Survival of twin-born lambs on commercial farms can be improved by reducing mob size at lambing

At least 20% of lambs born will die in the first 3 days of life under extensive grazing conditions in southern Australia. Limited research has suggested that increasing the mob size or stocking rate of ewes at lambing may increase the risk of ewe-lamb separations and lamb mortality. Higher lambing densities are likely to exacerbate the attraction of foreign periparturient ewes to amniotic fluids and newborn lambs which may result in greater interference at lambing. This thesis aimed to quantify the impacts of mob size and stocking rate on the survival of lambs born on commercial farms. Furthermore, it aimed to determine whether poorer survival of lambs born at higher mob sizes was associated with greater interaction with foreign ewes and lambs at lambing. Survey data collected from 66 farms in south-eastern Australia indicated the survival of single- and twin-born lambs decreased by 1.4% and 3.5% per additional 100 ewes in the mob at lambing. Furthermore, it suggested that increasing stocking rate by 1 ewe/ha would reduce lamb survival by 0.7%, regardless of breed and birth type. Similar survey data collected from producers in New Zealand found the survival of Merino lambs decreased by 1.3% per additional 100 ewes in the mob at lambing, however there was no effect of stocking rate on lamb survival. Data collected from 60 on-farm research sites across southern Australia confirmed that higher mob sizes result in poorer lamb survival. In this study, the survival of twin-born lambs decreased by 2% per additional 100 ewes in the mob at lambing. However, stocking rate was not found to influence lamb survival suggesting that the number of ewes in the mob regardless of paddock size has a greater influence on lambing density. Detailed experimentation also suggests that the relationship between mob size and lamb survival may be influenced by feed-on-offer (FOO) at lambing. In one experiment, a higher mob size decreased the survival of twin-born lambs when FOO was below 400 kg DM/ha at lambing and ewes were being supplementary fed. However, another experiment found the survival of single- and twin-born lambs was not influenced by mob size when FOO exceeded 2700 kg DM/ha at lambing. Mob size was not observed to influence interaction with foreign ewes or lambs at the time of lambing. However, less than 20% of ewes were observed at lambing and these observations were restricted to within approximately one hour of birth. Therefore, the reasons for poorer lamb survival at higher mob sizes remain unclear. Nevertheless, this research demonstrates that reducing mob size at lambing by 100 ewes will increase the survival of twin-born lambs of Merino and non-Merino breed by at least 2%. Integrating guidelines for producers related to mob size at lambing with existing ewe condition score targets will therefore aid in improving lamb survival on commercial farms.