1. In an experiment with ninety-nine lambs the effects on the body composition of male and female lambs were examined when five diets containing different concentrations of crude protein (in the range 10-20 %) were given at three levels of feeding and lambs were slaughtered at two live weights (27-5 and 40 kg).2. With lambs slaughtered at 27-5 kg there were significant increases in the rate of both nitrogen and fat retention with increases in levels of feeding. There were also linear increases in the rate of protein deposition and decreases in fat deposition with increases in the concentration of crude protein. This effect was particularly marked at the high level of feeding.3. With lambs slaughtered at 40 kg live weight there were also linear increases in fat and in nitrogen deposition with increasing feeding level but the effect of increasing the protein concentration on increases in nitrogen retention departed from linearity.4. While at 27-5 kg there were no significant effects of feeding level on nitrogen and ether-extract content of the bodies at slaughter, with animals slaughtered at 40 kg there was a significant linear decrease in ether-extract content with increasing feeding level and a corresponding linear increase in nitrogen content with increased level of feeding.5. Male lambs deposited more nitrogen and less fat than females. This was true of both rate of deposition and of carcass composition at 40 kg live weight.

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SUMMARYThe live-weight gains of male and female lambs were measured during growth from 16 to 40 kg live weight when five cereal-based diets varying in dietary crude protein concentration (from 10 to 20%) were given at three levels in a 5×3×2 factorial design.Males grew faster than females by about 15% (P < 0·001); this difference increased with age and with the amount of feed given (P < 0·05). Growth rate responded linearly to increase in feeding level (P < 0·001) and curvilinearly to increase in dietary protein concentration (P < 0·01). A significant interaction (P < 0·01) occurred whereby growth increased with higher protein concentrations as feeding level increased.At the highest feeding level (near ad libitum) the results suggest that the optimum dietary crude protein concentration for growth was about 17·5, 15·0, 12·5 and 12·5% at body weights of 20, 25, 30 and 35 kg respectively. The overall optimum dietary crude protein concentration for growth between 16 and 40 kg body weight was about 17·0, 15·0 and 11·0% when the mean digestible energy intake was 3·0, 2·6 and 2·1 Mcal/day.The results agree reasonably well with estimates of protein requirements for lambs given by the Agricultural Research Council (1965).

In a comparative slaughter trial which covered growth from about 16 kg to 40 kg live weight, five groups of six Romney x Swaledale lambs were offered ad libitum five pelleted diets (A to E) in which rolled barley was gradually replaced by an 80 : 20 mixture of rolled oats : oat husks to the extent that the calculated metabolizable energy (ME) concentration fell from approximately 2-9 (A) to 2-5 (E) Mcal/kg DM. Although daily live-weight gains did not differ significantly between treatments, the daily carcass gain fell from 143 g/day on A to 105 g/day on E (P<0-05) whereas gut contents increased from 9-9% of live weight to 16-1% (P< 0-001) from A to E. Differences in efficiency of utilization of ME above maintenance were variable but for gross efficiency (i.e. retention/total ME intake), there was a significant decline from A to E. Dry-matter intakes for the whole experimental period were similar for all five diets, but initially intake was lower for diet E than for A (780 v. 927 g/day) and ultimately it was higher for E (1410 v. 1150 g/day). In a second experiment, involving diets A and E and a third diet of chopped dried grass, the pattern of intake and growth changed with live weight in a similar manner to that described above, except that appetite and growth on dried grass were consistently poorer. Daily variation in intake and rumen pH tended to be greater on the concentrate diets. It is suggested that, amongst other factors, the stage of maturity of an animal may influence the point at which the digestible energy content of the diet restricts dietary intake in preference to physical limitations.

Two successive grazing experiments were conducted over 12 weeks on perennial ryegrass pastures with 50 and 44 young cattle to study the effect of N fertilizer when applied at a daily rate of 1 or 3 kg N/ha. At each level of N two stocking rates differing hy 20% were imposed. At the liigher N level and stocking rate, three frequencies of grazing were imposed.At the stocking rates imposed N tended to reduce the daily liveweight gain per head, hut increased the total liveweight gain per ha hy from 0-79 to 1-58 kg/kg N. A 20% increase in stocking rate depressed gain per head in hoth years. In the first year it did not improve gain/ha, hut in the second year gain/ha was increased hy 12-17%.There was a tendency for performance per animal and per ha to increase as the grazing cycle was lengthened. In 1969 the highest yield in the whole season was 1880 grazing days and 1260 kg gain/ha. The overall response to N fertilizer was similar to that recorded in other reports, hut it is possihle that a lack of K had limited pasture growth.