Wetenschap - 27 januari 2011

Genomic selection brings supercow closer

DNA is now helping livestock breeders to establish the breeding value of a bull. This opens up new possibilities such as breeding dairy cows with a small ecological footprint and plenty of unsaturated fats.

The bull Sunny Boy was the star of Dutch livestock breeding. He had a combination of genes which ensured high milk production among his female progeny. Sunny Boy (1985- 1997) inseminated countless cows, as can be seen from the fact that his sperm was sold 1.8 million times. He was one of the reasons for the rise in the number of dairy cows producing at least 100,000 litres of milk during their lifetimes.
And yet it took some time before the value of Sunny Boy became clear. His owner certainly knew that his parents were excellent quality animals, but he did not know whether the calf had inherited their useful characteristics. After all, siblings vary. A bull calf only starts to produce enough sperm to inseminate a cow after one and a half years. Then the bull's daughter needs to calve herself and start producing milk. Only then can the breeder calculate the breeding value of the bull on the basis of its offspring's milk production. So in those days it took about five years to determine the breeding value of Sunny Boy. 

Trade secret
This process can be speeded up considerably. The latest techniques enable breeders to analyse a newborn calf's DNA day after its birth and see what its breeding value is. This is a revolution with far-reaching consequences for livestock breeding, says Sander de Roos, head of Breeding & Support at cattle breeding company CRV. De Roos is due to receive his PhD on genomic selection on 21 January from professor of breeding Johan van Arendonk. In the new method the breeding value of bulls and cows is established on the basis of DNA markers: pieces of DNA which predict the characteristics of the animal. Not only can breeders establish an animal's breeding value much earlier, they can also do so much more precisely. Dairy farmers send a little bagful of the bull calf's hair to De Roos's company soon after its birth. It is then analysed in a laboratory in Li├Ęge, and a DNA profile is made using 50,000 markers or positions on the DNA. What then follows is a trade secret: with a formula it developed itself, CRV estimates the animal's breeding value.
CRV assesses about 40 characteristics. 'It is not just a matter of the quantity of milk, but also of its composition: the amounts of fat and protein', explains De Roos. Even the cow's appearance, including the quality of her legs and udder, contributes to her breeding value. Thirdly, CRV assesses the animal's genetic disposition towards health, fertility and longevity. CRV can now select for all these aspects much more effectively than was previously possible. The new technique doubles the genetic progress made per year, says De Roos.

Improving characteristics
De Roos started his doctoral research in 2006. 'Then genomic selection still only existed in theory.' Because scientists mapped the cattle genome and found many DNA markers, the technique can now be applied by companies such as CRV. In his thesis, De Roos solves important statistical problems about, for example, the reliability of breeding values determined on the basis of DNA markers. He has also established how he can apply genomic selection in breeding programmes. An important issue in any breeding programme is inbreeding. 'I conclude that genomic selection leads to less inbreeding per generation. That is good news for the geneticist. But beware: because we use young bullocks as sires, we get through more generations in the same period of time. And that can cause an increase in inbreeding per year. And that is what practitioners look at.' In order to limit inbreeding you need to make sure there is enough variety in the offspring.
De Roos hopes to use genomic selection in future to improve a number of new characteristics. For example, he would like to investigate whether the ecological footprint of the dairy cow can be reduced through breeding, by selecting for higher efficiency. The composition of milk can also be modified more easily using genomic selection.

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