Return to cyclic ovarian activity following parturition in mature cows and first-calf beef heifers exposed to bulls

Abstract

The effect of exposure to bulls on onset of ovarian activity following parturition was studied in multiparous cows and primiparous heifers. In experiment 1, Poll Hereford and Poll Hereford-cross cows were exposed to bulls either from 3 to 85 days post-partum (bull exposed - BE; n = 62) or from 55 to 85 days post-partum (not exposed - NE; n = 55). In experiment 2, 2-year-old Angus heifers were exposed to bulls either from 3 to 120 days post-partum (BE; n = 48) or from 58 to 120 days post-partum (NE; n = 50). Single blood samples were taken from all cows and heifers at weekly intervals from day 20 post-partum until the end of the bull exposure period to determine plasma progesterone concentrations, which were used as an index of ovarian function. In experiment 1, onset of ovarian activity, determined by the first progesterone concentration of 1.5 ng/ml or above, occurred at 31 ± 1 and 32 ± 1 days post-partum in the BE and NE cows respectively. In experiment 2, onset of ovarian activity occurred at 46 ± 3 and 62 ± 4 days post-partum in the BE and NE heifers respectively. Therefore, first-calf heifers, but not mature cows, had a significantly reduced interval from parturition to the initiation of ovarian activity as a result of early post-partum exposure to bulls. The response in heifers appeared to occur in large part prior to day 40 post-partum.