Postpartum adrenal, pituitary and ovarian functions in dairy cows.

Postpartum stress and the resumption of pituitary and ovarian functions were investigated in 29 dairy cows during the first month after calving. Adrenocorticotropic hormone (ACTH, 25 i.u., IM) challenge tests for adrenal function at 8 and 22 days postpartum and gonadotropin releasing hormone (GnRH, 50 micrograms IM) challenge tests for pituitary function at 7 and 21 days postpartum were conducted. Ovarian function, particularly the interval to first ovulation, was evaluated through weekly rectal palpation and ultrasonographic examination, as well as thrice weekly milk progesterone levels. Elevated basal cortisol levels were found in 24.1% and 20.7% of cows at 8 and 22 days postpartum, respectively. Adrenal response to ACTH was similar at 8 and 22 days postpartum with low, medium and high peak responses in 13.8%, 62.1% and 24.1% of cows, respectively. A moderate correlation was observed between basal and peak cortisol levels. Peak pituitary luteinizing hormone (LH) response was low in 65.5%, medium in 27.6% and high in 6.9% of cows at 7 days postpartum. Pituitary responsiveness improved by day 21 with low in 34.5%, medium in 37.9% and high in 27.6% of cows. About a week after calving, a negative correlation between peak cortisol and peak LH levels was observed. First ovulation occurred in 28%, 48% and 17% of postpartum cows during the second, third and fourth week, respectively. These results indicate a negative effect of peak cortisol levels on pituitary responsiveness to GnRH stimulation during the first week, altered adrenal and pituitary function in a third of the cows until the third week and restoration of ovarian function in most cows (93%) a month after parturition.