RATE AND EFFICIENCY OF GAIN IN HEREFORD AND ANGUS BULLS FROM LINES SELECTED FOR RAPID GROWTH ON HIGH-ENERGY AND LOW-ENERGY DIETS

Abstract

Hereford and Angus bull calves were obtained from lines selected for an average of 18 yr for rapid growth on a high-energy diet (concentrate) or a low-energy diet (forage). They were given a typical feedlot diet (70% concentrate and 30% forage) in amounts about equal to 95% of the ad libitum intake and were slaughtered at a liveweight of 500 kg. Effects of breed, or of the selection diet given to the antecedents of the bulls, on rate and efficiency of gain of empty body, carcass, fat, protein, and energy were measured. Selection diet had no effect on rate or energetic efficiency of gain of the empty body or any of its constituents, nor on the distribution of the constituents of the gain between the carcass and the noncarcass portions of the empty body. This suggests that both selection diets exerted the same selection pressure for improved growth rate and feed efficiency when offspring resulting from selection were provided with a common high-energy diet and that either type of diet would, therefore, be appropriate for selecting animals for production systems that use high-energy diets. It was suggested that the main effect of selection was to increase the mature weight of the offspring. Comparing breeds, it was noted that Hereford bulls were heavier at birth, grew more slowly to weaning but more efficiently thereafter, and their carcasses were a smaller proportion of empty live weight than were those of Angus bulls. Key words: Diet, breed, growth, efficiency, selection, bull