Three rations containing different levels of energy were fed beef bull, steer and heifer calves

Abstract

The higher energy ration (nº 1) was the only one fullfed throughout the two 17-week periods of experiment (treatment 1). Cattle on the medium energy ration (nº 2) were switched to the lower energy ration (nº 3) after the first 17-week period (treatment 2). Cattle on treaty ment 3 had these same rations (nº 2 and 3) in an opposite order. All treatments were fed to an equal number of bulls, steers and heifers (a total of 36 animals). Bulls showed the highest gaining ability, heifers the lowest, and steers were intermediate. Treatment 1 gave the best results, followed by treatment 3. The results of treatment 2 were possibly favored by a slightly higher protein content or ration 3 (first period), and by a probable induced compensatory growth (second period). Bulls were significantly higher in PUN than steers and heifers, which were equivalent. Treatment 1 and 2 were similar and significantly lower than treatment 3. PUN increased rather consistently from the beginning to the end of the experiment. It seemed to reflect to adequacy of protein and, with more emphasis, of energy intake. PBI, contrary to the initial hypothesis, did not show significant differences either among sexes, or among ration treatments. There was a tendency to decrease as the experimental time increased; however, no correlation was found between PBI and PUN. Conclusions: The studied efficiency of feed utilization was 1) higher for bulls and lower for heifers; 2) proportional to the amount of energy fed (considering only the first period); 3) greater for treatment 3, probably due to a slightly higher protein content of ration 3 and a possible compensatory growth; 4) to some extent, reflected by the levels of Plasma Urea Nitrogen; 5) no correlated with plasma Protein Bound Iodine.