230 Reproductive consequences of young sire overnutrition in the bovine: II) effects on semen quality and sperm functional traits.

Abstract Overnutrition in young beef bulls has been reported to negatively impact semen quality during industry-standard breeding soundness examinations. The objective of the present study was to evaluate the impact of divergent planes of nutrition during bull development on semen quality using advanced semen quality assessments. Half-siblings Angus yearling bulls (n = 44) were utilized in a completely randomized design. Bulls were stratified by age, body weight, and body condition scores before being randomly assigned 1 of 2 dietary treatments for 114 days: 1) Moderate Gain (MG): diet formulated to promote an average daily gain of 1.2 kg/d, or 2) High Gain (HG): diet formulated to promote an average daily gain of 1.8 kg/d. Bulls were housed in a feedlot facility equipped with a Smartfeed Pro automated bunk system that allowed for individual feed intake assessments daily throughout the study. Serial semen samples were collected via electroejaculation at the end of the feeding period (days 109, 111, and 114), extended, and analyzed using computer assisted sperm analyses and flow cytometry. Bull was considered the experimental unit and ejaculate was considered a replicate within the same experimental unit. Per design, bulls in the HG had greater (P < 0.01) average daily gain and increased subcutaneous backfat thickness at the end of the feeding period compared with MG bulls. Ejaculate volume was decreased (P = 0.04) in HG compared with MG bulls; however, ejaculate concentration was not different between treatments (P = 0.53). Bulls exposed to the HG diet tended to have decreased (P ≤ 0.08) total and progressive motility compared with MG bulls. Total and progressive composite scores tended (P ≤ 0.09) to be decreased in HG compared with MG bulls. A decreased (P = 0.02) percentage of ejaculates from HG bulls met progressive motility standards (≥ 60% motility) for cryopreservation compared with MG bulls. At 0 minutes in vitro incubation (IVC), semen from bulls exposed to the HG diet tended (P = 0.08) to have a greater percentage of sperm with acrosome damage compared with semen from MG bulls. Moreover, a greater percentage of acrosome intact sperm had membrane damage in semen from HG compared with MG bulls (P = 0.05). At 30 minutes IVC, differences between treatments in the percentage of acrosome intact sperm with membrane damage were maintained (P = 0.05). In addition, HG bulls had a greater (P = 0.05) percentage of sperm with membrane damage and tended (P = 0.09) to have a greater percentage of sperm with acrosome damage compared with MG bulls. In summary, the young bull overnutrition model used in this study negatively impacted semen quality, suggesting that diet-induced overconditioning in young bulls may compromise key sperm functional traits and potentially affect bull fertility.