RESUMEN
Most feedlot animals in Australia experience 2 to 3 moderate heat waves during summer. This study aimed to gain understanding of the physiological drivers in response to and during recovery from such events with a view to designing strategies to ensure rapid and safe recovery. Two hypotheses were tested during thermal challenge and recovery in climate-controlled rooms (CCR): firstly, the feedlot steer on a grain-based diet mounts appropriate physiological responses during moderate heat load and in recovery so that its performance and physiology state after recovery is not different to the feed restricted thermoneutral (FRTN) steer. Secondly, commonly used indicators of increased heat load, e.g., respiration rate (RR), panting score (PS), body surface temperatures (ST), and water consumption (WC), reflect rumen temperature (RT) during thermal challenge and recovery at the level of daily means. In this study, 36 Angus steers (live weight (LW) 451.5 ± 22.6 kg) made up 3 cohorts of 12 animals that sequentially underwent the CCR phase. For this 18-d phase, the steers were allocated to either a moderate heat load treatment (thermally challenged, TC, n = 18) or a FRTN treatment (n = 18). The TC group underwent 3 periods, Pre-Challenge (4 d, temperature humidity index (THI) range of 68 to 71), Challenge (7 d, THI 73 to 84 with diurnal cycling), and Recovery (7 d, THI 68 to 71). The FRTN group were held at thermoneutral conditions in the CCR (THI 66.9 ± 0.3), and each animal was offered an amount of feed was based on the feed intake of its LW matched TC pair. Thus, as DMI fell in the TC group during Challenge, feed restriction was imposed on the FRTN group. The data were collected by trained observers were DMI, RT, RR, PS, body STs (forehead, shoulder, leg, rump), and WC. Challenge induced a heat stress response in the TC group with reduced DMI and LW, and elevated RT, RR, PS, body STs, and WC (P < 0.001). These measures were unchanged or reduced in the FRTN group (P < 0.001). At the end of Recovery, the TC and FRTN groups had converged on most measures including LW. Daily mean RT of both groups showed strong linear relationships with THI, RR, PS, head ST, and WC (P ≤ 0.0022) but opposing elliptical relationships with DMI; that is, as DMI fell with increasing RT for the TC group, DMI increased with rising RT for the FRTN group. In all, the feedlot steers in this study demonstrated sufficient homeorhetic capacity to adjust to moderate heat load and recover from it.
Over any summer, most feedlot steers in Australia will encounter 2 to 3 moderate heat waves lasting 5 to 6 d. The ability of these animals to adjust to the hot conditions and recovery from them is not well understood. The advent of obtaining near real-time body (rumen) temperature from feedlot steers has the potential to help feedlot managers, veterinarians and nutritionists make decisions on the handling of feedlots animals during and after heatwaves. Using climate-controlled rooms, this study demonstrated that feedlot steers are physiologically capable of dealing with moderate heatwave conditions. The co-ordinated increases in respiration rate, sweating rate, body surface temperature, and water consumption alongside reduction in feed intake as heat load increases limits the rise in body temperature. By the end of a 7-d recovery period in thermoneutral conditions, the heat stressed steers were not different physiologically and performance-wise from thermoneutral counterparts feeding on a similar amount of food. However, during the recovery period, all animals appeared to be metabolically and physiologically constrained, and did not return to their pre-heatwave feed intake, body temperature or respiration rate. These findings will assist the management of feedlot animals through recovery after a heat wave.
