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Intrasire and Intracow Regressions of Lactation Records on Herdmate Performance: Use, Estimators, and Biases¹

Department of Dairy Science, Michigan State University, East Lansing 48823

ABSTRACT

Herd-year-season effects are not individually adjustable and yet contribute nearly 50% of the total variation in production. Herdmate comparison method was designed to adjust records for this major source of environmental differences without knowledge of individual influences. Essentially, in evaluating sires the sire of a daughter group is the genetic unit, and daughter averages are regressed on their adjusted herdmate averages. In cow evaluation, however, the genetic unit is a cow; therefore, an intracow regression of her performance on her adjusted herdmate averages is pertinent.

Intracow regression coefficient (b_c) is difficult to estimate, since only those cows making records in more than one herd contribute anything to the estimation. Estimates for intrasire regression coefficient (b_s) are available. The relationship between b_c and b_s is b_c = 2b_s – 1 which provides a consistent estimator for b_c with the bias factor being simply the inverse of the sample size for estimating b_s. The value .9 is commonly used for bs in the sire proofs. The value for b_c in cow index procedure for the similar purpose is then .8.

¹ Michigan Agricultural Experiment Station Journal Article No. 6410.