Associations among progeny tests of single or pooled lactations.

Genetic correlations between Predicted Difference for first and second, first and third, and second and third lactations milk were .82, .62, and .94 and for Predicted Difference percent fat .94 for the three associations. Regression coefficients for following on previous lactation Predicted Difference were less than unity; thus, Predicted Difference from first lactation might be overweighted when age adjusted and pooled with second and third lactations. Regressions of Predicted Difference milk of sons on sires for first, second, third, and pooled lactation records were .32 +/- 18, .42 +/- .21, .56 +/- .31, .35 +/- .15, and intraclass correlations between half brothers were .09 +/- .08, .28 +/- .11, .25 +/- .11, and .12 +/- 10. Predicted Differences for second and third lactations were, thus, near the theoretical expectation of .5 and .25 for regressions of son on sire and between half-brother correlations but lower for Predicted Differences of first and pooled lactations. Variance of Predicted Difference of first lactation between sires of sons was small compared with the variance of sons within sires. It was postulated that effective selection on the sire-to-sire path on Predicted Difference of first 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 first and following lactations and from the reduction in the variance between sire in first but not in later lactations. Progeny tests of single lactation seem warranted for the production profile of dairy sires.