The effects of plane of nutrition, genotype and sex on growth, body composition and wool production in grazing sheep

Abstract

Summary Growth rates, chemical composition of the body and wool production were measured with wethers and ewes from two genotypes of sheep grazing at two planes of nutrition. The nutritional treatments were applied after weaning at 12 kg live weight. Animals were slaughtered at 6 kg intervals between 12 and 54 kg live weight. Dorset Horn × (Border Leicester × Merino) lambs grew 57% faster than South Australian Merino lambs before weaning, produced 22% more wool and contained more fat at weaning. Post-weaning growth rates on the high and low planes of nutrition were 168 and 78g/day for Merinos and 183 and 116 g/day for Cross-breds. Despite these differences, there was virtually no effect on body composition. Sheep which were starved and re-grown also had a similar body composition to sheep grown continuously. There was, however, a nutritional effect on wool production. On the high plane of nutrition the rate of wool production was 22% higher than on the low plane of nutrition. After weaning there were only small differences in the growth rate of the two genotypes but the Merinos grew 30% more wool than Cross-breds on the high plane of nutrition and 39% more wool on the low plane of nutrition. Genotype differences in the fat and protein content of the empty body were small, but Merinos contained more ash, Ca, P and Mg. After weaning the normal pattern of allometric growth was reversed; at 18 kg live weight the sheep contained a lower proportion of fat and higher proportion of water than at 12 kg live weight. Above 18 kg live weight normal allometric growth was resumed. The fat-free empty body became progressively less hydrated but the ratio of protein: ash remained constant at 79:21 for Merinos and 81:19 for Cross-breds. There was a trend towards ewes being fatter than wethers, but this was only significant with Merinos. Other sex differences in body composition, body growth and wool growth were not significant.