The biostimulatory effect of bulls on postpartum follicular wave development in postpartum, anestrous, suckled beef cows

Abstract

The objective of this experiment was to determine if bull exposure influences follicular wave dynamics in primiparous, postpartum, anestrous, suckled, beef cows exposed to bulls. In Experiment 1, cows were exposed (continuously 24 h/d), (EB; n = 5) to bulls or not exposed to bulls (NE; n = 5) throughout the experimental period. In Experiment 2, cows were exposed to bulls for either 12 h, (EB12; n = 15), 6 h, (EB6; n =14) or not exposed to bulls (NE; n = 10) from the start to the end of the experimental period. In Experiments 1 and 2, cows were 67 d ± 3.8 (mean ± SE) and 51.5 ± 2.3 d postpartum at the start of the experiment. Follicular characteristics of each cow were examined by transrectal ultrasonography. In Experiment 1, interwave interval for wave 3 was shorter in EB than NE cows. Maximum dominant follicle (MDF) diameter tended to be greater during wave 2 for EB than NE cows, while wave 3 was greater for EB than NE cows. However, MDF diameter for wave 6 was greater for NE than EB cows. In Experiment 2, EB12 cows had fewer follicular waves to the resumption of luteal activity (RLA) than NE cows, while the number of waves to RLA for EB6 cows did not differ from that of EB12 or NE cows. Normalizing follicular waves to the time of RLA for cows within the EB12 and EB6 indicated that those cows at RLA had larger MDF diameters for the wave that produced the ovulatory follicle than cows that did not RLA. These data show the effects of bull exposure in altering follicular growth and developmental patterns, shortening the inter-wave interval and increasing the MDF diameter. Though the mechanism through which bull exposure alters postpartum follicular development is not entirely understood, these data provide new understanding.