RELATIONSHIPS BETWEEN AGE AND BODY-WEIGHT AT CALVING AND PRODUCTION IN 1ST LACTATION AYRSHIRES AND HOLSTEINS

Individual cow test day records collected between December 1979 and June 1986 were used to generate 70, 90, and 305-d lactation yields of milk, fat, and protein for 11,457 Ayrshire and 112,371 Holstein cows in first lactation. Three separate analyses per breed were carried out according to a multiple-trait mixed model that included age and weight at calving with production traits for two of the three time periods. Sixty-six Ayrshire and 416 Holstein sires were treated as random in the analyses. Holsteins were on average younger and heavier at calving and averaged more milk per day than Ayrshires. Breed differences in daily fat and protein yield were small. Heritability estimates were higher for Holsteins than for Ayrshires for all traits, particularly 305-d production. Heritability of age at calving was very low. Weight at calving was more highly heritable than age but was less heritable than the production traits. Generally, phenotypic and genetic correlations between traits were very similar for Ayrshires and Holsteins. Phenotypically, both older cows and cows heavier at calving had higher yields of milk, fat, and protein. However, genetic correlations between weight at calving and production ranged from -.11 to -.30 and -.21 to -.38 in Holsteins and Ayrshires, respectively. Although not large, these correlations indicate that genetically heavier cows at first calving produce less milk, fat, and protein. Genetic and phenotypic correlations between yield traits were all positive and were .47 or greater. All genetic correlations between the same production trait measured for different lengths of lactation exceeded .90.