The contribution of the leucine/valine substitution at amino acid position 127 in the bovine growth hormone (GH) protein to variation in carcass traits was studied. The data included 109 Polish Friesian bulls slaughtered at 15 months of age. The traits measured were carcass gain, weights of meat, bones, intermuscular and subcutaneous fat in the carcass and meat, bones and fat in valuable cuts (fore and best ribs, sirloin, round of beef and shoulder). The bulls’ GH genotype was determined using the PCR-RFLP technique. The frequencies of leucine (Leu) and valine (Val) alleles were 0·64 and 0·36, respectively. The GH concentration was determined in serial blood plasma samples collected every 15 min starting from 15 min before to 135 min after intravenous administration of 0·15 µg thyrotropin releasing hormone (TRH) per kg live weight. Response GH variables were: baseline (the mean of samples collected at –15 and 0 min), peak (the sample taken at 15 min post injection of TRH) and disappearance rate (calculated as peak minus the sample at 60 min, divided by time interval 45 min). Mixed animal models were used for the statistical analysis. Differences were found between the Leu/Leu and the Val/Val genotypes for carcass gain and weight of meat in the carcass (P ≤ 0·05). Moreover, differences in the size of the GH peak between the two homozygotes approached significance (P ≤ 0·10). The effect of GH genotype accounted for a moderate part of the phenotypic variance in the carcass traits, corresponding to a reduction in the residual variance of ≤ 5·25% when included in the model, whereas the corresponding value for the effect of GH genotype on the variation in GH release was lower, ≤ 1·77%. In conclusion, the Leu/Val polymorphism seems to be associated with carcass traits in dairy bulls, although the effect was relatively small when compared with the effects of season and background genome.

Current selection objectives for dairy cattle breeding may be favouring cows that are genetically predisposed to mobilize body tissue. This may have consequences 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 score of Holstein cattle in their first lactation. They were given either a high concentrate or low concentrate diet and were either selected or control animals for genetic merit for kg milk fat plus milk protein. Orthogonal polynomials were used to model each trait over time and random regression techniques 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 energy balance were calculated from combined breeding values for either (1) food intake and milk yield output, or (2) live weight and condition-score changes.

When estimated from daily fluxes of energy calculated from food intake and milk output, the average genetic merit of bulls for energy balance was approximately -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 became positive at about day 80 and then rose to a peak of +10 MJ/day. The difference between the two methods may arise either because of the contribution of food wastage to intake measures or through inadequate predictions of body lipid from equations using live weight and condition score or a combination of both. Body energy mobilized in early lactation was not fully recovered 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 linear type measures, it may be possible to calculate breeding values for energy balance from national evaluations for production and type. Energy balance may be more suitable as a breeding objective than persistency.