Introduction to SRS Merino and fibre genetics

James Watts

The SRS breeding system is based on producing wools of exceptional quality by selecting Merino sheep, alpacas and angora goats for low primary fibre diameter, high fibre density and high fibre length. It has been applied to flocks and herds in  Australia since 1988.

Genetic selection for these fibre traits, both visually and objectively, is done with the expectation that fleece weight will increase, fibre diameter will decrease and fibre quality will improve.

The animals are environmentally fit, easy to shear and easy to manage. The Merino sheep have been bred to be free of skin wrinkle (plain bodied) and naturally resistant to fly strike, and consequently, are not mulesed.  

Wools of exceptional quality  are destined to command  high prices in today’s consumer  world.  The wools provide luxury comfort and appeal. 

Wool follicle patterning

There are three types of wool follicles in the skin, Figure 1. In the foetal lamb, the primary follicle develops from about 65 days in groups of three, called trio groups.  The primary follicle is distinguished by having a sweat gland and a muscle attached to it. At about 85 days, secondary follicles appear, adjacent to each trio group of primary follicles. From about 105 days more secondary follicles develop as branches from the original secondary follicles. These are known as derived secondary follicles.

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Figure 1. Follicles in adult sheep skin (from Maddocks and Jackson, 1988).

Figure 2. Skin structure in two Merino sheep with different secondary follicle to primary follicle ratios. Top. S/P ratio = 43 to 1. Above. S/P ratio = 4 to 1. (from Maddocks and Jackson, 1988).

A follicle group comprises three primary follicles and the associated cluster of secondary follicles.  Figure 2 shows a cross section through follicle groups from two sheep containing high and low numbers of follicles. The number of follicles per group is characteristic for a particular animal. It is referred to as the secondary follicle to primary follicle (S/P) ratio.

Moore and colleagues (Moore, 1984; Moore et al, 1989, 1996, 1998) proposed that the genetic capacity for wool growth is largely determined by the population size of a specific cell line, the pre-papilla cells, committed to the formation of the dermal papillae of the follicles. Cells from this population form the papilla of primary follicles and the remaining cells of this cell line multiply as the foetus grows to form the original secondary and derived secondary follicles.

Moore and colleagues first suggested that selection for low primary fibre diameter may lead to a more advanced Merino sheep with improved wool quality. They showed that the number and size of the primary follicles are strongly inherited. They predicted that if fewer and smaller primary follicles were initiated, a lesser proportion of the pre-papilla cells will be used in the induction of primary follicles and a greater number of cells will remain and multiply for subsequent secondary follicle formation. This, in turn, will increase fleece weight.

Moore’s hypothesis also predicts that an increased number of follicles per group (increased S/P ratio) will be associated with finer fibres. The fibre diameter is related to the number of pre-papilla cells which are used to make the dermal papillae of the follicles.  If the follicles are larger, more of the specialized population cells will be used and the number of secondary follicles formed will be less. Conversely, if the follicles are smaller, less of the pre-papilla cells will be used per follicle and more follicles are formed to use up the available follicle-forming cells.

Low primary fibre diameter

If we are to breed wools of high quality, it is important to select for low primary fibre diameter, either directly or indirectly. This fact was demonstrated by Jackson et al (1988). They showed that Merino sheep selected for increased follicle depth (mimicking “thick  skins”) responded genetically by increasing primary fibre diameter without any change in secondary fibre diameter . They described the changes as the animals regressing toward the primitive sheep with a two-coated fleece; the outer coat consisting of coarse, medullated primary fibres and the undercoat, finer secondary fibres.

Selecting for low primary fibre diameter prevents primitive regression. It eliminates “hairy birthcoats” from Merino lambs, “kemp” from angora goats and “guard hair” from alpacas. It also leads to increased fibre density and lower fibre diameter.

The mean primary fibre diameters of Merino sheep, angora goats and alpacas bred by SRS selection are 6 to 8 microns finer than in the general populations of these animal species. The SRS bred Merino sheep have primary fibres that are 4 microns finer, on average, than the secondary fibres. The SRS bred angora goats and alpacas still have primary fibres that are 7 to 8 microns higher in diameter than the secondary fibres; indicating that selection for low primary fibre diameter needs to continue for these animals.

In the three animal species, the breeding goal is the same, namely to select for primary fibres that are finer than the fine secondary fibres.

Jackson et al (1988) showed that the increase in primary fibre diameter associated with selection for thick skins was not corrected by selecting for high fibre density. However, it can be offset by selecting for high fibre length. There is a strong and negative, genetic correlation between fibre length and skin wrinkliness (thick skins) in Merino sheep.  In other words, as fibre length increases, the skin gets thinner and the sheep gets plainer and primary fibre diameter decreases.

High fibre density and length

Ideally, high fibre density is the result of many follicles being laid down in compact follicle groups in the skin, and the follicle groups being packed closely. For the three animal species, fibre densities above 85 follicles per square millimetre are sought.

The goal for high fibre length in the crimping fleece of the Merino sheep and the Huacaya alpaca is at least 0.6 millimetres per day. The fibres have high crimp amplitude (“deep” crimp), and invariably have (because the fibres are so long) low crimp frequency (“bold” crimp). The fibre length to fleece length ratio is 1.5 to 1 or greater.

High fibre length in the coiling fleece of the Angora goat and the Suri alpaca is 1.0 millimetres per day or more. The fibres have twist and low coiling frequency. The fibre length to fleece length ratio is as close to 1 to 1.

In high density animals, the distance between the wool follicles is low. Consequently, the fibres emerging from each follicle group into the fleece are likely to be highly aligned and form a fibre bundle. There is also a high number of fibres per fibre bundle, and the fibre bundles are packed closely in the fleece.

In the Merino sheep and the Huacaya alpaca, the closely packed, deeply crimped and silky soft fibre bundles, containing high numbers of fibres, is used as the visual indicator of high fibre density. If these high fibre density animals also produce long fibres, the fibre bundles will be long and usually have low crimp frequency, Figure 3.

The Suri alpaca and the Angora goat produce coiling fleeces. The closely packed fibre bundles of high density animals are visible from the skin surface to the point where the first coil appears, Figure 4.

High fibre density and length in the Suri alpaca (top) and the Angora goat (above) is expressed as thin and gently coiling staples.
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Figure 4:    High fibre density and length in the Suri alpaca (top) and the Angora goat (above) is expressed as thin and gently coiling staples.

Fibre examination

Key fibre properties can be observed by drawing individual fibres from a fleece sample onto a velvet board. Black velvet is ideal for white and fawn fleeces. A white velvet board is suitable for dark colours. It is easy to scan as many as 500 fibres from a fleece in quick time.

The observer can gauge the degree of alignment (or entanglement) of the fibres, estimate the diameter of the primary fibres (when fibres are drawn from the birth coat tip), the diameter of the secondary fibres, the variation in diameter and length between the fibres, the silky texture and elasticity of the fibres, whether coloured fibres are present in white fleeces, and whether white fibres are present in dark coloured fleeces. An example is shown in Figure 5.

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Figure 5.     Individual fibres from Angora fleeces are being examined. Top. Fibres are fine, uniform in diameter and length. Above. Fibres are uneven in diameter and length with coarse, medullated fibres present.

This fibre examination is important to do for Angora goats and alpacas. Relying simply on fleece inspection can be deceiving. For example, the two alpaca fleece samples shown in Figure 6  have the same frequency and coarseness of primary fibres. It is simply that the primary fibres in the sample on the right are shorter and hidden from view.

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Figure 6.     Guard hair protruding from the alpaca fleece sample (left) but not from the sample (right).

Follicle and fibre measurements

Measurements of primary fibre diameter, secondary fibre diameter, fibre (follicle) density, the number of follicles per group, fibre length and fibre length to fleece length ratio, are valuable aids for sire selection. The measurements are made on horizontal skin sections prepared from mid-side skin samples.

Fibre length and fibre length to fleece length ratio are measured on mid-side fleece samples.

Figure 7 shows magnified views of horizontal skin sections under the microscope from a high density alpaca (left) and an average density alpaca (right). The high density alpaca has 73 follicles per square millimetre, a mean primary fibre diameter of 28.9 microns and a mean secondary fibre diameter of 19.9 microns. The average density alpaca has 35 follicles per square millimetre, a mean primary fibre diameter of 39.6 microns and a mean secondary fibre diameter of 26.3 microns.

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Figure 7.     High density alpaca (left) and average density alpaca (right).