Genomics Revisited

January 29, 2013 07:59 PM

Proven bulls prove their worth

The hype and promise of genomics four years ago are now running into the test of time as daughters of those early, genomic young sires enter—and leave—the nation’s dairy herds.
The results are a bit mixed, says Chad Dechow, an associate professor of dairy cattle genetics at Pennsylvania State University. "Genomics gives us some reasonable accuracy of genetic merit at a young age, which lowers the genetic interval between generations and increases the rate of genetic gain," he says.

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But because the bovine genome is more complex than originally first thought, genomics is not perfect.

The first official genomic proofs were released by USDA in 2009. Prior to that, producers were using Holstein young sires for less than 30% of their matings. As the promise of genomics caught on, that number climbed and very likely exceeds 50% today, Dechow says.

Consequently, USDA now has data on more than 500 bulls that were used as genomic young sires in 2009 that now have more than 100 daughters in their proofs. The correlation of predicted milk yield and actual yield has held up well, with a reliability of about 69%.

But the story isn’t as positive for productive life. Here, the correlation is only 34%. "It’s still a positive correlation, but not as strong as milk production," Dechow says.

There are lots of reasons for that. First, culling decisions differ among herds. Plus, producers are culling heavier, Dechow says. But it’s also true that productive life is more heavily influenced by non-additive effects, such as inbreeding depression, than milk yield.

Body size and somatic cell score now have greater negative correlations with productive life, while udder and feet and legs are less positively correlated with productive life. Daughter pregnancy rate is the only positively correlated trait that has seen its correlation increase. This past December, USDA tweaked its productive life formula to account for some of these changes.

Another way to view genomic performance is to look at the top 25 genomic young sires versus the top 25 proven bulls. In 2009, genomics suggested the top genomic young sires had a net merit of about $750 compared to $500 for top proven bulls. Once genomic bulls had daughters, there was no difference in the average of the two groups.

But the very best genomic young sires from 2009 had daughter proofs that were $100 better than the very best of the proven bulls. The problem is that genomics alone was unable to identify all of these top bulls when they were first sampled, Dechow says.

As a result, if you are a commercial producer who isn’t selling top-end breeding stock, you might want to consider using more proven bulls in your sire portfolio. "You will miss out on some of the very best young sires initially, but you will avoid those that don’t turn out," Dechow says.
In addition, proven sires have more accurate calving ease data. If you use young sires, using more of them will spread your risk.

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