Short communication: thermotolerance and residual feed intake in <i>Bos-indicus</i> crossbred beef heifers

This study investigated the relationship between thermotolerance and residual feed intake (RFI) in <i>Bos indicus</i> crossbred replacement beef heifers. In the summer of 2019, vaginal body temperatures were recorded from 654 heifers to identify the most and the least heat-tolerant animals based on changes in body temperature throughout the day. From this group of heifers, 27 were ranked in the summer as either heat-tolerant (n = 14; thermotolerant) or least heat-tolerant (n = 13; non-thermotolerant) to enroll in a 42-d feed efficiency test conducted in the fall. The maximum body temperature minus the 0700 h body temperature difference between the groups was 0.19 °C for thermotolerant and 1.2 for non-thermotolerant heifers (<i>P</i> < 0.01). In the fall, selected heifers (398 ± 11.3 kg of body weight [BW]; approximately 720 d of age) were randomly allocated into four concrete-floored pens, each with two GrowSafe feed bunks, and fed a common ad libitum total mixed ration (16.9% crude protein, 52% total digestible nutrients on a DM basis) for a 56-d period (14 d of adaptation followed by 42 d of evaluation). Body weight was collected on d -1, 0, 14, 28, 41, and 42, while fall body temperature was recorded (10 min intervals for 4 consecutive d) from d 29 to 32 of the study, using temperature probes placed intravaginally with a hormone-free controlled internal drug release device. Final BW, average daily gain (ADG), dry matter intake (DMI), DMI as a percentage of BW, RFI, and body temperature were analyzed as a completely randomized design. Final BW, ADG, DMI, and body temperature collected during the feeding period were not affected by thermotolerance phenotype (<i>P</i> ≥ 0.42); however, heifers previously classified as thermotolerant had decreased RFI (<i>P</i> = 0.01; -1.32 vs. 1.27 ± 3.31 kg, respectively) and DMI as a percentage of BW (<i>P</i> = 0.01) when compared with non-thermotolerant heifers. In conclusion, <i>Bos indicus</i> crossbred replacement beef heifers previously classified as thermotolerant based on multiple body temperature measurements collected throughout the summer had decreased RFI when measured in the fall, indicating the potential of feed efficiency in heat-tolerant beef heifers.