Prolificacy and lamb survival at birth in Awassi and Assaf sheep carrying the FecB (Booroola) mutation

Prolificacy up to the fifth parity and lamb survival at birth were investigated in ≥31/32 Awassi and ≥31/32 Assaf sheep belonging to the ++, B+ and BB genotypes at the FecB locus. In the Awassi, prolificacy of ++, B+ and BB ewes was 1.28, 1.90 and 1.92 lambs born/lambing (LB/L), respectively. In the Assaf, prolificacy of ++, B+ and BB ewes was 1.68, 2.40 and 2.55 LB/L, respectively. Lamb survival at birth in the ++ Awassi and the ++ Assaf averaged 0.98 and 0.94, respectively. It declined to 0.93 and 0.86, and 0.85 and 0.78 in the B+ and BB Awassi and B+ and BB Assaf, respectively. For singles, twins, triplets, quadruplets and quintuplets, lamb survival rate at birth was 0.98, 0.92, 0.86, 0.78 and 0.65, respectively. FecB genotype-litter size interactions were not significant (P < 0.05). A compilation of study results in which prolificacy of the ++ and B+ genotypes at the FecB locus were investigated in a range of breed–environment combinations revealed that the B allele has a multiplicative effect on prolificacy as B+ prolificacy was significantly (P < 0.05) linearly associated with ++ prolificacy.