371. The Effects of Yearling Bull Rate of Gain on Fertility and Performance.

Abstract Bulls represent a significant financial investment in beef cattle operations with their reproductive success being fundamental to herd productivity. Over a three-year study, yearling MARC II bulls [n = 114; 292 ± 30 d of age; BW = 347.64 ± 2.38 kg] were used to determine whether bull rate of gain prior to breeding alters bull reproductive traits at turnout. Bulls had ad libitum access to water and were individually fed the same basal TMR to gain either 1.8 kg/d (HIGH) or 1.13 kg/d (MOD) for 112 days. On d 112 each year, all bulls were subjected to a breeding soundness exam. Data were analyzed using the MIXED procedures in SAS with day, treatment, year, and their interactions as fixed effects. Average daily gain was greater (P = 0.04) for HIGH compared with MOD bulls (1.7219 vs. 1.1617 kg/d, respectively) which was divergent in weight by d 14 of the feeding period (P < 0.0001). While HIGH bulls had larger scrotal circumference (P = 0.003) than MOD bulls, there were no treatment × day or main effect of treatment differences in total, motile, or progressive sperm (total or per mL of ejaculate; P ≥ 0.23). A main effect of day was observed (P ≤ 0.014) for ejaculate volume, motile, and progressive sperm, where all increased from d 0 to 112. At the end of the feeding period, there was a treatment × day interaction (P = 0.0183) where circulating concentrations of NEFA were equivalent at d 0, but by d 112, HIGH bulls were lower compared to MOD gain bulls. In contrast to the NEFA data, a treatment × day interaction (P = 0.0294) was detected, where the concentration of glucose in plasma was equivalent on d 0, but by d 112, MOD bulls were lower compared to HIGH gain bulls. The concentrations of glucose (P = 0.32) and NEFA (P = 0.12) in seminal plasma were not affected during the feeding period. Overall, developing bulls at either rate of gain did not negatively affect total sperm production. The increase in scrotal circumference in the HIGH bulls without the concomitant increase in total sperm production suggests increased scrotal fat deposits rather than testicular size. Furthermore, the differences in glucose and NEFA support the increase in body weight and positive energy balance by bulls in the HIGH treatment. These data show no negative effects of developing bulls at either rate of gain on sperm production prior to the breeding season. However, further work should be done to determine the effects of bull management from high concentrate to high forage rations, as is typically seen in production from bull sale to turnout. USDA is an equal opportunity provider and employer.