INFLUENCE OF FENCELINE BULL EXPOSURE ON DURATION OF POSTPARTUM ANESTRUS AND PREGNANCY RATE IN BEEF-COWS

Two experiments (1991 and 1999) were conducted to test the hypotheses that fenceline exposure of cows to bulls following parturition shorten s duration of postpartum anoestrus and improves pregnancy rate to arti ficial insemination (AI). In Experiments 1 and 2, 75 and 125 postpartu m crossbred beef cows, respectively, were stratified by calving date a nd while maintaining equivalent primiparous cows in each treatment gro up were randomly assigned to be exposed to bulls through the fenceline (BE) or not exposed to bulls (NE). In each experiment, twice weekly b lood samples were collected for 13 weeks beginning 1 to 4 weeks postpa rtum, These samples were used to assess concentrations of progesterone and determine time of onset of ovarian luteal function following calv ing. In Experiment 2, a 23 day program of Al was included in the study in which signs of behavioral oestrus were detected twice daily and co ws 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 postpa rtum anoestrus in each experiment than primiparous cows in the NE grou p (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 e xposure on the duration of postpartum anoestrus of multiparous cows in either experiment. Pregnancy rate to Al in Experiment 2 was not impro ved by fenceline bull exposure. We conclude that fenceline bull exposu re was effective in shortening length of postpartum anoestrus in primi parous cows but this improvement did not translate into improved pregn ancy rates during the subsequent breeding season.