Energy expenditure, urea kinetics, and body weight gain within a segregating resource family population

Beef and dairy cattle represent divergent metabolic types that disseminate nutrients into either meat or milk and differ in nutrient accretion. To investigate nutrient flow and turnover in an animal model combining beef and dairy cattle, a crossbred experiment has been started. An F2 resource population was generated from Charolais (beef breed) sires and German Holstein (dairy breed) cows as P0 founders by consistent use of embryo transfer to establish the F1 and F2 generations, which accordingly comprised half- and full-sib offspring. In 64 bulls of 5 F2 families, dry matter intake and growth performance were measured monthly, and carcass composition was determined after slaughtering at 18 mo of age. Energy expenditure and urea kinetics were investigated via stable isotope tracer techniques using an intravenous single bolus dose of sodium [13C]bicarbonate [2.5 μmol/kg of body weight (BW), 99 atom% 13C] at 8 and 18 mo of age and of [15N]urea (0.28 mg/kg of BW, 99 atom% 15N) at 8 mo of age, respectively. Insulin responses were measured via glucose tolerances tests at the age of 8 mo. The results revealed significant differences between families for growth performance, energy expenditure, and urea kinetics. In summary, low energy expenditure was associated with high average body mass gain and high insulin response. A greater urea loss was associated with reduced muscle protein in carcass. In addition, corresponding half-sib and full-sib sisters from bulls with highest growth rate indicated highest milk production. In conclusion, we have demonstrated that differences in energy expenditure and urea kinetics result in differences in average daily gain and carcass traits and vice versa in F2 crossbred bulls with common beef and dairy genetic backgrounds.