Evidence against chronic teat stimulation as an autonomous effector of diminished gonadotropin release in beef cows.

ABSTRACT

The current experiment was performed to examine the acute and cumulative effects of chronic manual teat stimulation on the tonic pattern of luteinizing hormone (LH) secretion in beef cows. Additionally, we characterized the plasma profile of prolactin and cortisol release to evaluate whether changes in the concentrations of these hormones occurred in response to teat stimulation and whether such changes were related to gonadotropin secretion. Six weeks after ovariectomy, nonlactating beef females were paired by age and assigned randomly to a control group (n = 3) or a stimulation group (n = 3). Stimulated cows were subjected to 20 min of continuous manual teat stripping every 4 h for 24 h. Blood samples were collected from all cows at 10-min intervals beginning 1 h before and continuing for 2 h after the onset of each stimulation period in treated animals. Numerous episodes of prolactin and cortisol release were observed in control and treated animals throughout the 24-h experiment. The percentage of stimulations accompanied by prolactin and cortisol releases for each of the three treated animals was 100 and 100, 16.7 and 50, and 50 and 100, respectively. The number of prolactin peaks observed the hour after onset of teat stimulation was greater (P less than .06) than the number observed the hour before. However, the number of cortisol peaks was not statistically related to teat stimulation. Overall, mean concentrations of prolactin and cortisol were not increased by teat stimulation. Luteinizing hormone pulse frequency (1.6 +/- .1 pulses/h) and mean LH concentrations (12.1 +/- .6 ng/ml) were not acutely or chronically affected by teat stimulation and were not related to prolactin or cortisol release. We conclude that mechanical stimulation of the teat is not singly effective in altering the pattern or quantity of tonic LH release in ovariectomized cows.