Associations Among Progeny Tests of Single or Pooled Lactations

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Abstract
Genetic correlations between predicted difference for 1st and 2nd, 1st and 3rd, and 2nd and 3rd lactations milk were 0.82, 0.62 and 0.94 and for predicted difference percent fat 0.94 for the 3 associations. Regression coefficients for following on previous lactation predicted difference were less than unity; thus, predicted difference from 1st lactation might be overweighted when age adjusted and pooled with 2nd and 3rd lactations. Regressions of predicted difference milk of sons on sires for 1st, 2nd, 3rd and pooled lactation records were 0.32 .+-. 18, 0.42 .+-. 0.21, 0.56 .+-. 0.31, 0.35 .+-. 0.15 and intraclass correlations between half brothers were 0.09 .+-. 0.08, 0.28 .+-. 0.11, 0.25 .+-. 0.11 and 0.12 .+-. 10. Predicted differences for 2nd and 3rd lactations were, thus, near the theoretical expectation of 0.5 and 0.25 for regressions of son on sire and between half-brother correlations but lower for predicted differences of 1st and pooled lactations. Variance of predicted difference of 1st lactation between sires of sons was small compared with the variance of sons within sires. Effective selection on the sire-to-sire path on predicted difference of 1st lactation had reduced predictability on the sire-to-sire improvement path. An interaction of sire by number of lactations may be inferred from the incomplete genetic correlations between predicted differences of 1st and following lactations and from the reduction in the variance between sire in 1st but not in later lactations. Progeny tests of single lactation seem warranted for the production profile of dairy sires.