Genetic and Environmental Factors in Dairy Sire Evaluation. II. Uses and Limitations of Deviation Records and the Role of Dams

Abstract

Deviation records now constitute the basic data used by the USDA in compiling sire summaries. Such a record consists of the difference between a mature-equivalent (ME) lactation record of a daughter and what is termed the regressed adjusted contemporary herd-mate average. The latter is derived from the simple herd-mate average by two adjustments designed to correct the herd-mate average for (a) unequal numbers of herd-mates included in the various deviation records in a proof and (b) differences in the average breeding values of the herds in which the bulls were used.Adjustment (a) is based on the ratio of the variance among records within herd-year-season groups to the component of variance between such groups. In the present data, this adjustment is approximately n/(n+2) for milk and fat yields and n/(n+4) for test, where n is the number of herd-mates included. The second adjustment (b) is the linear regression of a daughter's record on her contemporary adjusted herd-mate average, or approximately .95 for milk and milk fat yields and .80 for test, in the present data.The variances among herd-year-seasons were only 21, 26, and 47% as large for milk, milk fat, and test, respectively, with deviation records as they were with nondeviated ME lactation records.With deviation records, the extra genetic gains that might be expected by including the records of the dams in sire proofs were only .7, 2.5, and 2.0% for milk, milk fat, and test, respectively.