The biostimulatory effect of bulls on the hypothalamic-pituitary-adrenal and ovarian axes and on temporal aspects of resumption of ovarian cycling activity in primiparous, postpartum, anestrous, suckled, beef cows

Abstract

Exposing cows to bulls or excretory products of bulls stimulates resumption of ovarian cycling activity in postpartum, suckled, anestrous cows. This biostimulatory effect may be mediated by pheromones produced by bulls that stimulate physiological changes in the hypothalamic-pituitary-ovarian (HPO) and/or -adrenal (HPA) axes of cows. In Experiment 1, the hypothesis tested was that the biostimulatory effect of bulls is associated with adrenal regulation and/or function in anestrous cows. The biostimulatory effect of bulls was associated with mean concentrations of cortisol in postpartum cows. Experiment 2 was designed to determine if acute (5-h daily) bull exposure alters characteristics of patterns of cortisol and luteinizing hormone (LH) concentrations in postpartum, anestrous cows. Cows exposed acutely to bulls exhibited fewer pulses of cortisol and more frequent pulses of LH than cows exposed to steers. However, it was not known if these changes were related to resumption of ovarian cycling activity in postpartum, anestrous cows. Experiment 3 was designed to test the hypothesis that patterns of cortisol concentrations are altered by continuous, 24-h daily, bull exposure, before and after resumption of ovarian cycling activity in postpartum, anestrous cows. Continuous bull exposure decreased cortisol pulse frequency before cows resumed ovarian cycling activity. Experiment 4 tested the hypothesis that the overall shape of patterns of cortisol and/or LH concentrations may differ between cows exposed acutely to bulls or steers in Experiment 2. Cows exposed acutely to bulls had more uneven patterns of LH concentrations than cows exposed to steers and as patterns of cortisol concentrations became smoother, patterns of LH become more uneven in cows exposed acutely to bulls. In Experiment 5, the hypothesis tested was that interval to resumption of ovarian cycling activity may depend upon duration of daily bull exposure. Cows resumed ovarian cycling activity sooner as duration of daily bull exposure increased. In conclusion, as duration of daily bull exposure increases, the biostimulatory effect of bulls alters activity of the HPA axis and this change may facilitate or support the function of the HPO axis and accelerate resumption of ovarian cycling activity in primiparous, postpartum, suckled, anestrous cows.