Genetic evaluation of dairy bulls for energy balance traits using random regression

Current selection objectives for dairy cattle breeding may be favouring cow s that are genetically predisposed to mobilize body tissue. This may have c onsequences for fertility since cows may resume reproductive activity only once the nadir of negative energy balance (NEB) has passed. In this study, we repeatedly measured food intake, live weight, milk yield and condition s core of Holstein cattle in their first lactation. They were given either a high concentrate or low concentrate diet and were either selected or contro l animals for genetic merit for kg milk fat plus milk protein. Orthogonal p olynomials were used to model each trait over time and random regression te chniques allowed curves to vary between animals at both the genetic and the permanent environmental levels. Breeding values for bulls were calculated for each trait for each day of lactation. Estimates of genetic merit for en ergy balance were calculated from combined breeding values for either (1) f ood intake and milk yield output, or (2) live weight and condition-score ch anges. When estimated from daily fluxes of energy calculated from food intake and milk output, the average genetic merit of bulls for energy balance was appr oximately -15 MJ/day in early lactation. It became positive at about day 40 and rose to +18 MJ/day at approximately day 150. When estimated from body energy state changes the NEB in early lactation was also -15 MJ/day. It bec ame positive at about day 80 and then rose to a peak of +10 MJ/day. The dif ference between the two methods may arise either because of the contributio n of food wastage to intake measures or through inadequate predictions of b ody lipid from equations using live weight and condition score or a combina tion of both. Body energy mobilized in early lactation was not fully recove red until day 200 of lactation. The results suggest that energy balance may be estimated from changes in body energy state that can be calculated from body weight and condition score. Since body weight can be predicted from l inear type measures, it may be possible to calculate breeding values for en ergy balance from national evaluations for production and type. Energy bala nce may be more suitable as a breeding objective than persistency.