Early weaning of grazing sheep. 2. Performance of ewes

Abstract

Two similar groups of Corriedale ewes were formed from a flock of 231. These were grazed on separated phalaris (Phalaris tuberosa) pasture at 8.6 or 17.3 ewes/ha, respectively, high and low nutritional planes, from 7 wk before their 1st lambing in spring 1968 until after their 2nd lambing. Within each group the 1968 lambs were weaned by removal to other pastures at 6 or 12 wk of age, or were left unweaned for 21 or 29 wk 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 7 wk of pregnancy of 0.22 .+-. 0.11 g/day. Pregnancy appeared to reduce by about 1/2 the expression of genotypic differences between sheep in wool producing ability, and reduced 12 mo. clean fleece weight by about 5%. 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 lactation appeared to reduce expression of genotypic differences in wool production. Clean fleece weights were reduced by about 5 and 10% by lactations of, respectively, 6 wk and 5 mo. 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. When there was a shortage of feed at and after lambing the performance of ewes would be improved by weaning their lambs at about 6 wk. Ewe liveweights were consistently greater after 6 wk weaning than after later weaning, a result confirmed in other experiments on weaning within fixed total areas of pasture.