Expression of reproductive and production traits in commercial merino ewes having 0, 1 or 2 copies of the 'FecB' mutation

Analysis of 3 years of performance data from a commercial flock of Merino ewes having 0, 1 or 2 copies of Booroola fecundity (FecB) mutation determined by direct DNA test showed that the mutation does not have deleterious effects on growth and wool production beyond those associated with a higher incidence of multiple births. Consistent with earlier studies, the effect of FecB on ovulation rate was shown to be approximately additive with a dominant effect on litter size. There was a non-significant decrease rather than increase in litter size with the second copy. Homozygous BB ewes also had lower fertility rates and increased lamb losses between scanning and lamb marking relative to heterozygote and wild-type ewes, independent of litter size. This represents a significant limitation on the utility of the gene for increasing reproductive rate in Australian Merinos.