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.

This experiment examined the effects of sustained levels of undernutrition applied to grazing Brahman heifers on subsequent lifetime reproductive performance. Weaned heifers were raised to a target weight of 270 kg at three different stocking rates to obtain contrasting weight gains (0·097 kglday, L = low; 0·215 kg/day, M = medium; 0·259 kg/day, H = high). They were then subjected to common grazing in a low-quality Brachiaria humidicola pasture, where mean weight changes ranged between -0.100 and +0·300 kg/day depending upon season and physiological condition. Differences in age at first calving between L and the other treatments (200 days) were maintained in subsequent calvings. Calf weaning weights were higher in L and M than in H. The mean weight of calf weaned per cow per year was higher in M than in either L of H. Cow weight at conception increased with parity, and was higher in L than in the other treatments. Similar trends were observed for the dam's weight preand post calving, and at weaning. This trend was associated with higher weight loss during lactation in I..We conclude that in tropical grazing systems that allow only low growth rates, differences established early in reproductive life may become perpetuated for the lifetime of the animal. Management implications in the context of extensive tropical grazing systems are discussed.

Fortin et al. (1980b).

Digestibility (DE) and metabolizability (ME) of a single pelleted diet was examined with growing Angus and Holstein cattle. One hundred and ninety-one individual digestion trials were conducted with bulls, steers and heifers. Food was offered ad libitum or restricted with a daily meal frequency of four. Data were analysed by two covariate models. The first included relative dry matter intake (RDMI) defined as the ratio of DMI to body weight (M 073 ). In the second model, covariates were defined by genetic-size scaling; the first covariate scaled DMI by mature weight (A) raised to the power 0-73 (DMI/A" 7 ') and the second covariate was degree of maturity (n) defined as body weight divided by A. Model 2 also allowed separation of size and DMI effects. No breed differences were detected. There was a consistent trend for increasing ME and ME/DE from heifers to steers to bulls which was significant between heifers and the average of bulls and steers (575-3 v. 589-3 and 832-8 v. 844-3 J/kJ for ME and ME/DE, respectively). Independent of animal effects, DE, ME and urinary energy (UE) decreased and ME/DE and faecal energy (FE) losses increased as RDMI or DMI/A 073 increased. Per unit rise in RDMI equivalent to maintenance, the corresponding changes for DE, ME, ME/DE, FE and UE were -33-9, -13-6, 21-3, 34-1 and 9-6 J/kJ, respectively. DE increased 1-5 and ME increased 0-8 J/kJ GE for each 01 unit increase in degree of maturity.