Body condition score (BCS) affects feed requirement. Genetic size scaling theory, the subject of recent columns, finds a stronger relationship across species between feed requirement and body weight if we adjust to constant body fat percentage.
This is because fatter animals have a smaller proportion of their body as lean tissue. It is lean body tissue that largely determines maintenance energy requirement. Lean body tissue burns energy all the time in warm-blooded animals.
By contrast, fat tissue is more inert. Once deposited fat has a very low cost per gram of tissue to maintain it. This makes sense – nature has selected an efficient way to store energy. Fat is both energy dense per gram and has a low maintenance cost compared to lean tissue.
Those who argue a low BCS animal has a higher feed requirement than a high BCS animal of similar liveweight are correct. The low BCS animal has a higher lean body mass since the high BCS animal has more fat.
In both sheep and beef cattle breeding our evaluation systems consider adult size a net cost to the system. Keeping a cow or ewe to produce a calf or lambs each year is an overhead to the system because of the maintenance feed requirement for the maternal animal. With maternal breeds, maintenance feed requirement of the cow or ewe must be accounted for in profit-focused indices.
Some argue that adult liveweight is a crude estimate of maternal feed requirements because animals differ in condition score. It is not a crude estimate – it is the best we have until we also measure BCS.
Not having data for adult liveweight or BCS is not a good reason to remove adult size from estimates of economic merit. To ignore adult size is to focus on product value and ignore a big cost of production, one associated with maintenance of the cow or ewe.
BCS alone is insufficient when considering the feed requirement of ewes or cows. We need both adult liveweight and adult BCS to best do this. So what is the best strategy for taking account of this when seeking genetic improvement of maternal breed types?
We need to take account of adult feed requirement as best we can. With no adult liveweight data we do our best by predicting adult size from liveweight data of the young animal.
This has been criticised as “a prediction too far”. This is the wrong conclusion. It is the best we can do in the absence of adult liveweight data.
The best alternative to this is to measure and use data on adult liveweight, not to ignore adult liveweight. To gain more precision we should bring in BCS as well.
SIL will soon introduce an estimated breeding value for BCS. We need maternal sheep breeders to collect BCS data to feed into evaluations that produce these breeding values and consider how they relate to their “better” maternal bloodlines. As data accumulates we will assess how BCS affects other production traits and integrate it into maternal indices. SIL recommends all maternal breeding flocks measure, at least once, adult ewe liveweight and BCS.
To increase farm profit through genetic improvement we must consider costs as well as returns. Considering both liveweight and BCS in ewes and cows will better characterise feed costs of adults and the value of body condition to the maternal animal.
Fat is valuable where it helps a cow or ewe sustain productivity. To best account for maternal feed requirements breeding programmes should record both liveweight and BCS in adult cows and ewes. BCS is one of the “Goldilocks traits” I have described previously.
We don’t want too little or too much, we want it to be “just right”. The challenge for our animal breeding scientists is to devise systems that do this. Fat is also important to meat quality but that is the subject of a future column.
If you have any questions about this topic email: silhelp@sil.co.nz or leave a message on 0800-silhelp (0800 745 435).
- Mark Young is senior geneticist with B+LNZ Genetics and SIL.
