Relationships Between Age and Body Weight at Calving, Feed Intake, Production, Days Open, and Selection Indexes in Ayrshires and Holsteins

Records collected between December 1979 and June 1986 provided measures of age and weight at calving, days open, 305-d measures of milk, fat, and protein yield, milk value, feed cost, and total energy intake for 8156 Ayrshire and 80,604 Holstein cows in first lactation. Phenotypic and genetic variances and covariances were estimated from separate analyses per breed using a multiple trait mixed model that included traits of body weight at calving, energy intake, 3.5% FCM, and days open with (fivetrait model) and without (four-trait model) age at calving, which was treated as a fixed effect in the four-trait model. Sixty-five Ayrshire and 410 Holstein sires were treated as random in the analyses. The resulting parameters were used to develop four-and five-trait selection indexes, which were then paired with a number of traits in a series of two-trait analyses. Heritability estimates were higher in Holsteins than in Ayrshires for all traits, especially for 3.5% FCM. Also, with the exception of days open, which had a very low heritability (.01 and .03), heritability estimates were higher when traits were adjusted for age at calving as a fixed effect. Phenotypic correlations between traits were almost identical for the two breeds, whereas ge-