"Estimates of genetic and phenotypic covariance functions for postweaning growth and mature weight of beef cows"

K. Meyer

Journal of Animal Breeding and Genetics 116 : 181-205.


Annual weights of of cows from 19 to 119 months of age in two herds were analysed fitting a random regression model, regressing on orthogonal polynomials of age in months. Estimates of covariances between random regression coefficients were obtained by restricted maximum likelihood, and resulting estimates of covariance functions used to construct covariance matrices for all ages in the data. Analyses were carried out fitting regression coefficients corresponding to overall animal effects only and fitting regressions for animals' additive genetic and permanent, environmental effects. Different definitions of fixed effects subclasses were examined. Models were compared using likelihood ratio tests and estimated standard deviations for the ages in the data.

Cubic regressions were sufficient to model both population trajectories and individual grow curves. Random regression coefficients were highly correlated, so that estimation forcing their covariance matrices to have reduced rank (2 or 3) did not reduce likelihoods significantly, allowing parsimonious modelling. Results showed that records were clearly not repeated measurements of a single trait with constant variances. As cows grew up to about 5 years of age, variances increased. Estimates of genetic correlations between three year old and older cows were close to unity in one herd but more erratic in the other. For both herds, genetic correlations between weights on two year old cows and older animals were clearly less than unity.

Key words : Growth, beef cattle, covariance function


K.Meyer, July 2000