Why are some calves better than their parent...and some not?

Breeding for improved genetic merit is a form of gambling – and like gambling it can be addictive! Successful gamblers on horse racing invest time in studying the form of horses so that their bets have the best chance of success.   You can win now and again by picking a horse with a pretty name but to be consistent you need to buy the Racing Post and study the form! It’s the same with cattle breeding , now and gain you will be lucky and discover or breed a high genetic merit bull without the aid of EBVs but to consistently breed good animals and make genetic progress you need to study the EBVs (the form!) and through that knowledge you can shorten the odds of producing good animals.

If we consider the “jackpot” is an animal of high genetic merit – much higher than the average of the parents – then the key to success is to have a good plan and then stack the odds in your favour. Once you are “in the game” then it’s important to stick to the plan because if you have got everything right a “jackpot” result will occur sooner or later.

Now the science bit. The wonders of DNA mean you can produce calves that are better than the average of the parents AND, equally likely, worse than the parents. Most offspring from a single mating will be somewhere near the average of the parents. Why is this case? Well, what happens is as follows. As each new offspring is made it gets half it genes from the dam and half from the sire but there are 1000s of genes all with two copies in the dam and two copies in the sire. Each offspring gets a unique selection of the available copies which is why siblings of the same sex perform differently. On average full siblings will have 50% of genes in common, but in theory it could be 100% or 0% in common. Because most matings from the same parents don’t produce many offspring you don’t always feel the variation. In the cattle breeding world where this variation in siblings is most noticeable is when embryos are produced on a large scale from one set of parents.

MPV average

Hence the need to performance record to see how successful your mating “gamble” has been.

So we have established that when you make a mating the most likely outcome in terms of EBVs from the parents is the average of the parents. So if you have a cow with MPV 20 and a bull with MPV 30 then the most likely outcome is that the calf will be MPV 25. But, and it’s a big BUT the range of possibilities from that one set of parents could be 15 to 35, for example. The extremes are very unlikely – but they are possible. When you get a good outcome you are a genius breeder, and performance recording helps identify the exceptional performance. When you get a bad outcome – which is just as likely – you need performance recording to make sure you identify that individual and take it out of the breeding programme!

The US bull Leachman 18 Karat is a good example of how things can work out well. Karat has a $Profit of 18,000 but his parent average was predicted to be 13,000.   Karat is a good example of the joy an “outlier” can bring to a persistent, well organised (or lucky) breeder.  In a nutshell 18 Karat inherited a really good combination of genes from his parents that came together to give outstanding performance.

So next time you are pleased (or disappointed) with the off-spring of a particular mating remember that you have done your best by putting the right parents together and then the random nature of DNA inheritance will play its part in how the EBVs stack up for a jackpot, a flop or, most likely the average of the parents.

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