Influence of fenceline bull exposure on duration of postpartum anoestrus and pregnancy rate in beef cows

Two experiments (1991 and 1992) were conducted to test the hypotheses that fenceline exposure of cows to bulls following parturition shortens duration of postpartum anoestrus and improves pregnancy rate to artificial insemination (AI). In Experiments 1 and 2, 75 and 125 postpartum crossbred beef cows, respectively, were stratified by calving date and while maintaining equivalent primiparous cows in each treatment group were randomly assigned to be exposed to bulls through the fenceline (BE) or not exposed to bulls (NE). In each experiment, twice weekly blood samples were collected for 13 weeks beginning 1 to 4 weeks postpartum. These samples were used to assess concentrations of progesterone and determine time of onset of ovarian luteal function following calving. In Experiment 2, a 23 day program of AI was included in the study in which signs of behavioral oestrus were detected twice daily and cows were artificially inseminated 12 h following detection of oestrus. Pregnancy rate to AI was determined by concentrations of progesterone, detection of pregnancy by rectal palpation, and confirmed by calving date. Primiparous cows in the BE group had shorter durations of postpartum anoestrus in each experiment than primiparous cows in the NE group (Experiment 1: BE = 78 days, NE = 92 days, P < 0.05; 2: BE = 109 days, NE = 117 days, P < 0.10). There was no influence of fenceline bull exposure on the duration of postpartum anoestrus of multiparous cows in either experiment. Pregnancy rate to AI in Experiment 2 was not improved by fenceline bull exposure. We conclude that fenceline bull exposure was effective in shortening length of postpartum anoestrus in primiparous cows but this improvement did not translate into improved pregnancy rates during the subsequent breeding season.