Impact of genetic selection for postweaning weight on the reproductive development of young Nellore bulls.

This study aimed to evaluate the impact of genetic selection for increased postweaning weight on the reproductive development of young Nellore bulls. Seventy-six young Nellore bulls from two distinct selection lines in two experimental years were used: 23 young Nellore Control (NeC) bulls (14.1 ± 0.9 months old), and 53 young Nellore Selection (NeS) bulls (13.9 ± 0.8 months old). The animals were evaluated from 14 to 24 months of age at a mean interval of 58.4 ± 8.5 days. In each assessment, testicular biometry, B-mode testicular ultrasound, Doppler ultrasound of pampiniform plexus, and seminal evaluation were performed. Ten young bulls from each selection line underwent testicular fine-needle aspiration and were subsequently castrated at 18 months old for histological analysis of testicular tissue. Young bulls from the NeS line exhibited greater mean scrotal circumference (P < 0.0001), testicular volume (P < 0.0001), and end diastolic velocity (P < 0.0001) compared to the NeC line. The mean vascular resistance (P < 0.0001) and pulsatility (P < 0.0001) indices were greater in NeC animals than in NeS animals. Genetic selection of young Nellore bulls for increased postweaning weight resulted in greater scrotal circumference, testicular volume, and end diastolic velocity, indicating more pronounced testicular growth. No differences were observed in the seminal, cytological, or histological parameters between selection lines. These findings suggest that selection for increased postweaning weight gain did not compromise reproductive development, promoted an increase in biometric testicular parameters without negative effect on sperm quality or testicular tissue structure.