THE EFFECT OF THE INTRODUCTION OF THE THOKA GENE FOR FECUNDITY ON LAMB PRODUCTION FROM CHEVIOT EWES

A flock of Cheviot ewes carrying the Thoka gene for fecundity was esta blished in southern Scotland through the importation of frozen semen f rom two Icelandic yams. Ewes considered to be probable carriers of the Thoka gene were identified on the basis of measurements of ovulation rate as juveniles at 1.5 years of age and again as adults. Analysis of 3 years' records of 8926 Cheviot lambings and lambings of 351 Iceland ic X Cheviot ewes considered to be probable carriers showed that fecun dity increased with age in both genotypes, the number of lambs bon? pe r ewe mated being 1.07, 1.19, 1.31 and 1.41 in the Cheviots and 1.55, 1.74, 2.33 and 2.30 in the Icelandic-Cheviot crosses at first to fourt h lambings respectively. The apparent effect of tile crossbreeding on fecundity thus also increased with age, values at first to fourth lamb ings being 0.48, 0.55, 1.02 and 0.89 lambs per ewe mated, giving a wei ghted mean value of 0.61 (1.20 v. 1.81 lambs per ewe mated in Cheviot and Icelandic X Cheviot ewes respectively). There was a positive effec t of the proportion of Icelandic ancestry on the number of lambs bent per ewe mated and thus the observed increases in fecundity, although a ttributed principally to the effects of tile Thoka gene, are also due in part to other generic factors introduced from the Icelandic sheep i n the crossbreeding programme. Lamb mortality to 6 weeks of age was on ly marginally higher in Icelandic X Cheviots (0.10) than in Cheviots ( 0.08). The increase in fecundity was achieved without al I excessive p roportion of large litters (<0.02 of probable carriers gave birth to q uadruplets). The results demonstrate that the introduction of the Thok a gene to breeds such as the Cheviot can increase fecundity and hence the potential for more efficient lamb production.