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The intake of pasture by nine groups of 40 Border Leicester x Merino ewes was controlled to achieve three levels of liveweight at mating and three levels at the end of the 15th week of pregnancy. Over the last 6 weeks of gestation all groups gained about 7 kg per head, and after lambing the intake of pasture was unrestricted. Ewes that weighed 58 kg at mating produced 152 lambs per 100 ewes compared with a lambing percentage of 107 by ewes that weighed 44 kg. However, when the lambs reached 10 weeks of age there was little difference in their numbers. This was partly because of the higher death rate of young lambs when the liveweight of the ewe fell during mid-pregnancy, and partly because of the loss of single lambs following dystocia in ewes maintained at a high weight throughout pregnancy. The percentage of lambs reaching marketable weight (32 kg) before the end of the pasture season was related directly to the liveweight gain of the ewe during mid-pregnancy, and inversely to the proportion of lambs reared as twins. The weight of lamb marketed per unit weight of food eaten by the ewes over the autumn and winter was much greater from the ewes that increased from 51 to 58 kg during mid-pregnancy than from those that followed the reverse pattern. In general, a fall in ewe weight over this period led to inefficient use of food and a reduction in the weight and soundness of the fleece. These results suggest that in areas that are marginal for prime lamb production because of a short pasture season, it may be better to ensure a rising plane of nutrition throughout pregnancy than to aim for a high weight at mating.

Fodder oats (Avena sativa L, cv. Cooba) and Sudan grass (Sorghum sudanense cv. Greenleaf) were grown in pots in glasshouses with day/night temperatures of 27/22 and 21/16C. Plants were harvested at the emergence of each leaf and at various stages after anthesis. Growth rates and changes in dry matter content, nitrogen, cellulose and in vitvo digestibility in oats were similar under both temperature regimes, but more primary leaf and less panicle were formed at the high temperatures. Development of Sudan grass was more rapid at the high temperatures, but production of main shoot, panicle and tillers was much less and the nitrogen contents and in vitvo digestibilities were consistently lower than at the low temperatures. Sudan grass produced much more dry matter than oats. Differences in composition were in general not large, though stem digestibility remained higher in Sudan grass after anthesis. Tropical species, because of their efficiency of photosynthesis and water use, warrant further study of their nutritive value as forage crops in temperate regions.

New and published data on the chemical composition of 149 Merino and Merino crossbred sheep covering a wide range of age were used to derive regressions of body water, protein, ash and fat on parameters measured in the live animal. Allometric models were tested with fasted liveweight (W), tritiated water space (T) and maturity (M) as predictors, maturity being the age of the sheep as a proportion of the age at which mature size is reached. For all body components the inclusion of maturity in the following equations substantially reduced the residual standard deviation (RSD), compared with allometric or multiple linear regression models that included only Wand T as predictors. In addition it largely eliminated bias from the predicted values for the six separate groups of sheep from which the data were obtained. Total body water (kg) - - 1.179T0.926M0.031 (RSD = 0.354) Protein in empty body (kg) = 0.408 W0.708M0.092 (RSD = 0.264) Ash in empty body (kg) = 0.305 W0.460M0.290 (RSD = 0.100) Fat in empty body (kg) = 0.117(W - T)1.592M-0.225 (RSD = 0.444) It is suggested that in future work a measure of skeletal size in the live animal may increase the precision of these equations.

Mature Merino wethers were changed in liveweight by grazing treatments or by feeding in pens and their intake of food over the next 5 weeks was then measured at two levels of pasture availability or in pens. Of nine groups of six sheep with mean initial fleece-free liveweight of 33 kg, three groups were increased to 37 kg (389 MJ energy) in 6 weeks by grazing abundant pasture, another three were reduced to 28 kg (220 MJ) on sparse pasture, and those remaining were reduced to 30 kg (268 MJ) by restricted feeding in pens. The three groups from each treatment were then moved to abundant pasture, to sparse pasture or to pens where food was offered ad lib. Energy status, within the range imposed here, had no effect on the intake of pasture or on the time spent grazing; both of these variables depended solely on the herbage currently available. Nor did energy status affect food intake when sheep were removed from pasture and compared in pens with a standard food. Sheep moved from sparse pasture to abundant pasture gained at least as much liveweight, but only 20% as much energy as sheep in better condition because more of their gain was water. These results suggest that short-term changes in the body condition of mature grazing sheep will have no effect on their intake of food and that their subsequent changes in fasted liveweight may give a misleading impression of changes in energy status.