Genetic Change in Milk Yield Estimated from Simultaneous Genetic Evaluation of Bulls and Cows

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Abstract
Genetic trend for milk yield in the artificially sired Holstein population in the northeastern United States was estimated from solutions for simultaneous genetic evaluations of bulls and cows for an animal model using mixed model equations and including all known relationships among the population by years of birth of 1960 to 1980 (1978 for bulls). First lactation milk records of 1,074,971 artificially sired cows sired by 6000 bulls in 20,065 herds were used. Averages of estimated genetic value by year of birth were similar from solutions after 10, 20, and 30 rounds of iteration of the mixed model equations. The trend in genetic value of bulls that entered artificial insemination was marked by slight negative change for three periods totaling 9 yr and substantial positive change averaging 105 kg/yr for the other 9 yr, which suggests that the dairy industry sacrificed milk yield in genetic value of bulls put into artificial insemination during those periods to selection criteria other than milk yield. There was little change in average genetic value of registered cows from 1960 to 1970. Gain in nonregistered cows totaled 177 kg for the same period. Gain from 1970 to 1980 was similar for both registered and nonregistered cows, 39.5 and 38.1 kg/yr, respectively. Average genetic value of nonregistered cows exceeded that of registered cows each year except 1960.

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