Effects of progesterone treatment on ovarian and estrous activity and on LH pulsatility and PGF 2α concentration in suckling and non-suckling Texel ewes

Abstract

Two experiments investigated the effects of low amounts of exogenous progesterone (P 4) on the resumption of estrous and ovarian activity and on luteinizing hormone (LH) pulsatility and prostaglandin F 2α (measured as 13,14-dihydro-15-keto-PGF 2α, PGFM) concentrations in postpartum Texel ewes early after lambing. In Expt. 1, six single-suckling ewes received intravaginal devices containing 210 mg of native P 4, from d 4 ± 2 to d 16 ± 2 after parturition during the breeding season, and six others served as controls. The P 4 supplementation induced earlier return to estrus in single-suckling ewes (23 ± 7 vs. 37 ± 3 d, P < 0.01) and was not associated with subsequent inadequate luteal function. Duration of the first luteal function was shorter in three of the six control ewes. LH pulse frequency (per 6 h) was decreased during the period of P 4 supplementation and increased 1 d after removal of P 4 devices ( P < 0.01). Concentrations of PGFM were lower in P 4-treated ewes than in the control group ( P < 0.01). In Expt. 2, ewes either suckled one lamb or weaned soon after parturition and received intravaginal devices containing 210 mg of native P 4 from d 16 ± 3 to d 28 ± 3 post partum. The incidence of estrous behavior after P 4 supplementation was lower in suckling than in dry ewes ( P < 0.01), while ovarian activity resumed to the same extent in both groups and was not associated with inadequate luteal function as in Expt. 1. The P 4 treatment raised circulating P 4 to higher levels in weaned than suckling ewes ( P < 0.01). In both categories, LH pulse frequency was suppressed during the P 4 treatment and increased 1 d after the P 4 withdrawal ( P < 0.01). Weaning reduced significantly plasma PGFM concentrations ( P < 0.01). These results indicate that the positive effect of P 4 on the re-establishment of hypothalamo-pituitary functions prevents the premature regression of the CL after lambing. They strongly suggest that P 4 treatment reduces the interval between parturition and return to estrus and is more effective in dry than in suckling ewes.