The energy metabolism of young Jersey and Friesian calves fed fresh milk

SUMMARY 1. Eight British Friesian and eight Jersey calves were fed fresh milk during the period from 3 days to 40 days of age, at two levels of feeding. 2. Energy and nitrogen balances were measured twice for each calf. 3. Fasting heat production (FHP) (MJ/day), which was measured on these occasions, was related to live weight (kg) by FHP = 0·435 W 0·75 . There was no significant difference in FHP/kg 0·75 between the breeds but previous level of feeding and the duration of fast had significant effects. 4. The data for heat production, metabolizable energy (ME) intake and energy retention were interpreted to provide estimates of ‘true’ net energy required for maintenance of 0·23–0·27 MJ/kg 0·75 daily. 5. Pooled values for ME required for maintenance and for the net efficiency of utilization of ME above maintenance were 0·39 or 0·42 MJ ME/kg 0·75 daily, and 0·67 or 0·72 determined by simple or multiple regression techniques respectively; there were no significant differences between the breeds. Pooled values for the partial net efficiencies of utilization of ME for the synthesis of protein and fat were 0·54 and 0·79 respectively. 6. ME required above maintenance was estimated to be 11·3 and 13·6 MJ ME/kg of live-weight gain for the lower and higher rates of gain respectively. 7. A small experiment revealed no major differences between diets of milk alone or milk plus solid food, with regard to maintenance requirement or net efficiency of utilization of ME by calves.