In the April issue of Graze we made an attempt to stimulate some discussion about the optimal cow in a “no grain” grass fed grazing dairy. In the summary paragraph I mentioned that “we can only breed one cow to one bull at a time”. Lots of feedback was received for this article, asking for specifics on the ideal grass cow physique and how to breed that, but no one mentioned this statement.
e transforming data into trait indexes and composite ranking, it is the conclusion of geneticists that your best selection strategy is simple: breed all your cows to that bull who ranks highest. (Oh, you worry about inbreeding? Then use the second highest bull as well.)
Your conclusion: variation in my cows doesn’t matter. Breed the group, not the individual.
So let’s consider how we provide data. You are a grazier who participates in DHIA testing and keep track of sire and dam ID. That qualifies your data for genetic evaluations. Heifer “x” calves along-side a couple dozen other heifers. They get to walk across the pastures every day. Unlike the others, however, “x” does not walk gracefully, she gimps and limps. She brings up the rear at every milking.
As soon as she is out from milking, she lays down. Over the next few weeks, she milks off her body and then production declines. She is not breeding back. You clean and trim her feet, but nothing you do appears to improve her mobility. Eventually you mark her “DNB” and at the point her production falls below your cutoff, you send her to town.
Once a month that DHIA tester comes back. He notes this cow has no milk weight. He needs to code a reason for this: did you sell her for dairy? (no) Dry her up early? (no) Did she die? (yes, but no, not here) Was she shipped as a cull? (yes) What caused you to ship her??
The biggest issue with accurate genetic mating is that you must determine between Causality and more simplistic Associative relationship. The cause of “x” being a cull was mediocre feet. The associated reason for culling was she had fallen below minimum production requirement (as a result of a chronic negative energy state, reducing appetite, caused by feet incapable of sustained grazing).
As for the accuracy of DHI data for genetic purposes, we have already heard that on average, 15% of all cows on test are misidentified by sire. I would propose that there is even more error in the coding of any discretionary question that has a multiple choice answer (“why did you sell this cow?”). Most DHI systems are now owned by AI studs, and they favor the herds that maximize production per cow, as the genetic evaluation ranking systems are based on pounds. Large percentages of graziers do not use DHI testing because you get tired of being put on the bottom of the DHI herd ranking publications.