A study of the growth of sheep to maturity

Abstract

Two groups of weaned sheep weighing about 30 kg, one born in March and the other in September, were each divided into two and given ad libitum one of two pelleted diets, ruminant diet A or ruminant diet AA6. The March-born sheep commenced experiment in July and those born in September in January. The experiment continued for 4£ years. At intervals sheep were killed and the fat protein and ash contents of their digesta-free bodies determined.The voluntary intake of feed showed a seasonal periodicity with minima in the winter and maxima in summer. The amplitude was 30 % in the 1st year and in subsequent years averaged 13%. Those given the higher quality diet (AA6) consumed slightly less than those given the poorer one.Mean daily feed intake averaged over 6-month periods from January to July and July to January was invariant with age during 4 years of observation. In this time the sheep increased in weight from about 30 kg to about 130 kg. There were, however, considerable differences between individual sheep in the amount of feed they habitually consumed.The body weight of the sheep increased and eventually plateaued. The asymptotic weight defined as A in the equation W = A -Be~k t , where W is weight at time t and B and k are constants, was related to the mean daily feed intake averaged over 6 months; mean daily feed intake measured over 6 months was proportional to a fractional power of body weight indistinguishable from 0-75, the interspecies power to which metabolism is proportional. Growth of wool during successive 6-month periods did not vary with age of animal but differed significantly between animals.Fasting metabolism of the sheep was 316 kJ/kg W 0 ' 76 for wethers and 336 kJ for rams.Analysis of the bodily composition data showed that over a range of digesta-free body weight from 46 to 130 kg it was not possible, on statistical grounds, to distinguish between linear relationships between body weight and its fat, protein, ash and water content, and allometric ones. The linear relationships had marginally smaller residual standard deviations and the regression coefficients show that the gain of the empty body in these sheep consisted of 68 % lipid, 8 % protein, 1 % ash and 24 % water. The lipid in the carcass accounted for 88 % of the total lipid gain and half the accretion of protein and ash was in the carcass. These results confirm those of Searle, Graham & O'Callaghan (1972) based on tritium dilution which showed that post-puberal growth in sheep is of constant composition.The results of the metabolic studies are shown to be consistent with the growth studies.Growth to maturity, as affected by different hypotheses related to the determinants of maintenance energy expenditure and the regulation of appetite, was examined algebraically. It is shown that mature weight is the rate of feed intake divided by the rate of maintenance metabolism per unit weight, and for defined feeding systems the rate constant for the approach to mature weight is the rate of maintenance metabolism divided by the feed equivalent ...

Genetic selection, sex and feeding treatment affect the whole-body chemical composition of sheep

“…When this was done, the four estimates varied little around a mean of 3.45; all were within 1% of this value. The value of 3.45 is in good agreement with other estimates from sheep and other species (Blaxter et al, 1982;Emmans, 1988;Emmans and Kyriazakis, 1995). Jenkins and Leymaster (1993) estimated their sheep to have 11.7 kg of protein and 38.6 kg of water in their pelt-free empty body at maturity (their Table 4), giving a mature water : protein ratio of 3.30.…”

“…The extent to which fatness was increased on the lower protein diets decreased as the animals grew, presumably because their protein requirement was falling. Although we found that fatness was reduced by a lower level of feeding in only one case -the males at SP5 -there is evidence that feed restriction can reduce fat content at a given weight in sheep (Blaxter et al, 1982) as has often been observed in swine (e.g. ARC, 1981).…”

“…These are that fatness increases, the water content of the fat-free body decreases and the protein content of the lipid-free dry matter changes little. These effects are well illustrated for sheep by the data of Blaxter et al (1982) and Jenkins and Leymaster (1993). Emmans (1988) suggested how these changes could be qualitatively and quantitatively described.…”

Consequences of genetic change in farm animals on food intake and feeding behaviour

“…The proposal that maintenance is directly proportional to protein weight (Russel & Wright, 1983;Emmans & Fisher, 1986) is attractive. In at least one case, that of normally-growing sheep (Blaxter et al 1982;Emmans, 1987), this proposal is equivalent to the assumption that maintenance is scaled to live weight 0·73 , the same rule that applies between genotypes at maturity. However, this similarity must be seen as a fortuitous outcome of the particular fattening characteristics of sheep that make protein weight proportional to live weight 0·73 during growth.…”

Predicting the effects of body fatness on food intake and performance of sheep

“…Parameter C was, therefore, set at a value of 0·008 because this resulted in a realistic description of observed lean weights for the pellet-fed sheep. To check whether this was unique for the present data set, we fitted a Gompertz curve to the long-term data presented by Blaxter et al (1982) and Hou (1991). In these long-term experiments, sheep had access ad libitum to high-quality pelleted foods that allowed an accurate estimate of mature size of the sheep.…”