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 controlled climate rooms (CCR): firstly, the feedlot steer on a grain-based diet mounts appropriate physiological responses during moderate heat load and in recovery so that i 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 three 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.

中文翻译：

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与瘤胃温度的椭圆和线性关系支持在暴露于中等热负荷的饲养场牛恢复期间 DMI 的顺势轨迹

澳大利亚的大多数饲养场动物在夏季都会经历 2 到 3 次中等热浪。本研究旨在了解此类事件的响应和恢复过程中的生理驱动因素，以设计确保快速安全恢复的策略。在受控气候室 (CCR) 的热挑战和恢复过程中测试了两个假设：首先，以谷物为基础的饮食的饲养场在中等热负荷和恢复期间会产生适当的生理反应，因此恢复后的性能和生理状态不会受到影响。与进给限制热中性 (FRTN) 转向不同。其次，常用的增加热负荷的指标，例如呼吸频率（RR）、喘气评分（PS）、体表温度（ST）和水消耗量（WC），反映了热挑战和恢复期间的瘤胃温度（RT）。日常收入水平。在这项研究中，36 头安格斯公牛（活重 (LW) 451.5 ± 22.6 kg）组成 3 个组，每组 12 只动物，依次经历 CCR 阶段。在这个 18 天的阶段，阉牛被分配到中等热负荷处理（热挑战，TC，n = 18）或 FRTN 处理（n = 18）。TC组经历三个阶段：预挑战（4天，温度湿度指数（THI）范围为68至71）、挑战（7天，THI 73至84，昼夜循环）和恢复（7天，THI 68至71） ）。FRTN 组在 CCR 中处于热中性条件下 (THI 66.9 ± 0.3)，每只动物的饲料量基于其 LW 匹配的 TC 对的采食量。因此，由于挑战期间 TC 组的 DMI 下降，因此对 FRTN 组实施了饲料限制。由训练有素的观察员收集的数据包括 DMI、RT、RR、PS、身体 ST（前额、肩部、腿部、臀部）和 WC。挑战在 TC 组中引起热应激反应，DMI 和 LW 降低，RT、RR、PS、身体 ST 和 WC 升高（P < 0.001）。FRTN 组中这些指标没有变化或有所减少（P < 0.001）。在恢复结束时，TC 和 FRTN 小组在包括 LW 在内的大多数措施上趋于一致。两组的日平均 RT 与 THI、RR、PS、头 ST 和 WC 均表现出较强的线性关系（P ≤ 0.0022），但与 DMI 呈相反的椭圆关系；也就是说，对于 TC 组，DMI 随着 RT 的增加而下降，而对于 FRTN 组，DMI 随着 RT 的增加而增加。总而言之，本研究中的饲养场公牛表现出足够的顺势能力来适应适度的热负荷并从中恢复。