Relationship Between Milk Yield and Age at Calving in First Lactation

Precise, unbiased evaluation of sires for milk yield is dependent on appropriate consideration of herd-year-season and age at calving. Age-adjustment procedures presume that the effect of age at calving on milk yield is environmental and the same for all sires, herds, and years. The homogeneity of curves and the degree to which age effects may be genetic are examined. Data from 17,885 first lactations of Holsteins in Canadian Record of Performance were analyzed. Sire, herd, sire X herd, and residual components of variance and covariance among milk yield, age at calving, and its square were estimated by fitting constants. Residual quadratic curves of milk yield on age at calving were similar for all year-seasons. Herds accounted for 32.1% of the variance in milk yield, 26.7% of the variance in age, but only 6.3% of the covariance between them. Within-year-season heritability eliminating herd effects was .30 for milk yield and .34 for age at calving with a .27 genetic correlation. Although sire by herd interaction accounted for only 2.0% of the variance in milk yield, it was 16.0% of the variance in age at calving and 10.7% of the covariance. The sire, herd, sire X herd, and residual mean squares and products led to different curves. Sire and residual curves were similar but less quadratic than the interaction curve and much steeper than the herd curve.