Progesterone content in peripheral plasma collected from Romney and Border Leicester x Romney ewes at 20, 40, 60, and 120 days after mating was determined. The animals had heen injected with pregnant mares' serum gonadotrophin (PMSG) to induce superovulation lind the number of corpora lutea' determined a week after injection. In early pregnancy (days 20, 40, 60) progesterone concentration increased with advancement of gestation and with increase in the number 01 corpora lutea (from 1 to 6). Border-Romney ewes had lower progesterone levels than Romneys over this period. In late pregnancy (day 120), litter size and progesterone level were positively related but the difference between breeds was nonsignificant. There was a significant positive effect of the weight of lamb born per ewe on progesterone concentration, and after allowance for this Romney ewes were shown to have higher blood progesterone levels than the crossbreds. Diagnoses were made in early pregnancy of whether ewes had one or more than one ovulation (70-80% correct), or whether a singleor multiple-bearing pregnancy existed (60-70% correct). In late pregnancy diagnosis of exact litter size was 60% accurate, increasing to 71% when single or multiple births were assessed. It is concluded that diagnoses of litter size based on peripheral progesterone levels are unlikely to be of widespread practical value in sheep management.
MATERIALS AND METHODSBorder-Romney ewes and also on the applicability of using this hormone to diagnose the number of corpora lute a and offspring carried.
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, or 1200 Lu. 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.
Romney and Border-Romney maiden 2-year-old ewes were treated for 14 days with intravaginal progestagen sponges at the start of the breeding season. After sponge withdrawal and at the next cycle 80% and 90% respectively of all ewes were in oestrus, and of these 77% and 80% were marked over 2 and 3 days respectively. Pregnant mare's serum gonadotrophin (PMSG) (600, 1200 i.u.) injected on day 12 or 13 after first heat did not affect synchronisation of oestrus, although the mean onset of oestrus was earlier than in control ewes. Improved nutrition, treatment with PMSG, and injection on day 12 reduced mean cycle length.
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