Pituitary and ovarian function in postpartum beef cows. I. Effect of suckling on serum and follicular fluid hormones and follicular gonadotropin receptors

Abstract

The effect of suckling on serum and follicular fluid hormones and on follicular gonadotropin receptors was studied. Sixteen anestrous postpartum cows were assigned to 1 of 2 groups: suckled (S) or weaned (W). All calves were allowed to suckle ad libitum from parturition to 21 days postpartum when calves from W cows were weaned. All cows were ovariectomized on Day 25 postpartum. W cows had more (P<0.01) pulses of LH during the 96-h period from weaning until ovariectomy than S cows (6.3 vs 1.3 pulses). Serum concentrations of prolactin (Prl), estrone (E1), estradiol-17β (E2) and progesterone (P) were not different (P>0.10) between groups. Furthermore, there were no differences (P>0.10) in follicular fluid contents of luteinizing hormone (LH), E1, E2 and P between the treatment groups. However, follicular fluid content of Prl was greater (P<0.05) in the W cows than in the S cows (123 vs 65.1 ng/cow). The number of follicular LH receptors was greater (P<0.05) in the W cows than in the S cows (71.1 vs. 48.3 fmoles/mg protein) although the number of follicular follicle-stimulating hormone (FSH) receptors was not different (P>0.10) between W cows and S cows (1531 vs 1862 fmoles/mg protein). There were no correlations between serum hormone concentrations and follicular fluid hormone content; however, the numbers of follicular LH receptors and follicular fluid Prl content were highly correlated in the W cows (r=0.85; P<0.05). It is concluded that removal of the suckling stimulus increases pulsatile LH release and the accumulation of Prl in the follicular fluid. These factors, either together or separately, may at least in part be responsible for the increase in follicular LH receptor concentrations that were observed in the W cows.