VARIANCE-COVARIANCE COMPONENTS ASSOCIATED WITH TRIMESTER YIELDS OF MILK AND FAT AND MULTIPLE TRAIT SIRE EVALUATION FOR TRIMESTER YIELDS

Genotypic and phenotypic variances and covariances for milk and fat yields of 3 equally-spaced intervals in 1st lactation, days 1 to 90, 91 to 180 and 181 to 270, were estimated from 26,523 records of initial progeny-test daughters of 2086 Holstein sires. Henderson''s method 3 was used with a model that included fixed herd-yr, fixed age-mo. of freshening and random sire and residual effects. Heritabilities for milk yields were 0.21, 0.21 and 0.13 for the 1st, 2nd and 3rd 90 days in lactation and for fat yields were 0.19, 0.16 and 0.10. Heritabilities for milk and fat yields over the entire 270 days postpartum were 0.22 and 0.22. Genetic correlations among partial yields were high and ranged from 0.74 to 0.99 for milk and 0.86 to 0.99 for fat. Genotypic and phenotypic variances were used to predict breeding values for trimester milk yield by multiple trait, mixed model procedures for 3797 Holstein artificial insemination sires from 283,900 1st lactation milk records of their daughters during the first 90, second 90 and third 90 days in lactation. Correlations among proofs for the 3 trimesters ranged from 0.85 to 0.94. Correlation between the sum of 3 trimester multiple trait proofs and proof for milk yield for 270 days of lactation from single trait analysis was 0.99. Correlations among proofs indicate sires may rank differently for milk yield in the 3 periods.