Abstract:
Two similar groups of Corriedale ewes were formed from a flock of 231. These were grazed on separate phalaris (Phalaris tuberosa) pasture at 8.6 or 17.3 ewes ha-1, respectively high and low nutritional planes, from seven weeks before their first lambing in spring 1968 until after their second lambing. Within each group the 1968 lambs were weaned by removal to other pastures at six or 12 weeks of age, or were left unweaned for 21 or 29 weeks in the high and low planes respectively. Birth weights in 1968 did not differ between planes, and for all ewes an increase of 1 kg in lamb birthweight was associated with a reduction in clean wool production during the last seven weeks of pregnancy of 0.22 �_ 0.11 g d-1. Pregnancy appeared to reduce by about one half the expression of genotypic differences between sheep in wool producing ability, and reduced 12 months clean fleece weight by about 5 per cent. Low plane ewes produced significantly less milk containing less solids. Liveweight gains and rates of wool growth were reduced by lactation and increased after weaning, wool growth the more slowly. Both rates of recovery were less in the low plane where, unlike the high plane, lactation appeared to reduce expression of genotypic differences in wool production. Clean fleece weights were reduced by about 5 and 10 per cent by lactations of respectively six weeks and five months duration. At subsequent joining a smaller proportion of the low compared with high plane ewes was marked by the ram and of these a smaller proportion produced lambs. Within the low plane, the longer the preceding lactation the lower the liveweight at second joining and lambing, and the lower the mating and lambing percentages. The results indicated that when there was a shortage of feed at and after lambing the performance of ewes would be improved by weaning their lambs at about six weeks. Ewe liveweights were consistently greater after six week weaning than after later weaning, a result confirmed in other experiments on weaning within fixed total areas of pasture.