Prediction of the energy requirements for growth in beef cattle. 2. Hereford × British Friesian steers given dried grass or barley

SUMMARY 1. Hereford × British Friesian castrate male cattle, growing from about 80 to 500 kg, were given equal amounts of metabolizable energy (ME) from dried grass pellets or from a pelleted diet of barley and a protein supplement. Two animals on each diet were fed close to appetite (High) and two at a level mid-way between maintenance and appetite (Medium). The rations for all animals were reduced to approximately maintenance for 4 weeks on three occasions equally spaced throughout the experiment. Measurements of energy and nitrogen balance were made for each animal at intervals of 6 to 8 weeks. Total body-water content was estimated from D20 dilution at intervals of 110 kg live weight. 2. Values for the metabolizability of the gross energy of the dried grass and barley/protein diets were 0·55 and 0·67. 3. Metabolic heat production during growth was closely related to body weight to the power 0·75. Predicted basal metabolism was 395 kJ/kg 0·75 per 24 hr. 4. Estimates of the net efficiency of utilization of ME for growth were unaffected by small variations in the exponent of W used to describe the effect of body size on heat production. Both diets were utilized above maintenance with a net efficiency close to 0·62. 5. Nitrogen balance trials seriously overestimated nitrogen retention. Gains in fat-free body mass estimated from D 2 O dilution were combined with calorimetric measurements of energy retention to predict the relationship between energy retention and weight gain. The agreement between observed and predicted live-weight gain was good at the lower body weights and on the lower ration. For fatter animals eating the high ration at higher body weights, live-weight gains were less than predicted.