Estimation of genotype X environment interactions, in a grassbased system, for milk yield, body condition score,and body weight using random regression models

(Co)variance components for milk yield, body condition score (BCS), body weight (BW), BCS change and BW change over different herd-year mean milk yields (HMY) and nutritional environments (concentrate feeding level, grazing severity and silage quality) were estimated using a random regression model. The data analysed included records from 7478 multiparous upgraded Holstein–Friesian dairy cows. There were G×E interactions for BCS across all environments and for BW change across different concentrate levels and silage quality environments. There was a three-fold increase in the genetic standard deviation (S.D.) for BCS change to day 60 of lactation (CS60-5) and a doubling of the genetic S.D. for BCS at day 5 (CS5) as silage quality improved. The genetic variance for CS60-5 increased as concentrate level increased and as grazing severity became tighter. There was significant re-ranking of animals for milk yield, CS5 and CS60-5 over the different HMY environments; genetic correlations fell to -0.60 between extreme HMY environments for CS60-5 and were as low as 0.41 for CS5 across different HMY environments.