Twelve Merino lambs were separated from their dams at 2 days of age. Six were reared in isolation and 6 were inoculated with rumen fluid when 4 weeks old. Apelleted, mixed diet was available from 10 days of age; milk feeding was restricted to maintenance from 2 weeks and discontinued when liveweight reached 9 kg. Three lambs from each group were given a medium intake and 3 a high intake of the diet. The isolated lambs had virtually no rumen protozoa; rumen ammonia and volatile fatty acid concentrations were lower in these lambs but acetate:propionate ratios were higher. The rumen mean retention time (MRT) of a solute marker was longer but the microbial turnover time (TT) was shorter in the isolated lambs; both MRT and microbial TT were shorter on the high intake. Total microbial synthesis did not differ between groups. The digestibilities of energy, organic matter, non-structural carbohydrate, and crude protein were lower in the isolated lambs but those of cell wall constituents were not affected significantly; digestibilities were lower on the high intake. The isolated lambs emitted much less methane than the inoculated lambs; consequently, available energy (i.e. digestible less methane energy), was similar for the 2 groups. Recovery of reducing equivalents in the rumen fermentation was incomplete in the isolated lambs but could be accounted for if reductive acetogenesis provided 13% (medium intake) or 16% (high intake) of the acetate and butyrate formed.
Utilization of energy and nitrogen was assessed by serial slaughter and intake/loss balances, and wool growth was measured, as young sheep grew from c. 25 to 30 kg on either 700 or 1000 g/day of a high-protein diet. Comparisons were made between animals with low (Dorset Horn) and high (Corriedale) propensities to produce wool. Depending on feed intake, fleece-free liveweight gain was 102-215 g/day in the Dorsets and 88-172 g/day in the Corriedales; corresponding growth rates of clean dry wool were c. 5 and 10 g/day, there being little effect of feed intake. Digestibilities of protein and energy were 84 and 74% respectively in both breeds at the lower level of feeding; at the higher level the values were 80 and 71 % in the Dorsets and significantly lower, 77 and 69 %, in the Corriedales. Metabolizable energy was 83 � 0.3 % of digestible energy at the low intake and 86 � 0.5 % at the high intake in both breeds. It averaged 11.7 �. 0.07 MJ/kg feed dry matter, being 0.2 MJ/kg more at the low than at the high intake and, in the latter case only, 0.3 MJ/kg more in the Dorsets than in the Corriedales. At the low and high levels of feeding respectively, the fleece gained 0.18-0.16 MJ and 0.93-0.89 g nitrogen daily in the Dorsets compared with 0.31-0.34 MJ and 1.65-1.81 g nitrogen daily in the Corriedales. The corresponding average daily retentions in the body were 1.74-2.46 MJ and 1.80-3.49 g nitrogen in the Dorsets and 1.34-2.08 MJ and 1.25-3.07 g nitrogen in the Corriedales. Thus body growth accounted for 91-94% of the energy retention and 6640% of the nitrogen retention in the Dorsets, but only 81-86 % and 43-63 % respectively in the Corriedales. The composition of weight gain was the same in both breeds and 15-20% of the energy stored in the body was in protein. It is estimated that the marginal efficiency of use of metabolizable energy was 31-34% for body growth and 16-19% for wool growth. It is concluded that, given a high protein diet, animals with a strong propensity to grow wool diverted an appreciable amount of metabolizable energy from body growth to wool production, thereby restricting deposition of both fat and protein.
Artificially dried grass of 2 tropical species (Setaria spacelata and Digitaria decurnbens) from which most of the stem had been removed, and a temperate grass (Loliurn perenne) grown under the same subtropical conditions, were compared for energy and protein value. Data for a temperate clover hay (Trifolium resupinatum) that was tested concurrently are included because they provide useful contrasts. Each was evaluated by measuring energy, nitrogen (N) and carbon balances in 4 adult sheep at 2 levels of intake (ad libitum and near energy maintenance) and during fasting. The 3 grasses had similar leaf content (76-83%) and the clover much less (60%). The tropical forages had similar chemical composition but they contained less crude protein (10%) than the ryegrass (L. perenne) (1 5%) and more cell wall constituents (63-66% v. 58%). The clover had the most crude protein (19%) and the least cell walls (38%); it also was highest in carbohydrates, pectin and heat of combustion. Voluntary intakes of the setaria, pangola (D. decumbens) and ryegrass were similar and 20-30% lower than the intake of clover. Digestibilities of dry matter (DM), organic matter (OM) and energy were similar for the grasses and 8 percentage units lower than for the clover whereas digestibility of crude protein differed between the tropical and temperate species, the latter having digestibilities 12-15 units greater. Digestibility of cell walls did not vary. The metabolisable energy (ME) values of the grasses at maintenance were 8.3-8.5 MJ/kg DM while that of clover was 10.1; at maximum intakes, corresponding ME values were 7.9 and 10.0 MJ/kg DM. Net availability of ME for gain was 0.42 for the tropical grasses and the persian clover and 0.36 for the ryegrass. By contrast, the tropical forages supported higher gains of N relative to N supply than did the temperate forages because urinary N loss was not affected by intake of the tropicals; these differences in N gain were drastically reduced when gain was expressed in relation to ME. These results do not support the contention that forages of tropical species have a lower energy value per kg than temperate forages of similar composition. Indeed, the tropical species tested here appear to have had an ideal balance of energy and protein insofar as efficiency of use of digestible N was close to 100% and their energy values were commensurate with their chemical composition.
Some aspects of digestion were studied in small groups of fistulated wether lambs from two breeds, Dorset Horn and Corriedale. The Iambs were given a ground and pelleted diet of equal parts hay and concentrates once daily at two levels of intake. At the same liveweight (25 kg), the higher intake was associated with reduced digestion of organic matter (OM) and fibre in the whole gastro-intestinal tract; rumen mean retention times were also reduced, as was the proportion of OM digestion which occurred in the rumen. An increase in the amount of protein entering the intestines appeared to be composed of both undegraded dietary protein and microbial protein. Despite a reduction in the digestibility of non-ammonia nitrogen in the intestines, more crude protein was digested there per unit of digestible OM intake (25.9 v..23.6%; P < 0.05). Differences between the breed groups were smaller than those associated with level of feeding. Nevertheless, particularly at lower relative intakes, rumen mean retention times were longer, rumen digesta content was greater, and rumen OM and fibre digestion tended to be greater in the Dorset Horn lambs; crude protein digestion in the intestines per unit of digestible OM intake also tended to be less at the low intakes. More than three-quarters of the cellulose digestion but less than half the hemicellulose digestion occurred in the rumen. The proportion of the urea synthesized that was degraded in the gastrointestinal tract (0.24-0.30) was consistently lower in these lambs than has been reported for adult sheep.
Merino and halfbred weaner sheep were fed ad libitum for 6 months and food intake recorded; some animals of each breed were given a roughage-concentrate mixture and others were given a wholly roughage diet. The chemical composition of the body was determined from time to time. The halfbred animals ate more of each diet and grew faster than the Mennos. The Merinos became progressively fatter, weight for weight, than the halfbreds; fatness was not affected by diet per se. There was no detectable difference between the breeds in energetic efficiency. Prediction of the energy content of the body from tritiated water space measured in vivo gave similar results to carcass analysis.
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