Breeding and Genetics

Genetic parameters for body weight (BW), body condition score (BCS) and dry matter intake (DMI) of dairy heifers were estimated using covariance function (CF) methodology. Data were from 469 Holstein heifers of the Langhill Dairy Cattle Research Centre and included observations at calving and at 3, 5, 10, 20, and 25 weeks of lactation. The experiment comprised a selection line and a control line. Heifers were allocated to a high or to a low concentrate diet, which was offered as a complete mixed ration ad libitum. Compared to multivariate REML analyses the number of parameters to estimate variance components could be reduced when covariance functions (CFs) were used. Genetic variances for the traits clearly depended on the lactation stage. For all traits the genetic variance at all stages during lactation could adequately be described by a second order polynomial CF. Heritability estimates for observations measured at different stages of lactation ranged from 0.43 to 0.5 6 for BW and from 0.21 to 0.45 for BCS. For DMI heritabilities ranged from 0.17 to 0.32. The genetic correlation between repeated observations generally decreased with increasing time period between observations. The genetic correlation between repeated observations on BW and BCS ranged from 0.84 to 1.00. However, for repeated observations on DMI genetic correlations were lower. The genetic correlation between DMI in week 3 and week 25 was only 0.23. The eigenfunctions and their associated eigenvalues that were derived from the estimated CFs showed that repeated observations can be assumed to be observations on the same trait for BW and CS but not for DMI. The results imply that observations for DMI at multiple lactation stages are needed when the aim is to improve DMI during whole lactation.