Effect of level of energy intake and influence of breed and sex on the chemical composition of cattle.

Chemical composition of the empty body was determined in 159 animals slaughtered at weights ranging from 121 to 706 kilograms. Holstein and Angus bulls, steers and heifers were fed at two energy levels: ad libitum and 65 to 70% ad libitum. The allometric equation, Y = aXb, was used to determine the effect of energy intake and the influence of breed and sex on the accretion rates of the chemical components relative to the growth of the empty body or fat-free empty body. Group comparisons for chemical composition were made after adjustment by regression to a common empty body weight. The expression of the sex influence on the accretion rates of water, protein and ash relative to the empty body depended on the breed and the energy intake level considered. The accretion rate of chemical fat was not influenced by sex. Genetic differences in the accretion rate relative to the empty body were detected only among animals in the high energy intake group. Regardless of sex, the accretion rates of protein and ash were more rapid (P < .05) in Holsteins than in Angus. However, a breed influence on the accretion rate of chemical fat was detected only among bulls, where Angus had a more rapid accretion rate. Feeding animals at two energy levels resulted in different accretion rates relative to the empty body. In the Angus breed, regardless of sex, the accretion rates of water, protein and ash were more rapid (P < .05) in the low intake group, whereas the accretion rate of chemical fat was slower (P < .05). Among Holsteins, the low energy intake level had a less definite effect; for bulls, the accretion rates of water and chemical fat were more rapid (P < .05) and slower (P < .05), respectively; for steers, and accretion rate of protein was more rapid (P < .05), and for heifers, none of the accretion rates was altered.