Context Annually, millions of sheep are exported from Australia to the Middle East, typically during the southern hemisphere winter to the northern hemisphere summer. During these voyages, sheep can be exposed to relatively rapid changes in ambient conditions within a short period of time (≤29 days); therefore, excessive heat load concerns can arise. Aims The aim of this study was to define the responses of sheep to incremental heat load under simulated live export conditions. The study herein describes (1) the heat load imposed, and (2) the effect of this heat load on the growth, performance and behavioural responses of sheep during periods of incremental heat load. Methods A total of 144 Merino wethers (44.02 ± 0.32 kg) were included in a 29-day climate controlled study using two cohorts of 72 sheep (n = 2), exposed to two treatments: (1) thermoneutral and (2) hot (HOT). Ambient temperature (°C) and relative humidity (%) for the HOT treatment were modelled from live export voyages from Australia to the Middle East in July. Climatic conditions within the climate control chambers were recorded at 10-min intervals, then used to calculate a temperature humidity index. Sheep posture, rumination, eating, drinking and demeanour (calm, agitated or depressed) were observed four times daily at 3-h intervals between 0800 hours and 1700 hours. Feed intake was recorded daily and water intake was measured using an automated meter. Sheep were weighed on Day 0 and then at 7-day intervals. Sheep were weighed at slaughter and carcass weights were obtained, these data were used to determine carcass dressing percentage. Data were analysed using a repeated measures model, with a compound symmetry covariance structure. Key results Climatic conditions in the HOT treatment increased incrementally between Day 1 (temperature humidity index ≥19) and Day 29 (temperature humidity index ≤34.7). Behaviour, feed intake, average daily gain (g/day), carcass weight (kg) and dressing percentage were not influenced by treatment (P > 0.05). Sheep in the HOT treatment group showed a 137% increase in water intake (P < 0.01) and on average consumed 2.15 L/sheep.day, whereas the thermoneutral group consumed 1.67 L/sheep.day. Conclusions These results suggest that these sheep were capable of maintaining feed intake and growth despite exposure to heat load, albeit with a 137% increase in water intake. Implications These results highlight the resilience of the Australian Merino genotype, as these sheep were capable of maintaining feed intake and growth during exposure to heat load. As the climatic conditions in this study were modelled based on typical live export vessel conditions, these results may suggest that the climatic conditions experienced by sheep during voyages may not be as critical as previously thought.

中文翻译：

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热负荷对美利奴羊的影响。1. 成长、表现、行为和气候

背景 每年，数以百万计的绵羊从澳大利亚出口到中东，通常在南半球冬季到北半球夏季。在这些航行中，绵羊可以在短时间内（≤29 天）暴露于环境条件的相对快速变化中；因此，可能会出现过多的热负荷问题。目的 本研究的目的是确定绵羊在模拟活体出口条件下对增加的热负荷的反应。本文的研究描述了 (1) 施加的热负荷，以及 (2) 在热负荷增加期间，这种热负荷对绵羊的生长、性能和行为反应的影响。方法 共有 144 只美利奴羊 (44.02 ± 0.32 kg) 被纳入一项为期 29 天的气候控制研究，使用两组 72 只绵羊 (n = 2)，暴露于两种处理：(1) 热中性和 (2) 热 (HOT)。HOT 处理的环境温度 (°C) 和相对湿度 (%) 是根据 7 月份从澳大利亚到中东的实时出口航程建模的。以 10 分钟的间隔记录气候控制室内的气候条件，然后用于计算温度湿度指数。在 0800 小时和 1700 小时之间以 3 小时的间隔每天观察四次羊的姿势、反刍、进食、饮水和举止（平静、激动或沮丧）。每天记录采食量，并使用自动仪表测量水摄入量。绵羊在第 0 天称重，然后每隔 7 天称重。羊在屠宰时称重并获得屠体重量，这些数据用于确定屠宰率。使用重复测量模型分析数据，具有复合对称协方差结构。主要结果 HOT 处理中的气候条件在第 1 天（温湿度指数 ≥ 19）和第 29 天（温湿度指数 ≤ 34.7）之间逐渐增加。行为、采食量、平均日增重（g/天）、胴体重（kg）和屠宰率不受处理的影响（P > 0.05）。热治疗组的羊水摄入量增加了 137% (P < 0.01)，平均消耗了 2.15 升/羊.天，而热中性组消耗了 1.67 升/羊.天。结论 这些结果表明，尽管暴露在热负荷下，这些绵羊仍能保持采食量和生长，尽管饮水量增加了 137%。含义 这些结果突出了澳大利亚美利奴羊基因型的恢复力，因为这些绵羊能够在暴露于热负荷期间保持饲料摄入量和生长。由于本研究中的气候条件是基于典型的活体出口船舶条件建模的，因此这些结果可能表明，绵羊在航行期间所经历的气候条件可能不像以前认为的那么严重。