Title: Parameters associated with sexual precocity of nellore heifers in integrated systems

Abstract

This study evaluated how integrated crop–livestock–forestry systems affected the expression of sexual traits in Nellore heifers. The serum concentration of insulin-like growth factor type-I (IGF-I), follicular diameter, rump fat thickness (RFT), and weight gain were assessed in 48 prepubertal Nellore heifers (14–16 months old, initial average live weight of 270 ± 36 kg). Calf birth weight was assessed after parturition. Heifers were distributed into four production systems following a completely randomized block design, with 12 animals in each treatment: open pasture (OP); two silvopastoral systems (SPSs—single rows or SPSt—triple rows), and crop-livestock system (CL). Thermal comfort was evaluated using the black globe, as well as humidity index (BGHI) and radiant thermal load (RTL). Animals in all treatments were exposed for long periods to heat stress; however, thermal indexes got lesser values in the SPS treatments. Heifers from the SPSt achieved the greatest serum concentration of IGF-I, but the follicular diameter did not differ among systems, as initially expected. The greatest weight gain and RFT standards were found in heifers of the CL system. Calves born from females of exclusive livestock (OP) showed the smallest weight at birth. Therefore, we suggested the adoption of the SPS and CL systems for livestock beef ranches as Nellore heifers reached better zootechnical and physiologic parameters associated with sexual precocity.