1. Studies were conducted to measure the influence of perinatal undernutrition of twin-bearing ewes on ewe milk yields and lamb performance in early lactation. Comparative studies were conducted with lambs to measure the effects of postnatal nutrition on lamb performance and carcass composition. The major objective of this work was to determine the extent of the influence of postnatal nutritional deprivation of lambs.
2. From 8 weeks prepartum to 4 weeks postpartum each of forty twin-bearing ewes was given daily 0.30 MJ of ME/kg body-weight (W)0.75. Measurements were made of ewe body-weight changes, milk yields during the first 4 weeks of lactation, and of lamb performance.
3. There were three postnatal experiments with lambs of ‘light’ and ‘normal’ birth weights, wcaned from the ewes at 72 h. One hundred and four lambs were used and they were individually reared on separate allowances of lamb milk-substitute for 8 weeks. They were subsequently given a lamb concentrate ration. In Expts 1 and 2 the lamb concentrate was fed ad lib. to ‘slaughter’ live weight. In Expt 3 the concentrate was restricted for 8 weeks post weaning and then fed ad lib. A carcass analysis (bone, muscle, fat content) was conducted on all lambs after slaughter. In the first experiment, half the lambs were slaughtered at 8 weeks of age on being weaned off the milk substitute, and a carcass analysis was carried out.
4. The mean net body-weight loss per ewe from commencement of late pregnancy to 28 d post partum was 23.8 kg. The over-all mean daily milk yield was 843 g/d. The mean growth rate of the lambs was 97 g/d during the 4-week measurement of ewe milk yields.
5. Live weight gain of lambs subjected to restricted milk-substitute allowance (125 g/dry matter (DM) per d) was low, irrespective of lamb birth weight. The average daily gain over the 8-week postnatal period was 86.5 g/d on restricted milk allowance compared with 244 g/d for those fed ad lib. Conversely, the food intake and growth rate of these lambs were almost identical during feeding on ad lib. concentrates. This clearly reflected a response to compensatory food intake which was independent of either birth weight or postnatal milk allowance. The average daily gain of lambs, before weaning, on the restricted milk allowance was very similar to that of the lambs being suckled by ewes subjected to perinatal undernutrition. The effects of postnatal undernutrition of lambs was particularly reflected in the time required to reach ‘slaughter’ live weight. The period of time varied from 117 d for those on ad lib. food intake to 169 d for lambs on restricted food intake.
6. The major effect of postnatal undernutrition on carcass composition was evident in the lambs slaughtered at 8 weeks in Expt 1. The percentage carcass fat was 4.7 for those on the low milk allowance. The carcass composition of all lambs at ‘slaughter’ live weight was similar. It would appear that lambs on a restricted milk intake before weaning deposited more fat during the subsequent period of compensatory food intake.
7. It is evident from the present results that postnatal growth retardation in lambs reflects either perinatal undernutrition of twin-bearing ewes, or postnatal undernutrition of the lambs due to inadequate milk availability from the dam.