Liveweight, fleece weight and prolificacy of Romney ewes carrying the Inverdale prolificacy gene (FecX I) located on the X-chromosome

Abstract

The effect of the Inverdale prolificacy gene (FecX I) on liveweight, fleece weight and reproductive performance was measured in the 2-year-old progeny of three carrier (I) Romney rams and eight non-carrier (+) Romney rams. There was no effect of FecX I on liveweight at first mating (19 months of age) or fleece weight at 13 months of age. Records from 82 progeny of FecX I I rams and 221 progeny of + rams showed that one copy of the gene increased litter size by 0.58 ( P<0.001). Mean litter size of carrier ewes was 2.17, and 32% of litters had three or more lambs. FecX I had no significant pleitropic effects on barrenness, embryonic mortality or lamb mortality, although embryonic mortality and lamb mortality were both affected as a consequence of the effect of FecX I on ovulation rate. Embryonic mortality was 13% ( P<0.001) higher in carrier ewes but at a common ovulation rate (ovulation rate= 2) there was no significant difference. Neonatal lamb mortality was 14% higher ( P<0.001) from ewes carrying FecX I but when only singles and twins were compared there was no significant effect. The net result was that lambs alive at 1 day of age per ewe lambing averaged 1.36 for carrier ewes and 1.19 for non-carriers.