Modified Contemporary Comparisons for First and Second Lactations in the Same and Different Herds

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Abstract
Separate sire evaluations by Modified Contemporary Comparison for 1st and 2nd lactations were calculated for 200 widely used Holstein bulls from a file containing only 1st or 2nd records of all daughters and from 2 independent subsets of the same data. Sire evaluations based on both 1st and 2nd lactations also were computed. Correlations between 1st lactation evaluations in 1 independent data set and 2nd lactation evaluations in the other were 0.87 and 0.84. First lactation evaluations accounted for only about 75% of the variation in 2nd record evaluations, suggesting a need for some form of later record sire evaluation, at least for the selection of sires of sons. Within sire, daughter variation for 1st records was not useful in predicting difference between 1st and 2nd record sire evaluations. Second lactation evaluations increased relative to firsts as culling increased, suggesting culling bias in Modified Contemporary Comparisons for later records. An adjustment based on an empirical regression of later records on firsts produced 2nd evaluations with means and SD nearly identical to those of 1st records. Correlations between 1st and adjusted 2nd evaluations were 0.87 for both independent data sets. Difference between adjusted 2nd and 1st evaluations were correlated 0.15 with culling, compared to 0.25 for unadjusted 2nds. Important sire differences in 1st and 2nd lactation evaluations were detected in all data sets. Such differences were unaltered by adjustment for selection.