A Multivariate-t Threshold Model for Clinical Mastitis in Norwegian Cattle

Abstract

Clinical mastitis records from 30 days pre-calving to 300 days post-partum on 36,178 first-lactation Norwegian Cattle cows were analyzed. Heritabilities and genetic correlations between clinical mastitis in 4 periods of lactation were inferred using a multivariate-t threshold model. This was compared with a multivariate probit model via a pseudo Bayes factor and an analysis of residuals. MCMC algorithms were used to draw from posterior distributions. The pseudo Bayes factor favored the multivariate- t threshold model. This model also had smaller residuals when cows had mastitis. The posterior mean of the degrees of freedom for the t model was 6.4, pointing away from the assumption of a normal liability. In the t model, heritability of liability to clinical mastitis was 0.15 before calving, and ranged between 0.06 and 0.07 after calving. Genetic correlations ranged between 0.52 and 0.75.