Reducing mob size increases the survival of twin-born Merino lambs when feed-on-offer from pasture is limited and ewes are supplementary fed during lambing

Improving the survival of twin-born lambs is a high priority for the Australian sheep industry. Higher mob sizes at lambing have been suggested to decrease the survival of twin-born lambs. However, our recent work indicated that this effect may not occur when feed-on-offer (FOO) exceeds 2400 kg DM/ha during the lambing period. Increasing mob size at lambing when FOO is limited may exacerbate the risk of mismothering due to the need for ewes to seek feed and thus result in poorer lamb survival. Therefore, this study tested the hypothesis that (i) a higher mob size at lambing will decrease lamb survival when FOO from pasture is limited and ewes are supplementary fed and (ii) that this is related to a greater rate of interaction with foreign ewes and lambs during the periparturient period. Twin-bearing Merino ewes (n = 795) were allocated into one of two treatments on day 140 of pregnancy; high (n = 210) or low (n = 55) mob size, with three replicates of each treatment. Feed-on-offer from live pasture at lambing was below 390 kg DM/ha and ewes were supplementary fed until lamb marking. Behaviour during the periparturient period was observed and dead lambs were collected for autopsy for 14 days during the peak of lambing. The survival of lambs to marking was recorded for each mob. Lamb survival was 6.2% higher at the low compared to the high mob size (P < 0.05). The proportion of ewes which interacted with a foreign ewe or foreign lamb at lambing did not differ significantly between mob sizes. Whilst the relationship between mob size, ewe-lamb behaviour and lamb survival remains unclear, this study has shown that the survival of twin-born Merino lambs is improved by lambing ewes in smaller mobs when FOO from live pasture is limited and ewes are supplementary fed.