Abstract
A regression method based on variances and covariances of unextended herd-year-season means for each of the 10 lengths of lactation was used to calculate the adjusted herdmate average. Needed parameters were estimated from monthly Dairy Herd Improvement Association test information from 12,750 Holstein cows with 16,991 part and whole records. Weights generated under this method were compared with those from the usual method. Components of variance among and within herd-year-season and herd-year season components of covariance between the 10 lengths were computed for the regression procedure. Both sets of estimates were smoothed to adjust for aberrant components. Regressions were estimated for all combinations of herdmate lengths in the data. The weights for each length varied with the distribution of the number of herdmates at each length. Weights for different lengths of herdmate means were developed algebraically for each method and were compared for different herdmate distributions. The weights for the short length records of herdmates of a cow with a complete record were comparatively larger for usual method than those from the regression method. When herdmates with lactations of different length were present, the longer incomplete and complete records received considerably less emphasis in the usual method than in the regression method. Possibilities of implementing the regression method are discussed.
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