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 heif ers, multitrait covariance functions were used to describe genetic covarian ces among feed intake, live weight, and milk yield during the first 15 wk o f lactation (n = 628). Random regression models were used to estimate covar iance functions for the additive genetic and permanent environmental effect s. Fixed effects were date of the week that records were collected, a group effect, and week of lactation. Second or third order polynomials were suff icient to describe the additive genetic variation for milk yield, dry matte r intake, and live weight during the first 15 wk of lactation. Estimates fo r the genetic covariance function demonstrated that a high milk yield is on ly 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 fi rst 15 wk of lactation. Levels of weight and intake were correlated strongl y (0.81). The reduced fit covariance function was used to estimate genetic correlations between traits at different lactation stages. Estimates for th e genetic correlations between wk 1 and 15 were 0.62, 0.24, and 0.79 for mi lk yield, dry matter intake, and live weight, respectively. Feed intake dur ing early lactation was negatively correlated with milk yield, but feed int ake during the later weeks was positively correlated with milk yield. The i mplication 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.