THE INFLUENCE OF DIETARY ROUGHAGE LEVEL ON EFFICIENCY OF GROWTH AND MUSCLE DEPOSITION IN BULLS AND STEERS

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Abstract
Bulls and implanted steers fed three dietary roughage levels (20% 50%, and 80% alfalfa-brome hay) were compared for various measures of efficiency in beef production. Cattle were about 1 yr of age at the start of the trial and bulls were heavier than steers (290 kg vs. 269 kg). There were four pens, each containing three cattle in every sex × dietary roughage combination. Animals in a pen were slaughtered when pen means approximated 450 kg or 580 kg. Increasing the level of roughage in the diet (20% to 80%) resulted in a significant decrease (P < 0.05) in both daily gain and liveweight gain per 100 MJ DE. The 50% roughage diet produced intermediate results. At a constant liveweight there was no significant (P < 0.05) effect of dietary roughage level on carcass muscle. Bulls produced 9% more muscle than steers at a constant liveweight. Muscle gain per 100 MJ DE decreased significantly (P < 0.05) with roughage level, and was higher for bulls than steers. For a constant amount of feed energy (24 148 MJ DE) bulls fed the 20% roughage diets produced 26.8 kg more carcass muscle (P < 0.05) than bulls fed the 80% roughage diet. Similarly, steers fed the 20% roughage diet produced 16.1 kg more carcass muscle (P < 0.05) than steers fed the 80% roughage diet. Bulls produced 23% more muscle than steers for a constant digestible energy intake (24 148 MJ DE). The overall results thus indicate that dietary roughage and sex-type cause large differences in the amount of carcass muscle produced for a constant energy intake.