Abstract
Three experiments were conducted to determine whether and how weaning, quantity 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 (Gn-RH), 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 cessation of lactation. All three dosages of PMS (750, 1,500, or 2,250 IU) stimulated 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 µg) 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 more milk, and had a longer interval from administration of 2,250 IU PMS to occurrence of the preovulatory LH surge; both breeds had increased ovarian and pituitary stimulation with 2,250 IU PMS as compared with 750 IU. Effect of suckling intensity on LH secretion was evaluated in nonsuckled, restricted nursed (twice daily), and unrestricted nursed 2-year-old ovariectomized heifers (experiment 3) from which blood samples were collected at 3, 9, 16 and 30 days postpartum. Mean LH was not affected by treatment, but did increase linearly between 9 and 30 days postpartum. However, nonsuckled heifers had an increased incidence of spontaneous LH releases and increased minimal LH concentration at 30 days postpartum when compared to the nursed groups. These results suggest that lactation suppresses gonadotropin secretion in early postpartum cows.
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