No abstract

Effects of dietary energy and protein supply on liveweight (LW) gain and gain of protein, fat and ash in the carcass, and weight and gain of non-carcass organs were determined in 118 weaned crossbred lambs from two nutritional histories at Camden, NSW in 1991. Half of the lambs were fed to achieve and maintain LW at 35 kg (LOW group) and half of the lambs were fed ad libitum until they attained 50 kg LW (HIGH group), during a preliminary period of 126 days. In the subsequent experimental period, lambs were allocated to treatments providing 500, 800, 1200 or 1500 g/day of pelleted diets (123 g crude protein, 10 MJ ME/kg dry matter). Diets at each intake contained either 0, 30, 60 or 90 g of formaldehyde-treated casein (rumen escape protein, REP). This resulted in an experiment comparing LOW and HIGH group lambs at four energy intakes, within which were four rates of inclusion of REP. During the 90-day experimental period, LOW group lambs had higher rates of gain of LW, carcass weight and all non-carcass components than did HIGH lambs (P<0·001). At any rate of carcass gain, LOW lambs contained a significantly lower proportion of fat in carcass gain than did HIGH lambs (P<0·05). After adjustment to a common carcass weight, the carcass of LOW lambs contained a significantly lower mass of fat than did that of HIGH lambs at slaughter (P<0·05).Carcass fat gain in the experimental period was not affected by LW at the start of that period or by nutritional history once initial LW was accounted for as a covariate. Data were consistent with fat deposition being principally controlled by energy intake over the immediate pre-slaughter period. In contrast, responses to energy intake in the rate of gain of carcass muscles, ash, liver, head and feet and gut tissue were significantly greater in lambs of LOW compared to HIGH nutritional history. A significant component of this effect of nutritional history was attributable to LW differences between LOW and HIGH lambs; however, nutritional history still had a significant effect on these parameters once initial LW was accounted for as a covariate. Nutritional history may also have modified carcass composition by changing the partial efficiency of use of available energy for protein deposition without changing the partial energetic efficiency of fat deposition.

Two experiments were undertaken to study the effects of protozoa on sulfur and nitrogen availability and on fermentation and the composition of bacteria in the rumen of sheep. In Experiment 1, 12 faunated and 12 fauna-free sheep were offered a basal diet of chopped wheaten straw with or without sulfur (S) and urea-nitrogen (N) supplements. Sulfur supplementation increased the rate of straw digestion and the concentration of volatile fatty acids (VFA) in the rumen while reducing methane production. The presence of protozoa did not significantly affected this response, although it increased rumen H2S concentration. In contrast, the response of rumen fermentation to a urea supplement was affected by the presence of protozoa. Unsupplemented faunated sheep had a faster rate of in-sacco straw digestion in the rumen than did fauna free sheep (44 v. 36%DM/day). Supplementary urea increased the rate of in-sacco digestion of the basal ration in fauna free sheep (36 to 42%DM/day) but not in the faunated sheep (44 to 46%DM/day), suggesting that N availability was greater in the rumen of faunated sheep. Ammonia and total VFA concentrations in the rumen were not affected by protozoa, but the molar proportions of butyrate and isoacids in rumen VFA were greater in faunated sheep. Bacteria from the rumen fluid of faunated sheep contained a higher proportion of lipid and a lower proportion of N in their cell DM.In a second experiment, the chemical composition of rumen bacteria of faunated and fauna free sheep was further investigated. In both groups, bacteria associated with the particle-phase of digesta contained a higher proportion of lipid and a lower proportion of N than did fluid-phase bacteria. Fluid-phase bacteria from faunated sheep tended to have more lipid and less N in their DM than did those from fauna-free sheep.It was concluded that the presence of protozoa enables sustained fermentation of diets low in rumen available nitrogen and also increases the lipid content of rumen fluid-phase bacteria.

Variation in nutrition is a key determinant of growth, body composition, and the ability of animals to perform to their genetic potential. Depending on the quality of feed available, animals may be able to overcome negative effects of prior nutritional restriction, increasing intake and rates of tissue gain, but full compensation may not occur. A 2 x 3 x 4 factorial serial slaughter study was conducted to examine the effects of prior nutritional restriction, dietary energy density, and supplemental rumen undegradable protein (RUP) on intake, growth and body composition of lambs. After an initial slaughter (n=8), 124 4-month old Merino cross wethers (28.4±1.8 kg) were assigned to either restricted (LO, 500 g/d) or unrestricted (HI,1500 g/d) intake of lucerne & oat pellets. After 8 weeks, 8 lambs/group were slaughtered and tissue weights and chemical composition were measured. Remaining lambs were randomly assigned to a factorial combination of dietary energy density (7.8, 9.2, & 10.7 MJ/kg DM) and supplemental RUP (0, 30, 60, 90 g/d) and fed ad libitum for a 12 to 13-week experimental period before slaughter and analysis. By week 3 of the experimental period, lambs fed the same level of energy had similar DMI (g/d) and MEI (MJ/d) (P&gt;0.05), regardless of prior level of nutrition. Restricted-refed (LO) lambs had higher rates of fat and protein gain than HI lambs (P&lt;0.05) but had similar visceral masses (P&gt;0.05). However, LO lambs were lighter and leaner at slaughter, with proportionally larger rumens and livers (P&lt;0.05). Tissue masses increased with increasing dietary energy density, as did DMI, energy and nitrogen (N) retention (% intake), and rates of protein and fat gain (P&lt;0.05). The liver increased proportionally with increasing dietary energy density and RUP (P&lt;0.05), but rumen size decreased relative to the empty body as dietary energy density increased (P&lt;0.05) and did not respond to RUP (P&gt;0.05). Fat deposition was greatest in lambs fed 60g/d supplemental RUP (P&lt;0.05). However, lambs fed 90 g/d were as lean as lambs that did not receive supplement (P0, P&gt;0.05), with poorer nitrogen retention and proportionally heavier livers than P0 lambs (P&lt;0.05). In general, visceral protein was the first tissue to respond to increased intake during refeeding, followed by non-visceral protein and fat, highlighting the influence of differences in tissue response over time on animal performance and body composition.