LIVEWEIGHT, FLEECE WEIGHT AND PROLIFICACY OF ROMNEY EWES CARRYING THE INVERDALE PROLIFICACY GENE (FECXI) LOCATED ON THE X-CHROMOSOME

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.