Duration of daily bull exposure on resumption of ovulatory activity in postpartum, primiparous, suckled, beef cows.

The objective of this experiment was to determine if duration of daily bull exposure influences length of postpartum anestrus in primiparous, anovular, suckled, beef cows. The null hypotheses were that intervals from calving or the start of bull exposure (D 0) to resumption of ovulatory activity (OA), and proportions of cows that resumed OA during the experiment does not differ among cows exposed to bulls for 0h, 6h, or 12h daily, and that there is no relationship between the duration of bull exposure and interval to resumption of OA in cows exposed to bulls for 0h, 6h, or 12h daily. At 51.5+/-2.3d (+/-SE) after calving, cows were assigned randomly to be exposed for 12h (BE12; n=15) or 6h daily (BE6; n=14) to bulls, or not exposed to bulls (NE; n=10) for 45 d. Interval from calving or from D 0 to resumption of OA was shorter (P<0.05) and the proportion of cows that resumed OA during the experiment was greater (P<0.05) for BE12 than for NE cows. Interval from D 0 to resumption of OA did not differ (P>0.10) between BE6 cows and either BE12 or NE cows. However, interval from calving to resumption of OA was shorter (P<0.05) for BE6 than NE cows. The proportion of cows that resumed OA did not differ (P>0.10) between BE6 cows and BE12 cows; however, the proportion of cows that resumed OA during the experiment tended (P=0.08) to be greater for BE6 cows than for NE cows. There was a linear relationship between intervals from calving (b(1)=-7.64 d/h; P<0.05) and D 0 (b(1)=-3.3 d/h; P<0.05) to resumption of OA and duration of daily bull exposure. Thus, the duration of bull-pheromone stimuli that cows perceive each day is related to when primiparous, postpartum, anestrous, suckled cows respond to this stimulus and undergo the physiological changes necessary to resume ovulatory activity.