Does inadequate luteal function limit the establishment of pregnancy in the early post-partum ewe?

In Exp. 1 the effect of lactation versus early weaning on luteal function was examined in seasonally anoestrous Finn Dorset ewes that were induced to ovulate at 21 (N = 14) or 35 (N = 14) days post partum by using a CIDR device and PMSG. Prolactin concentrations were significantly higher (P less than 0.001) in lactating compared with early weaned ewes throughout the study. The proportion of lactating ewes with inadequate luteal function (as assessed by daily progesterone concentrations) in the 21-day group was 0.43 (3 or 7) compared with 0.67 (4 of 6) for those weaned within 2 days after parturition. Corresponding values for the 35-day group were 0 (0 of 4) and 0.14 (1 of 7) respectively. There was no evidence of abnormal luteal function in standard ewes (N = 8) for which the interval from parturition was greater than 150 days. In Exp. 2 we examined whether pregnancy can be successfully established during the breeding season following transfer of embryos into lactating or early weaned ewes in the early post-partum period. Embryos were donated from Border Leicester x Scottish Blackface ewes for which the interval from previous parturition was greater than 150 days. These embryos were transferred synchronously on Day 5 after behavioural oestrus to recipient ewes with the same breeding history as the donors (standard ewes, N = 15) or to lactating or early weaned recipients that had been induced to ovulate on Day 21 (N = 16) or 35 (N = 24) post partum. In the 21-day group inadequate luteal function was observed in 2 of 7 (0.28) lactating and 4 of 9 (0.44) early weaned ewes compared with corresponding values of 1 of 13 (0.08) and 2 of 11 (0.18) in the 35-day post-partum group. Luteal function was normal in all standard ewes. The proportion of successful pregnancies in the standard ewes was 0.80 (12 of 15) compared with 0 in lactating and early weaned ewes in the 21-day group and 0.08 (1 of 13) and 0.36 (4 of 11) respectively in the 35-day group. The incidence of inadequate luteal function is therefore independent of the suckling stimulus and is higher in ewes induced to ovulate on Day 21 than Day 35 post partum during breeding and non-breeding seasons.(ABSTRACT TRUNCATED AT 250 WORDS)