Tw Searle

Growth in sheep. I. The chemical composition of the body

Fifteen 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.

Change of skeletal dimensions during growth in sheep: the effect of nutrition

Measurements of skeletal size were made at 2–3-month intervals on 30 Border Leicester × 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.

Longitudinal studies of body composition during growth in male, female and castrate male sheep of two breeds with different wool growing capabilities

Dorset Horn and Corriedale ewes (breeds with low and high wool growing capabilities respectively) with single lambs were fed ad libitum on a pelleted roughage-concentrate diet. Following weaning at 6–7 weeks of age (live weight (LW) ca. 15 kg), ten entire male, ten female and ten castrate male lambs of each breed were housed in single pens and fed the same diet ad libitum . Body composition (fat, protein, water, ash and energy) was estimated from tritiated water space at 5 kg LW increments from 10 to 55 kg. Fleece weight was also estimated on each occasion. The relationship between the weight of body components and fleece-free fasted live weight ( W ) for each sheep was described by a two-phase piecewise linear model, the slope in each phase representing the average composition of weight gain. In the first phase, the composition of weight gain did not differ between breeds and contained similar amounts of fat and protein (14%), 66% water and 5% ash. Within breeds females tended to be fattest. There were significant differences due to breed and sex in all body components in the second (fattening) phase. The fat content of gain was lower in Dorset Horn ewes and wethers (45 and 51% respectively) than in corresponding Corriedales (64 and 58%) with the males being even lower (44 and 41% respectively). Dorset Horns entered the fattening phase at a significantly higher W (26 kg) than Corriedales (23 kg), there being no sex difference within breeds. The combined effect of lower slope and higher weight at the commencement of the phase change in the Dorset Horns was to produce a considerable breed difference in fatness in the second phase of growth in ewes and wethers. Possible physiological reasons for the differences are discussed.

Physiological state of sheep before and during fattening

Various physiological characteristics of castrate male sheep fed ad libitum were studied between 1986 and 1987 in Australia. The sheep were between 20 and 45 kg live weight (LW) before and during fattening. Sequential data on body composition, estimated from TOH space (starting at 10 kg), established that the weight gain contained 19% fat and 13% protein below c. 26 kg LW and 55% fat and 10% protein above. For a given feed intake, the rate of fat gain was constant but the rate of body protein gain was 45% lower above 26 kg LW