Overnutrition induced metabolic dysregulation and partially decreased semen quality in young beef bulls

The objective of this study was to characterize the systemic metabolic response to overnutrition in young bulls and to evaluate the effects of overnutrition on semen characteristics. Half-sibling yearling beef bulls (n = 44) were utilized in a completely randomized design, where bulls were randomly assigned 1 of 2 dietary treatments (n = 4 pens/treatment): 1) Moderate Gain (MG): diet formulated to promote an average daily gain of 1.2 kg/day, or 2) High Gain (HG): diet formulated to promote an average daily gain of 1.8 kg/day. Bulls were housed in a feedlot facility equipped with an automated individual feed intake monitoring system and fed their respective diets for 114 days. Body weight, carcass ultrasonography, and blood samples were collected on days 0, 36, 76, and 114. Blood samples were utilized to determine circulating concentrations of cholesterol, triglycerides, glucose, insulin, leptin, low-density lipoprotein, non-esterified fatty acids (NEFA), beta-hydroxybutyrate (BHB), testosterone, and haptoglobin. Serial semen samples were collected at the end of the feeding period (days 109, 111, and 114) and analyzed using computer assisted sperm analysis and image-based flow cytometry. Body weight and subcutaneous backfat thickness were greater (P < 0.01) at the end of the feeding period in HG compared with MG bulls. Similar results were observed for circulating concentrations of glucose, insulin, leptin, cholesterol, and low-density lipoproteins (P ≤ 0.02). Alternatively, circulating concentrations of NEFA and BHB were decreased (P < 0.01) in HG bulls at the end of the feeding period. Bulls exposed to the HG diet had greater (P < 0.01) insulin resistance at the end of the feeding period based on insulin: glucose ratio and revised quantitative insulin sensitivity check index (RQUICKI). HG bulls had greater (P = 0.02) plasma haptoglobin compared with MG bulls, whereas testosterone concentrations were similar (P = 0.69). Bulls exposed to the HG diet tended to have decreased (P ≤ 0.09) total and progressive motility compared with MG bulls. Moreover, the proportion of sperm with partially damaged acrosomes tended (P = 0.09) to be increased and the proportion of sperm with intact plasma membrane tended to be reduced (P ≤ 0.10) in HG bulls compared with MG bulls. In summary, HG dietary treatment promoted an obese-like metabolic profile that increased insulin resistance and circulating haptoglobin, and resulted in a subtle decrease in semen quality.