Cow evaluation at different milk yields of herds

Herds with higher milk production per cow tend to have used bulls with larger Predicted Differences and to have larger Cow Indexes. A larger portion of cows in these herds should attain elite status for production than in herds with lower production. However, even when average sire merit was similar, higher producing herds had substantially more elite cows, apparently because standard deviation of deviated production of cows is greater in herds with high milk production. This suggested that cow deviation in high producing herds may receive too much weight in the Cow Index. However, higher heritabilities likely are appropriate for cows in higher producing herds, which would favor weighting deviation more in high producing herds. Adjustments for these two effects would offset each other at least partially. Cow Indexes adjusted for either or both effects had similar correlations with son or daughter performance as did the current Cow Index that considers neither effect. Although high producing herds have more elite cows than can be justified by average sire merit, some of the apparent advantage may be justifiable by larger heritabilities.