Gootwine, E., Institute of Animal Science, A.R.O., The Volcani Center, P.O. Box 6, Bet Dagan, 50250, Israel
Reicher, S., Institute of Animal Science, A.R.O., The Volcani Center, P.O. Box 6, Bet Dagan, 50250, Israel
Rozov, A., Institute of Animal Science, A.R.O., The Volcani Center, P.O. Box 6, Bet Dagan, 50250, Israel

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. © 2007 Elsevier B.V. All rights reserved.