The influence of pasture species and exogenous oestrogen treatment on the live-weight gain and carcass composition of lambs

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Abstract
The live-weight gain, carcass weight, and carcass composition of wether lambs grazing two different pasture species have been studied. The 56 lambs (by Dorset Down rams out of Border Leicester × Corriedale ewes) of approximately 8 weeks of age were randomised to four treatment groups. Two of the groups were grazed on pure perennial ryegrass and two on pure lucerne for 7 weeks before slaughter. Seven of the lambs in each of the treatment groups received 336 μg diethylstilboestrol by subcutaneous injection weekly. Carcass composition was determined by chemical analyses. The lambs grazed on lucerne grew at 0.37 kg/day, 76% faster than those on perennial ryegrass (0.21 kg/day). The hot carcasses of the lucerne-grazed group were 7.0 kg heavier than those of the perennial ryegrass group and differed in composition (+ 3.7% fat, -3.1% water, + 0.8% protein, no difference in ash). This difference in carcass composition is explained by the increase in carcass weight. Oestrogen treatment enhanced live-weight gain (+ 0.04 kg/day); did not affect hot carcass weight; reduced the fat content of the carcass by 2%; and increased the water content by a similar figure. These results are discussed in relation to previous work on pure pasture species and implantation of diethylstilboestrol, and brief reference is made to the relative rates of growth of the carcass components.

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