Effect of transport stress on luteinizing hormone released by GnRH in dairy cows.

Stress is thought to reduce gonadotrophin secretion in ruminants. So far, only prolonged administration of ACTH or cortisol have been shown experimentally to reduce LH release. The present experiment studied the effects of an acutely stressful situation. Fifteen min after the start of a 30 min truck journey for dairy cows plasma cortisol values exceeded 55 nmol/l, as measured by RIA. Values began to decline 15-45 min after return to the barn and reached baseline (2.5 to 20 nmol/l) 1-2 h later. Responses to 20 or 40 micrograms GnRH administered on day 10 of the cycle, or 48 h after prostaglandin analogue, were significantly reduced on a within-animal basis when given during the truck journey. Hence acute endogenous stimulation of pituitary-adrenal activity can interfere with pituitary LH release.