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.