Genetic studies of prolificacy in New Zealand sheep

Data on ovulation rate, embryo survival and litter size have been analysed from the 1988-96 records collected at AgResearch's Woodlands Research Station on 2180 elite (highly prolific) ewes of three breeds: Romney, Coopworth and Perendale. The Romneys included a subgroup found to carry the sex-linked Inverdale high prolificacy gene. Foundation animals for all breeds were screened on high litter size (lambs born, alive or dead, per ewe lambing) from industry flocks. Prior to being transferred to Woodlands between 1979 and 1984, their average litter size was 2.66. Ovulation rates for descendants of the foundation ewes averaged 2.15 (Romney), 2.43 (Coopworth), 2.15 (Perendale) and 2.96 for the Romney subgroup carrying the Inverdale gene. Embryo survival averaged 0.84 except for a low value in Inverdales (0.76). Using a restricted maximum likelihood model, with full relationship matrix, the within-breed heritability of ovulation rate over all ages and breeds (excluding Inverdale) was 0.14 (s.e. 0.03), with repeatability 0.26 (s.e. 0.02). Litter size averaged 1.80 (Romney), 2.05 (Coopworth), 1.76 (Perendale) and 2.l7 (Inverdale). The heritability estimate for litter size (excluding Inverdale) was 0.073 (s.e. 0.018), from data for all ages. The genetic correlation between ovulation rate and litter size teas 0.91 (s.e. 0.08). The standard deviation for ovulation rate in the Inverdales was 1.75 times as great as the average of the other three breeds. Embryo survival to term for ewes conceiving to two, three or four ovulations averaged 0.89, 0.77 and 0.64, respectively, and was significantly different among breeds (P < 0.001). The greater ovulation rate in Inverdales than Romneys was sufficient to explain the Inverdale effect on reduced embryo survival. The highest embryo survival was observed in the Coopworth breed (P < 0.001).