506 Residual feed intake and thermotolerance in grazing lactating first-calf beef heifers: Effects on metabolism, performance, rumen temperature, and activity behavior

Abstract Beef cattle have been selected for feed efficiency to reduce feeding costs and environmental impact, but there is a knowledge gap regarding residual feed intake (RFI) and thermotolerance in grazing beef females. Therefore, this study evaluated blood parameters, rumen temperature (RT), activity behavior, and performance of grazing lactating first-calf beef heifers with divergent RFI during summer. Crossbred beef heifers n = 35; 432 ± 8.40 kg of body weight (BW) and 26 ± 1 mo of age previously tested and classified for RFI as more (n = 17; LOW-RFI = -0.8 ± 0.214) or less efficient (n = 18; HIGH-RFI = 1.5 ± 0.220) were used in a completely randomized design, grazing continuously a single pasture during summer (June to August). An accelerometer-based sensor recorded lying, standing, and step counts, while RT was automatically recorded using a rumen bolus throughout the 64-d experimental period. Body weight and blood samples were collected on d 0, 13, 28, and 64, while body fat deposition was assessed via ultrasonography on 0, 13, 28, and 50 d. Plasma was analyzed for concentrations of blood urea nitrogen (BUN), non-esterified fatty acids (NEFA), free triiodothyronine (fT3), heat shock protein 70 (HSP70), and whole blood used to perform a complete blood cell count. Environmental conditions were assessed by calculating the Climate Comprehensive Index (CCI) using temperature, wind speed, solar radiation, and humidity from a weather station within 1 km of the pasture. The resulting CCI index categorized environmental stress from non-existent to imposing extreme danger risk. An RFI × day interaction was observed for NEFA (P = 0.08), with LOW-RFI showing greater concentrations vs. HIGH-RFI on a period classified as not imposing environmental stress. An effect of day (P 0.01) was detected for BUN, fT3, and HSP70. An RFI × hour (P = 0.02) interaction was detected for RT, where RT was greater from 1400 h to 0600 h in HIGH-RFI. White blood cell counts (P = 0.04) and mean corpuscular volumes were greater (P = 0.05), while plateletcrit was less (P = 0.01) in LOW-RFI. An RFI × day interaction was found for monocytes and monocytes% (P = 0.04), with greater concentrations 24-h after severe stress in HIGH-RFI. The HIGH-RFI tended to have greater RIB fat at d 50 (P = 0.06) despite no differences in gain or BW between groups (P ≥ 0.26). An interaction between RFI × hour (P 0.01) showed that HIGH-RFI had a greater number of steps and standing time and less lying time during the hottest hours of the day. In summary, this study revealed that RFI influenced physiological and behavioral reactions to environmental stress, favoring the more feed-efficient first-calf beef heifers.