PSIX-26 Performance and body composition of early-maturing Zebu heifers under different supplementation levels on pasture during pre-puberty.

Abstract Genetic selection and nutritional management have advanced puberty onset in Zebu cattle, enabling first calving at two years. To achieve this, heifers must reach 55–60% of their adult body weight (BW) in the peri-pubertal period. Nutrition influences weight gain rate and through neuroendocrine modulation. Achieving the target weight is essential; however, in grazing systems, the weight gain rate can vary. This study evaluated supplementation levels during the dry season on the performance and body composition of Nellore heifers. A total of 44 heifers (298 days average age) were evaluated over seven months: a four-month of growing phase, a one-month follicular evaluation to assess corpus luteum presence, and a two-month of reproductive phase, during which heifers underwent fixed-time artificial insemination (FTAI). During the growing phase, heifers received one of four treatments: a control group (ad libitum mineral mix) or supplementation at 3, 6, or 9 g/kg BW (330 g crude protein (CP)/kg dry matter). The 6 g/kg level met 100% of CP requirements to growth, while the 3 and 9 g/kg levels provided 50% and 150%, respectively. After this phase, all heifers received the same nutritional management. BW and carcass traits, including Longissimus dorsi rib-eye area (REA) and subcutaneous fat thickness at the rib (SFTrib) and rump (SFTrump), were measured at the end of the growing, follicular evaluation, and reproductive phases. Data were analyzed using a completely randomized design with the GLIMMIX procedure of SAS (version 9.4), adopting a 5% significance level. A linear effect (P = 0.01) was observed for BW, average daily gain (ADG), REA, SFTrib, and SFTrump. At the end of the growing phase, heifers supplemented at 6 and 9 g/kg BW had greater BW (P = 0.001). All groups reached the target BW before the reproductive phase, with greater weight in the 6 and 9 g/kg groups (P < 0.001). By the end of the reproductive period, heifers supplemented at 3, 6, and 9 g/kg exhibited similar BW (P = 0.007). A significant interaction (P < 0.001) between supplementation and ADG showed that heifers receiving 9 g/kg BW had the highest ADG during the growing (P < 0.001) and follicular (P = 0.001) phases but the lowest at the end of the reproductive phase (P < 0.001). No corpus luteum was detected during the follicular evaluation. Final pregnancy rates were 17%, 53%, 81%, and 50% for the control, 3, 6, and 9 g/kg BW groups, respectively. Despite all groups reaching the target BW before reproduction, differences in conception rates and fetal losses suggest that a steady weight gain, with corresponding body composition modifications, enhances productive and reproductive performance.