are carrying a full or nearly full fleece, and visually assess a number of characteristics of the sheep including wool characters before deciding whether the sheep should be retained or culled.