Effects of bull exposure on postpartum ovarian activity of dairy cows

The effects of bull exposure on postpartum reproduction have been previously studied in beef, but not in dairy cattle. The objective of this experiment was to investigate effects of bull exposure on postpartum reproductive characteristics during early lactation in high-producing Holstein dairy cattle. Forty-five multiparous dairy cows (9764±140 kg milk/lactation) were randomly grouped at the time of calving. Treatment 1 (NBE) was never exposed to the bull, treatment 2 (2×BE) experienced fenceline contact with a mature Holstein bull twice daily during routine detection of oestrus, and treatment 3 (CBE) was in continuous proximity to the bull. All cows were observed for oestrous behaviour twice daily for 1/2 h following milking. Blood samples were collected every other day for progesterone (P4) assay. Postpartum ovarian reactivation, indicated by the occurrence of the first sustained rise in P4, was later (P=0.02) during the postpartum period in both bull-exposed (2×BE, CBE) treatments compared to the NBE treatment. Time postpartum until completion of the first ovarian cycle tended (P=0.08) to be shorter in NBE cows compared to bull-exposed cows. Peak P4 concentrations were higher (P=0.05) in NBE cows compared to bull-exposed cows prior to the second postpartum ovulation. Results indicate an extended period to ovarian reactivation in dairy cattle exposed to a bull, but bull exposure had no effect on long-term reproductive performance of exposed cows.