Semen Quality, Sperm Morphology, and Reproductive Development in Young Nellore Bulls Managed on Different Nutritional Planes

Abstract Reproductive success in beef cattle depends on genetics, reproductive biotechnologies, and effective nutritional management, especially for breeding bulls. This study evaluated the effects of different nutritional planes on growth, semen quality, oxidative stress in spermatozoa, and reproductive traits in young Nellore bulls. Forty-two bulls, averaging 440 kg of body weight (BW) and 20-mo-old, were randomly assigned to three nutritional treatments (n = 14 per group): 1) control (CON), grazing pasture and receiving 0.25 g·kg⁻¹ BW of mineral salt daily; 2) supplemented (SUP), grazing pasture and receiving 1 g·kg⁻¹ BW of a protein-energy supplement plus 0.25 g·kg⁻¹ BW of mineral salt daily; and 3) feedlot (FLT), fed a total mixed ration with a 40:60 corn silage to concentrate ratio. The CON and SUP groups were kept in paddocks of Urochloa brizantha cv. Marandu. The 140-d experiment included three semen collections and scrotal circumference measurements on days 1, 68, and 138. BW and body condition score (BCS) were recorded every 14 days, and average daily gain (ADG) was calculated. At slaughter, testes, epididymides, and seminal vesicles were weighed. As expected, there was an interaction (P 0.001) between nutritional plane and time for BW, ADG, and BCS, with mean differences of approximately 51 kg (CON vs. SUP), 100 kg (CON vs. FLT), and 49 kg (SUP vs. FLT). Scrotal circumference was affected by the interaction (P 0.001); FLT bulls showed a greater increase over time than SUP and CON, but final values at day 138 did not differ (P = 0.4379). Testes and epididymis weights did not differ, whereas seminal vesicles were heavier (P = 0.005) in FLT bulls compared with CON and SUP. A diet × time interaction was detected for the plasma membrane damage index (P = 0.0217), with greater values in SUP and FLT bulls at days 68 and 138 compared with CON. A trend for a diet × time interaction was observed for sperm vigor (P = 0.0918), showing a transient reduction on day 68 in CON bulls. The percentage of isolated pathological heads increased late in CON bulls at day 138 (P = 0.0711). No other sperm morphological traits or reproductive organ characteristics were affected by nutritional plane (P ≥ 0.454), except for a trend (P = 0.094) toward greater lipid peroxidation in FLT bulls, with the lowest value in SUP. In conclusion, confinement promoted greater growth and testicular development but was also associated with increased oxidative stress and sperm damage. Protein–energy supplementation on pasture offered a more balanced alternative, supporting growth while preserving semen quality. The observed interactions highlight the dynamic nutritional effects on reproductive traits. Moreover, potential physiological and molecular impacts on offspring should be considered when designing bull development programs.