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To assess the effects of diet (roughage vs. concentrate) on energy utilization and protein metabolism, growing Holstein steers were given equal daily intakes of digestible energy from one of three diets: hay alone (diet 1; 12% protein), 85% concentrate and 15% hay (diet 2; 16% protein), and 85% concentrate and 15% hay (diet 3; 12% protein). The ratio of protein to digestible energy was the same in diets 1 and 2. At 100, 200, 300, 400, and 500 kg liveweight, estimates were obtained of digestibility and metabolizability of the diets, of weight of gastrointestinal digesta, of body composition based on the volume of distribution of tritiated water, of the concentration of a number of constituents in blood and of VFA in rumen fluid, of fractional degradation rate of skeletal muscle protein based on the urinary excretion of N T -methyl histidine, of the concentration of ammonia in rumen fluid and bacterial urease associated with the reticulo-rumenal epithelium, and of the concentration of total protein, and of the myofibrillar and sarcoplasmic protein fractions, in skeletal muscle. As a proportion of the weight of the empty body, water and protein were higher and fat was lower in steers on diet 1 than in those on diets 2 and 3. Empty liveweight gain in the steers on the concentrate diets (diets 2 and 3) was 35% greater than in steers on the hay diet (diet 1), and steers on concentrate required only 73% as much metabolizable energy per unit empty liveweight gain and 57% as much energy per unit energy gain as steers on hay. Daily gains of fat and energy were greater in the steers on concentrate than in those on hay and most of the extra gain of the steers on concentrate was attributable to gain of fat. Steers on concentrate had lower levels of triglyceride and higher levels of insulin and glucose in blood, and higher levels of propionate and lower levels of acetate in rumen fluid, than did steers on hay. There were no diet effects on the composition of skeletal muscle nor on the fractional degradation rate of muscle protein. Nitrogen metabolism in the rumen, and associated effects on blood urea and urinary nitrogen excretion, were the same in steers on diets 1 and 2 but different for steers on diet 3. However, fecal nitrogen output was the same for steers on diets 2 and 3 but much higher in steers on diet 1. These results are consistent with the view that the higher rate of gain of fat in steers on concentrate, as compared with those on hay, was not due to effects on the availability of protein for productive purposes or on protein metabolism in skeletal muscle, but was more likely to have been due either to lower rates of protein turnover in other tissues, such as those of the gastrointestinal tract, thus sparing energy for fat synthesis, or to a less efficient utilization of the energy available for fat synthesis. The latter possibility appears to be compatible with the finding of higher levels of circulating insulin in steers on the concentrate diets. Key words: Growth, efficiency, body composition, cattle

canadian journal of animal science

RATE AND EFFICIENCY OF GAIN, BODY COMPOSITION, NITROGEN METABOLISM, AND BLOOD COMPOSITION OF GROWING HOLSTEIN STEERS GIVEN DEITS OF ROUGHAGE OR CONCENTRATE