Suckling and four-times daily milking: influence on ovulation, estrus and serum luteinizing hormone, glucocorticoids and prolactin in postpartum holsteins.

: Holstein cows were assigned at calving to be (1) milked at 12-hr (n = 5) or (2) 6-hr (n =5) intervals or (3) suckled ad libitum by one calf and milked at 12-hr intervals (n = 6). Suckling resulted in an increase in the postpartum (PP) interval to first ovulation (39.7 vs 21.2 days), but increased milking frequency did not. Delayed resumption of episodic luteinizing hormone (LH) secretion was associated with the increased PP interval to ovulation in suckled cows. Both frequency and amplitude of episodic LH peaks were reduced (P less than .05) on days 7 and 14 PP in suckled cows compared to nonsuckled cows (1.2 vs 2.5 peaks/4.5 hr and 1.8 vs 2.8 ng/ml, respectively). Suckling did not significantly modify basal of milking-induced concentrations of prolactin or total glucocorticoids. Nor did serum concentrations of progesterone or estradiol-17 beta differ between suckled and nonsuckled cows, or between day 7 and day 14 PP. Milking-induced secretion of both prolactin and total glucocorticoids were greater (P less than .05) on day 14 PP than on day 7 PP in both suckled and nonsuckled cows. These results suggest that suckling may delay the first PP ovulation by suppressing episodic LH secretion. Alternations in prolactin, total glucocorticoids, progesterone or estradiol-17 beta do not apper to mediate directly the effects of suckling on PP episodic LH secretion and (or) ovulation.