SUMMARYTritiated water (TOH) space was determined in sixty-one sheep of known age (3 days to 18 months). Representative samples of the minced carcass were subsequently analysed to determine body composition (water, fat, protein, ash and energy). The regressions of the weight of the various body components on TOH space and body weight reported here gave equations suitable for prediction. These were similar to equations derived from published results for adults. Combining data on young sheep and adults gave broadly based equations that could be used to predict the body composition of healthy sheep of all ages from 3 days to adults.

SUMMARYFifteen sheep were fed ad libitum from 2 days to 27 months of age, and another 15 sheep were each fed exactly half the average amount consumed by the first group, age for age. The body composition of each sheep (water, fat, protein, energy) was estimated from tritiated water space on 13 occasions during this period.To describe the course of growth in individual sheep in terms of the relationships between the various body components and body weight, a model was set up in which 4 phases of growth were distinguished, viz. the milk-feeding phase, the period of rumen development, and a prefattening followed by a fattening ruminant phase. Each phase was represented by a linear equation.Except for phase 1, mean composition within each phase differed significantly between well-fed animals and those which had been given a restricted diet. Individual animals differed in the body weight at which the final phase commenced; the average weight was ca. 31 kg. Fat storage was zero or negative during the main period of rumen development; otherwise the fat and therefore energy content of weight gain increased from phase to phase. The protein and water content of gain was high in phases 1 and 2 and decreased subsequently.Calculations based on data in the literature indicated that, in phase 4, the composition of weight loss was the same as that of weight gain. It is also suggested that the body weight at which this fattening phase commences is related to mature weight, with animals of large ultimate size starting to fatten at heavier body weights than those of small ultimate size.The application of the results to the determination of nutrient requirements is discussed.

Basal metabolic rate (BMR) was determined in 56 crossbred sheep, up to 10 observations being made on each animal between 1 week of age and 2$ years. The level of feeding was varied amongst the sheep so that there was a wide range in growth rate at each age. BMR was estimated as heat production under standard conditions of fasting. Trends during fasting were studied in four sheep at ages 3 weeks, 2 months and 9 months.The effects on BMR of body weight (or fat-free weight), age, prior growth rate and prior nutrition were examined statistically by estimating the parameters of a series of model equations by a least squares iterative method. Analysis of lamb and sheep data separately and combined showed that all these variates contributed significantly to BMR.Of the variance of BMR, 89 % was accounted for in a body weight term, kgX, in which the value of x was not significantly different from if one or more of the other variates were in the model;x was unity when fat-free weight was used instead of body weight. If body weight was used alone, x was smaller for both lambs and weaners, being c. 0.60; with fat-free weight the values for lambs and weaners were 0.71 and 0.96 respectively. Age, growth rate and level of feeding were of approximately equal importance, together accounting for a further 6% of the total variance. BMR declined by c. 8 % per annum and was affected to the extent of 2.8 kJ per gram body weight gain and 46 kJ per MJ digestible energy intake before fasting (all values per 24 hr). Thus an increase in growth rate in a lamb from zero to maximal (0.3 kglday) caused BMR to increase by 50%, and an increase of food intake by 1 kg/day in an averLge adult sheep caused BMR to increase by 10%. For any given set of these variates, BMR was 23 % higher in milk-fed lambs than in weaned sheep.An equation was derived for sheep in general; the residual standard deviation was c. 300 kJ/24 hr, or 7-8 % of BMR in an average adult sheep. Some evidence was cited to show that this equation may be used to predict BMR in growing and adult cattle by multiplying the whole expression by 1.3.

S U M M A R YMeasurements of skeletal size were made at 2-3-month intervals on 30 Border Leicester x Merino castrate male (wether) sheep between 2 and 27 months of age. Fifteen sheep were fed ad libitum on a high-quality diet and the other 15 half the average amount consumed by the first group, age for age. The ad libitum group grew faster and were larger in all body dimensions on each occasion, except for leg length at 27 months which showed no statistical difference between groups. When the groups were compared over the live-weight range common to both (16-44 kg) the unrationed animals were consistently wider at the shoulders but smaller in leg and chest dimensions.The relationship between each body component and age is described by a Mitscherlich equation and the relationship with live weight by a linear equation in which both variables are log transformed. Separate relationships were determined for each sheep and tested for differences within and between groups.

Wether sheep (4 months old) were held at 20 kg liveweight for either 4 or 6 months by restricting their food intake, and then fed ad libitum to recover weight for age. Voluntary food intake during recovery was recorded for several months in 20 penned animals. Basal metabolic rate was estimated in eight other sheep at intervals during weight stasis and once after a month of recovery. Complete balances of energy, carbon and nitrogen were measured continuously in the latter sheep during the first month of recovery and in immediately preceding and succeeding short periods at 'maintenance'. Control data were derived from an earlier experiment. Voluntary food intake was greater during rehabilitation than in controls of the same weight. Basal metabolic rate per kg¾ tended to fall during the period of weight stasis. It rose in the first month of recovery but by a smaller amount than predicted for controls growing at the same rate. Net energetic efficiency was probably higher than normal, and maintenance requirement lower, in the first week of recovery, but otherwise was the same as for the controls. Hence, apart from this first week, gross efficiency was unusually high solely because voluntary food intake was high relative to maintenance requirement. Nitrogen utilization was also more efficient in the first week or two of compensation than predicted for control sheep of the particular body weights and energy intakes.