Exploring phenotypic, nutritional and metabolic traits associated with early sexual development in young Nelore (Bos indicus) heifers.

This study explored the associations among phenotypic, reproductive, nutritional, and metabolic traits with early sexual development in Bos indicus heifers. A total of 443 prepubertal Nelore heifers (11.1 ± 0.07 mo old) were evaluated during a feedlot (automated feeders) period from day -70 to day 0. Parameters assessed on day -70 (beginning of the feedlot period) included body weight (BW), body condition score (BCS), presence of CL, uterine (UT) and largest follicle (LF) diameters, as well as blood collection for LC-MS/MS metabolomics (subset of 139 heifers). On day 0 (end of the feedlot period), all heifers were submitted to a timed artificial insemination protocol (TAI). In addition, on that day, ribeye area (REA) and rump fat thickness (RFAT) were assessed by ultrasound. Residual feed intake (RFI), average daily gain (ADG), and dry matter intake (DMI) during the feedlot period were calculated. Pregnancy diagnosis was performed on day 39 and heifers confirmed as pregnant were classified as early-conceiving (EC; n = 212), while those non-pregnant were classified as late-conceiving (LC; n = 231). Parameters evaluated on days -70 and 0 were compared between these groups. On day -70, no differences in BW (P = 0.30), BCS (P = 0.11), UT (P = 0.32) and LF (P = 0.39) diameters, and presence of CL (P = 0.98) were observed between groups. On day 0, no difference between groups were observed in BW (P = 0.46), BCS (P = 0.82), LF (P = 0.56) diameter, and REA (P = 0.19). However, EC heifers had greater UT diameter (P = 0.05), RFAT (P = 0.02) and presence of CL (P = 0.04). Additionally, although no differences between groups were observed in DMI (P = 0.15) and ADG (P = 0.40) during the feedlot period, EC heifers had higher RFI (P = 0.05) than LC heifers. Metabolomic analyses of samples collected on day -70 indicated a marked separation between LC and EC heifers, with an accuracy of 0.74. Significant pathways altered were alanine, aspartate and glutamate metabolism, TCA cycle, and pyruvate metabolism. In conclusion, none of the phenotypic traits assessed on day -70 reliably predicted early sexual development in heifers. By day 0, EC heifers demonstrated increased fat deposition and uterine development. Moreover, feed efficiency was negatively associated with early sexual development in yearling Bos indicus heifers. Metabolomic analyses revealed key changes in amino acids and energy metabolism and provided moderate predictive accuracy between EC and LC heifers. Collectively, the phenotypic, nutritional, and metabolic traits evaluated in this study contribute to a more integrated understanding of the physiological mechanisms underlying sexual development in Nelore heifers.