Effects of hormonal manipulation on the resumption of postpartum reproductive activity in Texel ewes

Abstract

Three experiments were conducted on Texel ewes to study the influence of prostaglandin F 2α (PGF 2α), prolactin (PRL), estradiol (E 2), and gonadotrophin releasing hormone (GnRH) on postpartum reproductive activity. In Experiment 1, oral administration of indomethacin (25 to 50 mg/day/ewe) from Day 3 post partum to the first detected estrus inhibited plasma 13, 14-dihydro-15-keto, PGF 2α (PGFM) concentrations (P < 0.0001). This treatment resulted in an earlier rise in the frequency and amplitude of luteinizing hormone (LH) pulses and a resumption of estrous behavior (P < 0.05), while ovarian activity estimated by progesterone (P 4) concentrations resumed to the same extent in treated ewes and controls. Bromocriptine treatment (2.5 mg/day/ewe) reduced plasma PRL levels (P < 0.0001) but had no effect on ovarian activity as evidenced by P 4 and resumption of estrus or on either the frequency or amplitude of the LH pulse. In Experiment 2, a single injection of GnRH agonist (42 mcg of buserelin/ewe) on Day 16 post partum resulted in an abrupt elevation of plasma LH concentrations; mean LH values were 18 to 27 times higher when compared with those of the control ewes. Two days after this treatment, ovulations occurred in 5 of the treated ewes and in 2 of the control ewes. This induced ovarian activity was not associated with estrous behavior; however, after an adequate subsequent luteal phase all the treated ewes displayed estrus, the resumption of estrus thus being earlier in treated than in control ewes (P < 0.01). In Experiment 3, E 2 supplementation from Day 16 to Day 28 post partum increased the number of LH pulses per 6 hours in suckling ewes (P < 0.05) and induced earlier resumption of estrus in dry ewes but not in suckling ewes (P < 0.01). Luteal function was detected about 5 and 8 days after the insertion of E 2 implants in 4 dry ewes and in 2 suckling ewes, respectively.