Stimulation of estrogen and luteinizing hormone secretion in postpartum beef cows.

Three experiments were conducted to determine whether and how weaning, quanti ty of milk produced, suckling intensity, age of dam, or a combination of these factors influence ovarian and/or pituitary response to pregnant mare serum gonadotropin (PMS), synthetic gonadotropin-releasing hormone (GnRH), or ovariectomy in postpartum beef cows. Ovarian and pituitary responses, as determined by plasma estrogen, progesterone, and luteinizing hormone (LH) concentrations, were evoked in both 3-year-old and mature anestrous, suckled cows (experiment 1) at 42 days postpartum with either PMS, Gn-RH, or cessat ion of lactation. All three dosages of PMS (750, 1,500, or 2,250 IU) st imulated estrogen secretion and a subsequent LH surge in anestrous suckled and nonsuckled cows; whereas, PMS only stimulated estrogen secretion in the cows already cyclic at 42 days postpartum. Cows given Gn-RH (75, 150, or 300 ug) had a dose related, biphasic increase in peripheral plasma LH concentration. A comparison between 2-year-old suckled Brown Swiss and Angus heifers at 42 days postpartum (experiment 2) indicated that Brown Swiss produced