Progesterone content in peripheral plasma collected from Romney and Border Leicester #xD7; 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#x2013;80 correct), or whether a singleor multiple-bearing pregnancy existed (60#x2013;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.
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