Exogenous PGF 2α enhanced GnRH-induced LH release in postpartum cows

Abstract

This study evaluated the effect of exogenous PGF 2α on circulating LH concentrations in postpartum multiparous (n = 32) and primiparous (n = 46) Brahman cows. The cows were randomly allotted within parity and calving date to receive 0, 1, 2 or 3 mg im PGF 2α (alfaprostol)/100 kg body weight (BW), with or without GnRH on Day 30 after calving. Blood samples were collected at weekly intervals from calving through treatment. Serum progesterone concentrations were determined using RIA procedures to assure that only anestrous cows were treated. Sterile marker bulls were maintained with cows on Coastal bermudagrass pastures until the first estrus was detected. Multiparous cows had a shorter (P < 0.05) interval from calving to estrus than did primiparous cows. Serum LH was affected by time (P < 0.0001), PGF 2α dose (P < 0.0002), GnRH (P < 0.0001), parity by PGF 2α dose (P < 0.0003), PGF 2α dose by GnRH (P < 0.0009), parity by GnRH (P < 0.0008), and by parity by PGF 2α dose by GnRH (P < 0.0005). Multiparous cows not receiving GnRH had higher mean serum LH (P < 0.02), LH peak pulse height (P < 0.03), and area under the LH release curve (P < 0.03) compared with primiparous cows. The number of LH pulses/6 h was greater (P < 0.06) in multiparous than primiparous cows, and was greater (P < 0.02) in multiparous cows receiving 3 mg/100 kg BW than in cows receiving 2 mg/100 kg BW, but not in the controls or in cows receiving 1 mg/100 kg BW. Exogenous GnRH resulted in increased (P < 0.0001) serum LH concentrations in all cows, and LH was enhanced (P < 0.0009) by simultaneous treatment with PGF 2α. Primiparous cows had a greater response (P < 0.0005) to PGF 2α and GnRH compared with multiparous cows. Pituitary release of LH in response to GnRH was enhanced by simultaneous exposure to PGF 2α in Day 30 postpartum cows.