Beyond the scrotal circumference: Exploring phenotypic, nutritional and metabolic traits associated with early sexual development in young Nelore (Bos indicus) bulls

This study evaluated the associations among phenotypic, reproductive, nutritional, and metabolic traits with early sexual development in Bos indicus bulls. A total of 128 prepubertal Nelore bulls (11.1 ± 0.1 months old) were evaluated in an automatic feedlot system during a 70-day period. Parameters assessed on days −70 (beginning of the feedlot) and 0 (end of the feedlot) included body weight (BW), body condition score (BCS), scrotal circumference (SC), rectal temperature (RT), scrotal surface temperature (SST), testicular parenchyma (via transcutaneous ultrasound) and serum metabolomics. On day 0, bulls were submitted to a seminal analysis and categorized as late maturing (LM; n = 97; <50 × 10⁶ sperm cells per ejaculate) or early maturing (EM; n = 31; >50 × 10⁶ sperm cells per ejaculate, with >10% motility). In addition, feed efficiency traits (average daily gain [ADG], dry matter intake [DMI] and residual feed intake [RFI]) were evaluated during the feedlot period, and ribeye area (REA), rump fat thickness (RFAT), and subcutaneous back fat thickness (BFAT) were assessed on day 0. On day −70, no differences were observed in BW (P = 0.28), BCS (P = 0.38), RT (P = 0.11), SC (P = 0.17) and SST (P = 0.99) between LM and EM bulls. However, EM bulls exhibited more echogenic testicular parenchyma, with fewer black pixels (P = 0.003) and more white (P < 0.001) and gray pixels (P < 0.001). Despite similar seminiferous tubular density (P = 0.11), EM bulls had lower seminiferous tubular area (P < 0.001) and diameter (P < 0.001) than LM bulls. On day 0, EM bulls had greater SC (P = 0.01) and BFAT (P = 0.05), while other traits (BW, BCS, RT, SST, REA, RFAT) remained similar (P > 0.05) between groups. Additionally, no effects on testicular parenchyma were observed. Moreover, EM bulls had a similar ADG (P = 0.15) but tended to have higher RFI (P = 0.06) and DMI (P = 0.10) than those LM. Serum metabolomics on day −70 revealed a clear distinction between LM and EM bulls, with alterations in pathways such as glyoxylate and dicarboxylate metabolism, glycine, serine, and threonine metabolism, and TCA cycle. Likewise, on day 0, metabolic distinction persisted, involving lipoic acid metabolism, glutathione metabolism, and glyoxylate and dicarboxylate metabolism, reflecting energy production and redox regulation, which are critical for early sexual development. In conclusion, metabolomics analysis proved an effective tool for discerning metabolic profile differences between LM and EM bulls. EM bulls showed greater SC and BFAT only on D0, while testicular parenchyma echogenicity was unreliable for predicting early sexual development. Feed efficiency traits, such as RFI and DMI, may influence sexual maturation, highlighting the interplay between nutrition, metabolism, and reproductive development.