Heterogeneity in (co) variance structures of test-day yields

First-lactation test-day milk, fat and protein yields from New York, Wisconsin, and California herds were adjusted additively for age and lactation stage. A random regression model with third-order Legendre polynomials for permanent environmental and genetic effects was used. This model also included a random effect with the same polynomial regressions for 2-yr intervals within herd (herd, time period of calving effect). Phenotypic variances were modeled using a mixed model. Heritabilities and permanent environment showed the expected pattern. (...)