Pituitary and Ovarian Function in Postpartum Beef Cows. II.Endocrine Changes Prior to Ovulation in Suckled and Nonsuckled Postpartum Cows Compared to Cycling Cows1

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Abstract
Fifty-four anestrous postpartum beef cows were allotted to 1 of 6 treatment groups to determine the effects of suckling on pituitary and ovarian function. Calves were weaned (W) on day 21 postpartum from 4 groups but were allowed to continue to suckle (S) ad libitum in the other 2 groups. W cows were slaughtered at either 24, 48, 72 or 96 h after initiation of the experiment on day 21 postpartum and S cows at 24 and 96 h (groups were designated W-24, W-48, W-72, W-96, S-24 and S-96). In addition, 27 cycling (C) beef cows were allotted to 1 of 3 treatment groups to compare endocrine changes just prior to ovulation in C cows with endocrine changes in the W cows. Estrus was synchronized with a progestin ear implant and cows were slaughtered at 12, 24 and 36 h after implant removal (groups were designated C-12, C-24 and C-36). Weaning had no effect (P > 0.10) on either pituitary weight or pituitary luteinizing hormone (LH) and FSH concentrations. Gonadotropin-releasing hormone (GnRH)-induced release of LH in vitro in the W cows was not affected (P > 0.10) by duration of weaning; however, incubated pituitary explants of the W cows secreted 89% more (P < 0.05) LH in response to GnRH than did those of the S cows. The total amount of LH released by pituitary explants of the C cows was similar to the amount of LH released by the W cows (778 and 853 ng, respectively). The change in basal serum LH concentrations (posttreatment minus pretreatment; .DELTA. LH) of the W cows increased (P < 0.01) in a linear fashion as the duration of weaning increased and was greater (P < 0.01) than the .DELTA. LH of the S cows. Duration of weaning did not affect the .DELTA. FSH of the W cows although the .DELTA. FSH of the W cows was greater (P < 0.01) than the .DELTA. FSH of the S cows. Follicular fluid prolactin (Prl) concentrations were greater (P < 0.01) in the W cows than in the S cows (39.1 vs. 20.2 ng/ml) although follicular fluid LH concentrations were similar (P > 0.10). Follicular fluid Prl concentrations of the C cows (41.6 ng) were comparable to Prl concentrations in the follicular fluid of the W cows. LH receptors in the largest follicle of the W cows changed little from 24 to 72 h posttreatment but increased (P < 0.05) markedly by 96 h posttreatment. FSH receptors in the largest follicle of the W cows increased (P < 0.05) steadily from 24 to 72 h posttreatment, then declined. Follicular LH receptor concentrations in the C cows changed little from 12 to 24 h postimplant removal but increased sharply (P < 0.05) by 36 h postimplant removal. Follicular FSH receptor concentrations tended to decrease from 12 to 26 h postimplant removal. Removal of the suckling stimulus increases pituitary responsiveness to GnRH and increases basal serum LH and FSH concentrations and follicular fluid Prl concentrations which, together or separately, cause an increase in LH and FSH receptors in the largest follicle. A similar series of events occurs prior to ovulation in cycling beef cows.

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