The birthweight and survival of Merino lambs can be predicted from the profile of liveweight change of their mothers during pregnancy

The single largest influence on the survival of lambs in thefirst few days of life is their birthweight. Fetal growth and birthweight are regulated by genotype of the fetus, maternal genotype, maternal nutrition and the external environment. In this paper we report the extent to which the changes in maternal liveweight during pregnancy and lactation (liveweight profile) ofMerino ewes can be used to predict the birthweight and survival of their progeny toweaning.At two sites [Victoria (Vic.) ~700 ewes andWesternAustralia (WA) ~300 ewes] in each of 2 years, a similar experiment used adultMerino ewes to explore effects of nutrition from joining to Day 100 of pregnancy and from Day 100 of pregnancy to weaning. The average difference between extreme treatments atDay100of pregnancywere 7kg in ewe liveweight and0.7 of a condition score (CS) and at lambing 11.9 kg and 1.3 of a CS. This resulted in average birthweights of progeny from different treatments ranging from 4.0 to 5.4 kg and survival to weaning ranging from 68 to 92%. Across the four experiments between 68 and 85% of all lamb deaths to weaning occurred within 48 h of birth. Lambs born to ewes in CS 2 at Day 100 of pregnancy were lighter (P<0.05) inbothyears at theVic. site than those fromewes inCS3atDay100ofpregnancy.Lambsborn to theewesgrazing a feed on offer of 800 kg DM/ha during late pregnancy were also lighter than those from other levels of feed on offer between 1100 and 3000 kgDM/ha at theVic. site in both years and at theWAsite in 1 year (P< 0.001). Lambs from the 800 kgDM/ha treatment during late pregnancy at theVic. site had a lower survival thanother treatments, especially in the secondyear. There were no significant effects of treatments on lamb survival at theWAsite; however, the results were in the same direction. The birthweight of individual lambs was significantly related to the liveweight profile of their mothers. Their liveweight at joining, change in liveweight to Day 100 of pregnancy and change in liveweight from Day 100 to lambing all contributed (P < 0.05) to the prediction of the birthweight of their lambs. The responses were consistent across experimental sites and years, lamb birth rank and sex, and confirmed that the effects of poor nutrition up until Day 100 of pregnancy could be completely overcome by improving nutrition during late pregnancy. At theVic. site, survival to 48 hwasmost influenced by the birthweight of the lamb and survival was significantly higher in singlethan twin-born lambs and female thanmale lambs after adjusting for differences in birthweight. A higher chill index during the 48 h after birth reduced survival of both single and twin lambs to a similar extent, but reduced survival of male lambs more than female lambs. There were no effects of birthweight or chill index on lamb survival at the WA site where most lambs weighed more than 4 kg at birth and climatic conditions during lambing were less extreme. Overall, these results supported our hypothesis that improving the nutrition of Merino ewes during pregnancy increases birthweight and this leads to improved survival of their progeny.