SRS® Merino Sheep: The genetic alternative to mulesing

Figure 1. SRS® Merinos are large-framed and plain-bodied animals with open faces, mulesing is not required.

Introduction

Since Soft Rolling Skin (SRS®) Merino breeding began in 1988, the objective has been to breed a plain-bodied sheep with high levels of wool fibre density and wool fibre length and the best quality wool possible (Figure 1).

The logic here is that if a sheep has many wool fibres on its body and these fibres are long, then the sheep will produce a high fleece weight (Figure 1). Also, the fleece will be fine in diameter (high density is closely associated with low fibre diameter) and the fibres will be highly aligned, smooth-surfaced and uniform in size and shape.

Today, 40 Merino and Poll Merino studs across Australia are implementing the SRS® breeding system and supplying over 11,000 rams to commercial Merino breeders. These studs produce sheep that have high fleece weights and low fibre diameter, high lambing percentages and excellent body growth rates.

All of these studs have stopped mulesing as the sheep are plain-breeched (Figure 2) and naturally resistant to breech strike. Some of these stud breeders stopped mulesing four to eight years ago.

It is important to say that while Merino sheep have been bred that are naturally resistant to fly strike, the main focus of our sheep breeding programs has been to improve fleece quantity and quality, feed conversion efficiency, early body growth, muscle patterning and fecundity. Fly strike resistance comes as a bonus when selecting for a plain bodied sheep with high quality wool.

No mulesing

SRS® sheep are totally free of any skin wrinkling (Figure 2). When the sheep's skin is like this it remains dry at all times. Urine is not trapped in the breech regions and rain droplets are not trapped along the sheep's back. Mulesing is not required and neither is preventative insecticidal treatment.

It is when sheep have breech wrinkle and body wrinkle (Figure 3) that the weeping dermatites that produce conditions such as urine scald and fleece rot (from prolonged rain) occur and the sheep are predisposed to fly strike of any kind.

This fact is not new. In 1931, HG Belschner and HR Seddon studied different body types of non-mulesed Merino sheep and found that plain-breeched sheep were far less susceptible to breech strike.

Table 1. Incidence of breech strike in non-mulesed Merino ewes from a 1931 trial. A - plain-breeched, B - wrinkly, C - very wrinkly.

An important opportunity was lost at this time. Breeding plain-bodied sheep for fly strike resistance might have become accepted practice throughout Australia from the 1930s onwards.

Fleece weight

Wool producers often say they are unwilling to change the Merino sheep type from one with thick skin and body wrinkle to a plain-bodied type because they fear losing fleece density and fleece weight.

Many public demonstrations to wool producers have been conducted over the past 15 years to show that plain-bodied Merino sheep with thin and loose skins can produce more weight of fleece per animal at a lower fibre diameter than thick-skinned, wrinkly sheep.

Figure 9 shows the typical results of such demonstrations. In each comparison, the sheep were from the same mob and were of the same age, sex, shearing time and reared under the same nutritional and environmental conditions.

The sheep were assigned to either group on subjective assessment of the animal and its fleece structure. The fleeces were then weighed and tested for fibre diameter and the fleeces valued by experienced wool buyers, notably from Itochu Wool Limited.

Figure 9. Comparisons of fleece value, fibre diameter and fleece weight differences between plain-bodied SRS® Merino sheep and thick-skinned and wrinkly Merino sheep.

A sheep can be plain-bodied and have a high density of wool follicles in its skin, or have a low density.

If a plain-bodied sheep has high follicle density and high fibre output (length) from each wool follicle, the skin will be thin and loose. SRS® Merino sheep have thin and loose skins with follicle densities above 80 follicles per square millimetre and fibre length above 0.50millimetres per day. Average values for traditionally bred Merino sheep are much lower - namely about 55 follicles per square millimetre and 0.30 millimetres per day respectively.

If a plain-bodied sheep has low follicle density, the skin is thin but not loose. Often, the wool is long but the density is too low to allow the sheep to produce high fleece weights.

The photographs in Figure 4 show cross-sections of the wool follicles and fibres, as seen under a microscope. in the skin of two plain-bodied Merino sheep.

The high density sheep is from an SRS® Merino flock. It has 97.0 wool follicles per square millimetre of skin and produces a high fleece weight of fine diameter wool.

The other sheep is a plain-bodied sheep that has a low density of 53.0 wool follicles per square millimetre and a low fleece weights. It is often referred to as a 'flat skin' sheep. While it also does not need to be mulesed, the 'flat skin' sheep is light-cutting and does not pay the bills.

In a glance it is possible to see from Figure 4 that fleece weight of Merino sheep can be improved greatly by genetically improving the density and length of wool fibres on the sheep.

Wool quality

In plain-bodied Merino sheep with high levels of fibre density and fibre length, the fleece consists essentially of fibres that are highly aligned cylinders of uniform diameter and length.

The fibres are smooth surfaced (due to the long, cuticular scales of low scale height) and have a high crimp amplitude (deep crimp) and usually low crimp frequency (bold crimp). These fibre properties create a fleece composed of fibre bundles (not staples) of superb fineness, softness, lustre, high elasticity of deep and well-defined crimp (Figure 5).

In thick-skinned sheep, which have lower levels of fibre density and fibre length, the fleece consists of entangled fibres of uneven size and shape and low crimp amplitude. Thus, thick staples rather than thin fibre bundles are formed in the fleece.

Wool quality measurements were conducted on the sheep listed in Figure 9. In the 1995 comparison at Marnoo, Victoria, the plain-bodied ewes produced 1.2 kilograms more weight of fleeec and were 1.3 microns finer than the thick-skinned ewes. Also, the plain-bodied ewes had higher follicle density (92.5 versus 78.0 follicles per square millimetre), higher fibre length (0.43 versus 0.37 millimetres per day), better fibre alignment (71.2% versus 54.8%), more cylindrical fibres (86.5% versus 69.0%) and more orthocortex in the fibre substance (42.0% versus 30.0%).

In the 1999 and 2000 comparisons at Cooma, New South Wales, the plain-bodied ewes produced a mean of 0.1 kilograms more weight of fleece (for 8.0-8.5 months wool growth), higher wool yield (76.2% versus 68.6% in 1999 and 74.2% versus 70.2% in 2000), were 2.7 microns finer than the thick-skinned ewes, and longer fleeces (96mm versus 76mm in 1999 and 125mm versus 101mm in 2000). Also, the plain-bodied ewes in the Cooma trials had lower coefficient of variation (CV) of fibre diameter (17.1% versus 21.3% in 1999 and 16.0% versus 20.4% in 2000), and higher comfort factor (99.7% versus 98.8% in 1999 and 99.5% versus 97.1% in 2000).

In the 2000 comparison at Badgingarra, Western Australia, the SRS® ewes produced one kilogram more wool than the thick-skinned ewes, and the wool was 2.5 microns finer in diameter. The diameter variation along the length of the fibres was much lower in the SRS® sheep than in the thick-skinned sheep (means of 1.0 micron and 5.2 microns respectively).

In the 2002 comparison at Mungindi, New South Wales, a more detailed investigation of group differences was carried out (Figure 9). This data shows the SRS® sheep have higher follicle density and fibre length, and finer fibre diameters, than the thick-skinned Merino. The number of dermal papilla cells in each folicle was significantly lower in the SRS® sheep; a feature which is fundamentally important for producing fine diameter wool.

Figure 9. The effect of selecting for the SRS® phenotype on the wool follicle population, dermal papilla cell population and fibre growth in Merino sheep (mean ± standard error).

Bare breech

Bare breech is a different trait to the plain breech. It is a wool-free area that surrounds the perineum and can extend down onto the udder (Figure 6) or scrotum, on the inside of the back legs and also around the pizzle.

Sheep with this trait are a low risk of developing breech strike and may not need to be crutched.

However, if the sheep are also wrinky, such animals will remain susceptible to other forms of fly strike, especially body strike, and are likely to pass this susceptibility on to their progeny.

It follows that bare-breeched rams will need to be plain-bodied. It is an easy trait to select for in plain-bodied Merinos.

Breeding mules-free Merinos within five years

Andrew Michael, an SRS® Poll Merino breeder from 'Leahcim', Snowtown, SA, says that 25 of his ram-buying clients, all commerical wool producers, have converted their flocks to being unmulesed and resistant to breech strike within 3 to 5 years.

None of the 130,000 Merino lambs on these commercial properties were mulesed in 2008.

The same short transition period was achieved in Michael's “Leahcim” Poll Merino flock which has not been mulesed since 2004.

He calculates that stopping mulesing has added $9.90 per lamb to the value of each lamb at 9 months of age. The calculations are conservative estimates based mainly on the improved body weight gains and labour savings.

His ewes produced an average fleece weight of 5.4 kilograms at 19.5 microns fibre diameter and a length of 80 to 85 millimetres when shorn in only 7.5 months wool.

This is one example which indicates that wool producers should be relaxed and confident about their ability to be able to select productive Merino sheep that do not need to be mulesed, especially in the knowledge that the transition period can be brief.

Within the SRS® breeding system, all 40 Merino and Poll Merino studs, which are located in all mainland states of Australia, have ceased mulesing, some as long as eight years ago.

A survey Dr. Watts conducted in 2005 of about 300 commercial wool producers implementing the SRS® breeding system indicated that 30% had ceased mulesing and 33% were intending to stop the following year. These respondents considered that their Merino lambs had sufficiently smooth and wrinkle free breeches to no longer warrant mulesing.

The procedure for achieving this rapid transition from mulesing to no mulesing is specific. It can be done within five years if Merino breeders understand the skin and body traits that need to be selected for, and how much change is required for a particular flock.

Industry demonstrations

In July 2008, Dr. Watts and his colleagues held two industry days with Landmark in South Australia to demonstrate to wool producers how to breed a plain-bodied Merino sheep that did not need to be mulesed. The workshops were attended by about 280 wool producers.

Wool producers were presented with two groups of one year old Merino ewes, one the had been recently shorn and one that were in 10 months wool.

They were asked to class the sheep into those animals that could have been safely left unmulesed and those that could not. Both groups of sheep appeared to be plain-bodied, particularly as long-woolled sheep.

With guidance, nearly all of the large gathering of wool producers were able to select the animals correctly.

In short wool, the focus was on ensuring that the whole body surface of the sheep was wrinkle free, thin and loose. The tail had to be smooth and rounded and the breech and hindlegs free of any cross folds of skin.

The fleece tip was distinctive, consisting of small staple ends, resembling the heads of matchsticks, arranged as closely packed and beaded array with obvious lustre, all over the sheep's body.

In long wool the wool producers were able to pick the sheep types on their fleece structure.

The sheep with wrinkle free skins that did not require mulesing and appeared to produce high fleece weights of low fibre diameter wool, grew fleeces consisting of long and thin staples of high crimp amplitude (“deep crimp”) , softness and lustre.

Another type of sheep with wrinkle free skin that did not require mulesing but were likely to produce low fleece weights, had taut skins and shorter stapled fleeces with low crimp amplitude (less “character”) and lacking lustre.

And there was another sheep type which outwardly appeared to be plain-bodied but still had wrinkly skins, much like the shorn sheep shown in Figure 7.

These sheep grew fleeces consisting of thick and shorter staples that lacked softness, lustre and whiteness and were often black tipped. The “ribbiness” or skin wrnkled extended to the breech region, indicating that these sheep would be susceptible to breech strike if left unmulesed or unprotected.

Take Home Messages

It is not difficult to breed a plain-bodied Merino sheep that does not need to be mulesed, has superior meat traits and produces high fleece weights of low diameter wool.

Large gatherings of Merino wool producers at recent SRS® industry days clearly demonstrated that they have the capacity to do this, and were reassured that the process can occur within five years.

Figure 2. Merino ewes and lambs in an SRS® ram breeding flock. Mulesing was discontinued in this flock in 2002. The sheep are plain-bodied and naturally resistant to fly strike.

Figure 3. These Merinos are not plain-bodied. Note the skin wrinkle on the breech and body. Despite mulesing these sheep are still susceptible to fleece rot and body strike because persistent rain along the topline can keep the skin wet where skin wrinkle occurs.

Figure 4. Plain-bodied Merinos can have a high density (above) or a low density (below).

Figure 5. An SRS® Merino fleece testing 16.5 microns for fibre diameter. Note that fibre bundles (clusters of fibres originating from follicle groups in the skin) have replaced staples.

Figure 6. The bare breech trait is associated with wool free areas on the scrotum, udder and around the pizzle.

Figure 7. The smooth and wrinkle free skins of sheep that do not require mulesing are easily recognised when sheep are in short wool.

Figure 8. In long wool, the plain-bodied sheep that had high fleece production and were wrinkle free could still be identified visually from sheep that had 'ribby' skins.