It is just over 14 years since my new wife and I packed up the Mazda and headed across the Nullarbor to start my PhD with the late Dr Norm Adams in Perth. I was keen to understand the implications of selection for carcass traits in merinos and 14 years on, I’m still learning. The sheep we have today weren’t around 14 years ago, they have evolved at the hands of dedicated breeders who have seen the benefits of a merino with a long-range fuel tank. I thought it was timely to provide a summary of what we currently know about carcass traits in merinos.

The genetics of condition score

I think the single most important factor associated with fat and muscle is their close genetic correlation with condition score. That is, the higher the genetic propensity for laying down fat and muscle in an animal, the higher the condition score of the animal will generally be. This makes perfect sense; you condition score an animal at exactly the same spot as you would ultrasound scan it for fat and muscle depth.

We see this genetic correlation play out clearly in the New Zealand Merino, Central Progeny Test site (NZM CPT; Figure 1). Each dot represents a group of ewe progeny from an individual sire. The progeny had been running in the same group since conception, the sires were mated randomly to the same ewes, the only difference between the groups is the genetics of the sire. Importantly, there is 0.8 condition score units between the highest and lowest performing progeny groups. This is a sire-only effect, the actual genetic effect would be expected to be double this (ie only half of the genes come from the sire). Farming sheep that are naturally more likely to be in good condition is a completely different scenario than farming with sheep that are programmed to not hold condition.

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Fat and muscle and reproduction

There are now a lot of studies that show a positive link between fat and muscle and reproductive performance. Perhaps the most comprehensive recent analysis is that by Daniel Brown, Kim Bunter and Andrew Swan, published in the 2017 AAABG Proceedings and incorporating around 30,000 animals. These authors show a strong positive correlation between muscle and fat and number of lambs weaned and a moderate positive correlation between fat and muscle and lamb survival. There is no doubt that if you want to improve reproductive traits then you select for them directly. However, getting good information on reproductive traits isn’t easy and the majority of merino flocks in Australia do not currently generate this information.

Perhaps the most exciting aspect of the link between carcass traits and reproduction is around lamb survival. While there is clearly a lot of noise around these correlations, lambs with better carcass data are more likely to survive. In the 2018 drop of the NZM Central Progeny test, the correlation was negligible in single born lambs but evident in the twins (Figure 2). The other aspect of this graph is the massive variation that exists between sires for lamb survival.

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Trade-offs, limits and all that jazz.

I see and hear a lot about the trade-offs with wool associated with positive selection for muscle and fat and there is no denying an unfavourable genetic correlation between fat and fleece weight for example. There are a couple of points worth making. Firstly, by playing the long game and selecting within your preferred wool/sheep type, there need not be any compromise on wool quality or quantity. Introductions of external genetics of a different wool/type are obviously going to throw off the home type for a generation or two. Secondly, this is uncharted waters and we are dealing with exceptionally complex biology, there are no arbitrary cut-offs or limits. The combined selection strategy of carcass and wool has never been practised with the precision that it is today, everything is possible it just takes time.

To provide some support to this concept I thought it would be useful to provide the genetic trend graphs of a breeder that has been selecting for fat and muscle and wool for almost 20 years (graph below). This breeder improved fat by 1mm, muscle by 2mm and fleece weight by 10% over this period. I’ve also provided a graph of CV as one of the traits that is associated with wool quality. Over the same period of time, the CV of the wool has been reduced by over 1.5%. So yes, there definitely are trade-offs but none of them are significant enough to stop you making excellent rates of genetic gain across all traits.

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Short shanks

Finally, we get around to discussing the title of this article! One of the things that is important to note is the body shape change that is associated with positive selection for carcass traits in merinos. As we move to higher genetic merit for muscle and fat, we see reduced cannon bone length, increased width across the pins and increased depth through the twist. These animals tend to have deceptive weight and can be overlooked as being small, but a different story is told once they are on the scales.

The shape of the animal changes and it is important that if people are embarking on a carcass selection journey that they are aware that these changes will occur. Associated with this change is a shift to earlier maturity and a reduction in mature weight which all culminates in an improvement in maternal efficiency.

The most important thing in any breeding program is balance, different breeders have different perspectives on where the balance lies across different traits. Often this is associated with the type of country the sheep are running on and the stocking rate pressure they are put under. Carcass traits are much less valuable in low stocking rate environments where sheep are always in good condition. They are much more valuable where frequent periods of drought are experienced or where sheep are run under high stocking rates.

There are now breeders in all states of Australia and both islands of New Zealand who are putting positive pressure on muscle and fat in merinos. Their stories are very consistent, as the generations tick over they start to find the sheep are more forgiving to a tough period, they respond more quickly when the going is good, they have more lambs that stay healthier, they have less health problems and they stay in better condition. This doesn’t mean we should expect this to happen for ever, biological relationships are never linear, at some point there will be no additional gain for further improvements in muscle and fat and this selection pressure will be diverted to other aspects.

Let’s hope we learn as much in the next 14 years that we have in the last 14!

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