“There is no such thing as a free lunch,” is a quote you will have heard many hundreds of times. It echoes Newton’s Third Law, which states that for every action there is an equal and opposite reaction. When it comes to increasing fleece weight, there is a bit of theory circulating in the industry that there is no cost to growing this extra wool. 

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At first glance this is true. There is good evidence that on a genetic level, it is possible to improve fleece weight without an associated increase in voluntary feed intake. This improved efficiency is not the result of improved efficiency of digestion but rather an increased efficiency of getting amino acids to the wool follicle for conversion to wool. This has led geneticists and breeders to dial up the weighting on selection for fleece weight in many merino indexes over the years (a practice that continues to this day). Unfortunately, like anything in biology, there are consequences that come with this shift in wool growth efficiency. 

This has long been an interest of mine and I’ve written and talked about it a lot over the years.

To explain it further, I want first to introduce a key person into this story: this area was also a strong interest of my PhD supervisor, Dr Norm Adams, which incidentally is why I selected him as my supervisor. Norm unfortunately passed away before I finished my PhD (and before we got to the bottom of what was going on with fleece weight). However, he did run some insightful experiments that led him to a hypothesis that makes sense. 

The thing that confused Norm (and, by association, me) is that wool is largely a protein, yet there is a lot of evidence that producing more wool results in an energy deficit. Norm’s theory, backed by experimental evidence, is that sheep with high fleece weights have a higher rate of protein turnover in their organs and muscles and this is an energetically expensive thing to do, resulting in lower fat reserves in some instances. 

We often talk about partitioning nutrients to meat or wool when we talk about different types of sheep, particularly different types of merino. However, the reality is that wool growth is not mediated by a hormone signal. That is, there is no internal signalling that allows an animal to partition nutrients toward or away from wool growth. I often joke about the wool follicle being like a parasite embedded in the skin. It has no control over the amount of nutrients that will head its way, it just gathers the amino acids it needs from the circulating blood. 

Animals that mobilise and then lock away protein stores more often (protein turnover) make amino acids available to the wool follicle more often. The outcome of selection for high fleece weight has been a shift in the protein turnover of animals, which can result in a deficit in total energy stored in the sheep.

At the phenotypic level, the more feed (and the higher the quality of that feed) the more wool a sheep will grow. The increase in nutrient supply enables a higher level of circulating amino acids and carbohydrates in the bloodstream and hence higher wool growth. When you take this to the extreme and you house or contain animals - and you feed them very high-quality feed - they cut enormous amounts of wool. This leads some people to think that this is ‘expressing the potential’ and that animals can both be very fat and cut a heap of wool. The problem arises when you run sheep that have been selected to perform in very high-nutrition environments and then send those genetics out to face the realities of mob pressure, stocking rate, disease challenges and sub-optimum nutrition for at least part of the year.

What I see

I am fortunate to get to travel to lots of different areas in Australia and New Zealand and see sheep being asked to perform in some very different environments. Lots of these are commercial farms or stud enterprises running their stud ewes in a commercial way.

Consistently, I see sheep that have been loaded up genetically for fleece weight that struggle to maintain condition when run under a feed-limited environment. They are fine when things are good, but not so much when they are not so good. This isn’t an exact science. There is no clear cutoff point for where fleece weight can be pushed to or how high the genetics for condition score need to be to buffer the fleece weight. But I see this scenario play out all the time. 

The fleece weight to body weight ratio

The concept of fleece weight to body weight ratio is one that has been well studied in South Africa. It is calculated by dividing greasy fleece weight by the animal’s liveweight and then turning that into a percentage. For example, a 50kg ewe cutting 5kg of wool would have a fleece weight to body weight ratio of 10%. The South Africans clearly showed that this ratio was linked to reproductive efficiency: the higher the ratio (the more fleece weight at a given body weight) the lower the total weight of lambs weaned. 

Within an Australian context, from MLP data available within public reports, if you calculate the ratio of fleece weight to body weight within sire groups, and then correlate this number with lamb weaning percentages within sire groups, you will find a strong negative correlation. As the percentage of wool to body weight increases, reproduction is reduced.

Immune system competition

Another challenge we see with high fleece-weight animals is the potential for immune system suppression. The immune system requires protein and energy for it to remain effective. When either one is in short supply, the immunity of an animal to disease and parasitism can be threatened. This is regardless of whether the deficiency of protein is due to a lack of nutritional supply or losses through competing processes such as wool growth. 

Additionally, both the immune system and wool growth have a high requirement for sulphur-containing amino acids. This requirement exceeds the normal proportion of these particular amino acids in microbial protein leaving the rumen. These sulphur-containing amino acids can become limiting for both wool growth and the immune system.

Finding the balance

There is plenty of opportunity to select animals that don’t follow the correlations discussed, but it does need to be remembered that these biological relationships are likely to remain in place. So, if you aren’t purposeful about controlling them, they will be at play. 

What I believe to be true is that, in every production environment, there is an upper ceiling in terms of what fleece weight is easily supported. It can be selected beyond this ceiling, but it gets increasingly difficult to not have an impact on other aspects of production. In the mixed cropping zone, where sheep are run at low stocking rates, the ceiling is a lot higher than in the high rainfall zone, where sheep are run at high stocking rates.

We are yet to find out what happens when you select for positive condition score in addition to fleece weight. Potentially, this may shift the ceiling to a higher place within each production environment. That is yet to be seen. 

Generally, the industry has made significant shifts in fleece weight, with plenty of sheep that have been bred beyond the ceiling. But it can pay to take a step back and look at the bigger picture of the production system as a whole. In practice, these high-fleece weight sheep can end up cutting less wool per hectare than a sheep with genetics for lower fleece weight that is also easier doing, maintains condition, can be run at higher stock rate and produces more lambs to shear. 

If you are keen to head further down the rabbit hole and review all of the work undertaken in this area and written about by Dave Masters and me (largely Dave), send me an email and I will send you the paper.

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