Trends in Genetic Merit of Dairy Sires at Initial Proving

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Abstract
Initial modified contemporary comparison sire summaries from 1974-1978 were studied for 16,791 diary sires of 6 breeds. These sires were grouped into 5 classes. Sires sampled through artificial insemination units were in 1 group; natural service sires further classified by number of herds (1, 2-3, 4-7 and 7+) in the initial (1st published) proof were in the other 4 groups. Initial repeatabilities of the progeny test for sires in these 5 classes were .apprx. 40, 18, 23, 29 and 35%. Genetic merit (pedigree indices and predicted differences) was increasing over the 5 yr. Regression of either measure of merit on the year sampled averaged 19 kg of milk and .6 kg of fat. Bulls sampled through artificial insemination organizations were superior to natural service bulls in pedigree index and initial predicted difference. Genetic merit of widely sampled natural service sires showed no consistent advantage over natural service sires sampled in single herds. The intial sire summaries were compared to the most recent summary through 1978. There was a slight tendency for later summaries to average higher for all classes of sires. Sires sampled by artificial insemination organizations were more likely to increase in predicted difference.