A covariance function for feed intake, live weight, and milk yield estimated using a random regression model.

To enable investigation of genetic variation during early lactation in heifers, multitrait covariance functions were used to describe genetic covariances among feed intake, live weight, and milk yield during the first 15 wk of lactation (n = 628). Random regression models were used to estimate covariance functions for the additive genetic and permanent environmental effects. Fixed effects were date of the week that records were collected, a group effect, and week of lactation. Second or third order polynomials were sufficient to describe the additive genetic variation for milk yield, dry matter intake, and live weight during the first 15 wk of lactation. Estimates for the genetic covariance function demonstrated that a high milk yield is only moderately correlated with high feed intake (0.21) but is very strongly correlated to an increase of intake and a loss of live weight during the first 15 wk of lactation. Levels of weight and intake were correlated strongly (0.81). The reduced fit covariance function was used to estimate genetic correlations between traits at different lactation stages. Estimates for the genetic correlations between wk 1 and 15 were 0.62, 0.24, and 0.79 for milk yield, dry matter intake, and live weight, respectively. Feed intake during early lactation was negatively correlated with milk yield, but feed intake during the later weeks was positively correlated with milk yield. The implication is that when selection is for a linear combination of milk yield, feed intake, and live weight (i.e., energy balance or efficiency), it is important to consider when each trait is measured during lactation.