Reproductive differences in two‐year‐old Romney and Border Leicester ×Romney ewes after gonadotrophin treatment

Abstract Two hundred and seven Romney and first‐cross Border‐Romney 2‐year‐old ewes were glazed for 2 months at two planes of nutrition and treated with 0,600,or1200 i.u. pregnant mares' serum gonadotrophin (PMSG).Ewes were mated to Southdown rams after treatment and for a further two succeeding oestrous cycles.Each ewe was laparotomised within a week of injection and the number of fresh ovulations recorded. The number of lambs born was noted. High‐plane ewes gained 1.39 kg and low‐plane ewes lost 1.51 kg over the 2‐month period.The crossbred ewes had higher natural ovulation rates,lambing performances,and responses to PMSG.PMSG reduced (16%) the conception rate at first service,proportionately more ewes returning to service at prolonged intervals (> 20 days),and this “carryover” effect reduced the incidence of ewes that subsequently became pregnant.The proportion of “potential lambs ” born was related to ovulation rate through dose of PMSG and breed.Litter size increased with up to five ovulations per ewe despite an increasing percentage of potential lambs lost.Nutrition had no effect on natural ovulation rates or response to PMSG.It is concluded that lambing performance in these breeds is limited by natural ovulation rate,as a higher uterine capacity exists in both breeds.