Match birthweight ASBVs to flock fecundity for lamb survival

Selection of sires with high growth rates may unintentionally reduce lamb survival via dystocia due to the genetic relationships between high growth rates and birthweight. A range of Australian Sheep Breeding Values (ASBVs), including birthweight, lambing ease and gestation length, can be used as selection criteria to genetically increase lamb survival. However, their impact on lamb survival is likely to vary between birth types. Relationships between lambing ease scores, birth weights, gestation length and lamb survival of crossbred lambs born to Merino ewes from the MLA Resource Flock were quantified. Across all birth types, lamb survival was greatest for unassisted lambs; assisted lambs were of low incidence and above average birthweight. Increased lambing ease scores (i.e. more lambing difficulty) were associated with longer gestation length, higher birthweight and poorer lamb survival. Higher birthweight ASBVs were associated with increased lamb survival, but this was dependent on litter size and the lamb surviving parturition. Less fecund commercial flocks that experience dystocia related issues should place an upper limit on birthweight ASBVs and include lambing ease and gestation length ASBVs in their ram selection decisions. These flocks will also need to management ewe nutrition during late pregnancy, to ensure their single bearing ewes do not produce heavy lambs.