229 Reproductive consequences of young sire overnutrition in the bovine: I) Systemic metabolic and endocrine profile.

Abstract Overnutrition in young beef bulls has been previously reported to negatively impact semen quality during industry-standard breeding soundness examinations. The objective of the present study was to characterize the metabolic and endocrine profile of young bulls exposed to divergent planes of nutrition. Half-siblings Angus yearling bulls (n = 44) were utilized in a completely randomized design. Bulls were stratified by age, body weight (BW), and body condition scores (BCS) 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. Throughout the study, bulls were housed in a feedlot facility equipped with a Smartfeed Pro automated bunk system that allowed for individual feed intake assessments daily. Body weight, BCS, and blood samples were collected on days 0, 38, 76, and 114. Changes in body composition were estimated using carcass ultrasonography on the same days. Average daily gain (P < 0.01) and dry matter intake (P < 0.01) were greater in HG compared with MG bulls. Moreover, HG bulls had decreased feed to gain ratio compared with MG bulls. Body weight, BCS, subcutaneous backfat, and intramuscular fat were greater (P ≤ 0.01) at the end of the feeding period in HG compared with MG bulls (treatment × day; P ≤ 0.04). Similar results were observed for circulating concentrations of glucose, insulin, leptin, cholesterol, and low-density lipoproteins (treatment × day; P ≤ 0.01). Bulls exposed to the HG diet also had decreased (treatment × day; P ≤ 0.02) circulating concentrations of non-esterified fatty acids and beta-hydroxybutyrate at the end of the feeding period compared with MG bulls. Insulin:glucose ratio and HOMA-IR, which are estimates of insulin resistance, were increased in HG compared with MG bulls at the end of the feeding period (treatment × day; P < 0.01). Moreover, HG bulls had greater (P = 0.02) circulating concentrations of haptoglobin at the end of the feeding period compared with MG bulls. Scrotal circumference tended (P = 0.10) to be greater at the end of the feeding period in HG compared with MG bulls; however, scrotal skin thickness and paired-testicular volume were similar (P ≥ 0.18) among treatments. In summary, the HG dietary treatment utilized in this study promoted an obese-like metabolic profile along with increased insulin resistance and systemic inflammation compared with the MG diet.