The SRS Merino
A large skin surface area for growing wool is the starting point for sheep to produce high fleece weights.
When SRS breeding commenced in 1988, I was aiming to do this by selecting for a plain bodied sheep with a loose skin that was thin, Figure 18. I realized that this type of sheep would be wrinkle free, would not need to be mulesed (Belschner 1937) and would be easy to shear. Fortunately, it was this loose skin type that not only provided a large surface area for growing wool but often had high fibre density and good fibre length.
I knew that if the sheep has a thick and wrinkly skin, the surface area available for growing wool can still be much greater (Figure 18, inset) than on a plain bodied sheep with a loose skin. But I was not interested in this alternative.
CSIRO researchers, Jackson, Lax and colleagues had already shown that genetic selection of Merino sheep for increased wool follicle depth, mimicking “thick skins”, led to a rapid increase in primary fibre diameter whilst secondary fibre diameter remained unchanged, more “hairy birthcoat” lambs, yellower wool and more fleece rot ((Jackson et al, 1988; Lax, 1990). Wool quality could deteriorate well before any change in fleece fibre diameter was detected. And then there were the issues that these sheep would have to be mulesed and would be difficult to shear.
It is important not to confuse the plain bodied sheep with the thin and loose skin with the plain bodied sheep with taut or flat skin. The latter type of sheep will have less surface area for growing wool, low fibre density and low fleece weights
Figure 1: A SRS Merino sheep with a thin and loose skin Inset: Conventional Merinos after “chemical defleecing” demonstrating why the sheep can have up to 25 % more skin surface area for growing wool.
Whilst a lot of the SRS selection decisions are based on visual and tactile markers for low primary fibre diameter and high fibre density and length, it is still important to measure these traits on rams considered worthy of using as sires for ram breeding. Doing this in recent years has allowed us to try to push the density and length levels higher with each joining, and to avoid using poor testing animals.
Body and fleece characteristics
SRS Merino sheep are open faced, triple wedge shaped, well muscled with good fat cover and are bare on the lower legs (Figure 2).
Figure 2: SRS Merino ewes (left) and rams (right), recently shorn.
SRS® sheep have fleeces consisting of long and closely packed fibre bundles. The fibre bundles are very thin (1 to 2 millimetres wide) but contain high numbers of fine diameter fibres . The crimp is bold (low frequency) and deep (high amplitude). An example is the 12 months old ewe in 4 months wool (Figure 3), and is 14.2 microns for fibre diameter. The wool is white, lustrous and very soft.
Figure 3. Closely packed fibre bundles on an SRS Merino ewe with 4 months wool growth.
In long wool, the fleece surface of SRS Merino sheep looks dishevelled . The wool along the topline opens up (Figure 4), drawing the criticism that dust will get in the fleece and lower the yield of the fleece. This is untrue. These fleeces are high yielding and layer like a thatched roof on a house, keeping the dust and rain out of the fleece.
Figure 4. SRS Merino ram with 12 months wool growth.