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Herdmates versus Contemporaries for Evaluating Progeny Tests of Dairy Bulls

Animal Physiology and Genetics Institute, ARS, USDA, Beltsvile, Maryland 20705

ABSTRACT

Distributions of various kinds of herdmates, defined as nonpaternally related cows calving in the same moving 5- mo herd-year-season, were studied on 113,386 first and 70,044 second lactations initiated 1966 to 1968. These data were all the lactations available from 6,428 herds and 5 major dairy breeds. From 6 to 23% of progeny in first lactation sired by bulls in artificial insemination and 9 to 25% of the naturally sired progeny with a herdmate of some age did not have contemporary herdmates in first lactation. Similar values for second lactations were 10 to 28% and 15 to 29%. Only 1 to 5% did not have herdmates of some age. For those with herdmates, the average number of contemporaries ranged from 3 to 15 but were 5 to 10 for most groups. Numbers of herdmates ranged from 12 to 37, but most groups averaged over 20. Values for second lactation were lower. Biases against AI sired first lactations caused by comparing them to selected older cows were small in three breeds ( +7 to –10 kg) but were larger for Jerseys and Brown Swiss (–86 and –115 kg). Biases against non-AI sired first lactation cows were of similar magnitudes. Larger biases resulted from comparing first lactations to only their first and second lactation herdmates. Sire summaries based on first lactation vs. first lactation herdmates would have a larger sampling variance (about 5 to 40%) but would not be biased by culling. Sampling variances would be lower when first lactations are compared to herdmates of all ages, but biases resulting from older cows being the survivors of culling for yield would be present. A modified contemporary comparison that uses all cows with at least one herdrnate of any age but that is unbiased by culling is presented.

¹ Present address: Animal Science Department, North Carolina State University, Raleigh 27607.