148. Evaluating Bull Breeding Soundness Throughout the Breeding Season.

Abstract Currently, the breeding soundness examination (BSE) is the most common method to assess reproductive potential. In the United States, natural service remains the primary breeding method utilized on commercial beef herds as approximately 96% of cows are exposed to bulls during the breeding season. Sub-fertile bulls delay conception rates, prolong the calving season, and reduce calf weaning weights. Whether a decrease in bull fertility is related to changes in sperm quality throughout the breeding season is still unknown. Our objective was to investigate how individual bull BSE characteristics changes throughout the breeding season. We hypothesized that bull reproductive soundness would change throughout the breeding season, with a decline in semen quality parameters including motility and morphology. Hereford bulls (n = 38) were subjected to a BSE, following the guidelines of the Society for Theriogenology 60 days before the breeding season (BSE1), 30 days after the start of the breeding season (BSE2) and 2 weeks following the conclusion of the breeding season (BSE3). During each BSE, whole blood was collected via tail venipuncture, and semen samples were obtained via electroejaculation (Pulsator IV, Lane Manufacturing, Inc.). Sperm motility was evaluated both subjectively (phase-contrast microscopy) and via portable Computer Assisted Sperm Analysis system (Androscope, Minitube, Tiefenbach, Germany), while sperm morphology was evaluated using eosin/negrosin stain. Sperm concentration decreased between BSE1(622.94 ± 93.91) and BSE2 (55.59 ± 99.76) (P < 0.001) and BSE1 and BSE3 (P < 0.002). Likewise, a decrease in motility between BSE1 and BSE2 (P < 0.001) and between BSE1 and BSE3 (P < 0.001) was observed. There was an increase in abnormal cells as the breeding season progressed, when comparing BSE1 to BSE3 (161.82 ± 93.88) (P < 0.001) and between BSE2 to BSE3 (P < 0.001). Most relevant defects observed were folded and curled tails (BSE1), detached and pyriform heads (BSE2), and distal midpiece reflex/folded tails (BSE3). In conclusion, sperm quality parameters such as concentration and motility decreased as the breeding season progressed, while the incidence of abnormal sperm increased. Additional investigations across multiple breeds and breeding seasons are needed to further characterize bull field fertility resilience. The present work will assess endocrine profiles, DNA fragmentation, and sperm reserve exhaustion as potential contributors to the lack of resilience in bull reproductive soundness.