Evaluation of growth performance and reproductive traits at puberty and attainment of sexual maturity in Bos indicus Nelore bulls raised in pasture management systems

The objective of this study was to characterize the sexual development of 48 Nelore bulls by associating the findings of sperm analyses with growth traits and ultrasound-measured testicular traits. Bulls were evaluated from 14.3 to 24.6 months old at intervals of 28 days, totaling 12 observations per animal. Data were subjected to repeated-measures analyses of variance over time (PROC MIXED, SAS 9.2) and equations for semen traits were developed using stepwise regression (PROC REG, SAS 9.2). Regarding the growth performance on pasture, there was an increase in average body weight (388 to 500±4.59 kg), body condition (5-6), and hip height (139 to 145±0.52 cm) with increasing age. However, a lower average daily gain (0.64 to 0.29±0.05 kg) was observed during the dry season. Reproductive traits such as scrotal circumference (27 to 33.6±0.34 cm), testicular artery diameter (2.14 to 2.96±0.04 mm), blood testosterone concentration (8.3 to 22.7±1.48 mg/mL), sperm motility (total: 42 to 82.8±2.91% and progressive: 26.2 to 62.3±2.61%), percentage of rapid cells (38.1 to 79.8±2.9%), and sperm concentration (43.6 to 645±42.1 × 106/mL) increased with increasing age, while there was a reduction in sperm defects (major: 59.4 to 9.6±3.18% and total: 62.5 to 15.1±2.94%). Ultrasonography and growth traits have a moderate power to predict major and total sperm defects but cannot predict other seminal traits. By 19-20 months of age, Nelore bulls start to exhibit reproductive traits that indicate the attainment of sexual maturity.