Bos taurus indicus cattle have the superior ability for the regulation of body temperature during heat stress due to a number of physiological and cellular level adaptive traits. The objectives of this study were to quantify the change in body temperature in heifers with various proportions of Brahman genes per unit increase in heat stress as measured by temperature-humidity index (THI) and to assess how different breed groups responded to varying intensity and duration of heat stress. A total of 299 two-yr-old heifers from six breed groups ranging from 100% Angus to 100% Brahman were evaluated under hot and humid conditions during 2017 and 2018 summer days. Two strategies were used to estimate the plasticity in body temperature of breed groups in response to environmental challenges: 1) a random regression mixed model was used to estimate reaction norm parameters for each breed group in response to a specified environmental heat stress and 2) a repeated measures mixed model was used to evaluate the response to different environmental heat loads. The reaction norm model estimated an intercept and slope measuring the change in body temperature per unit increase in THI environmental heat stress for different breed groups of animals and allowed the identification of genotypes which are robust, with low slope values indicative of animals that are able to maintain normal body temperature across a range of environments. The repeated measures mixed model showed that Brahman cattle have an advantage under moderate or high heat stress conditions but both Angus and Brahman breed groups are greatly affected when heat stress is severe. A critical factor appears to be the opportunity to cool down during the night hours more than the number of hours with extreme THI. With heat stress conditions predicted to intensify and expand into currently temperate zones, developing effective strategies to ensure sustainable beef production systems are imperative. Effective strategies will require the identification of the genes conferring the superior thermotolerance in Brahman cattle.

中文翻译：

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婆罗门基因对热胁迫下牧场上小母牛体温可塑性的影响。

由于多种生理和细胞水平的适应性状，金牛座牛在热应激期间具有优异的体温调节能力。这项研究的目的是通过温度-湿度指数（THI）来量化每单位热应激增加下不同比例的婆罗门基因的小母牛的体温变化，并评估不同品种对不同强度和持续时间的反应热应激。在2017年和2018年夏季的炎热和潮湿条件下，对来自100％安格斯到100％婆罗门的六个品种组的299只2岁小母牛进行了评估。为了应对环境挑战，使用了两种策略来估计品种组体温的可塑性：1）使用随机回归混合模型来估计每个品种组对特定环境热胁迫的反应标准参数； 2）使用重复测量混合模型来评估对不同环境热负荷的响应。反应规范模型估计了截距和斜率，用于测量不同品种动物的THI环境热胁迫每单位升高的体温变化，并允许鉴定稳健的基因型，低斜率值表明动物能够在各种环境中保持正常的体温。重复测量混合模型表明，婆罗门牛在中度或高温胁迫下均具有优势，但是当严峻的高温胁迫条件下，安格斯和婆罗门品种都受到很大影响。一个关键因素似乎是，在夜间，降温的机会要多于极端THI的小时数。预计热应激条件会加剧并扩展到当前的温带地区，因此制定有效的策略以确保可持续的牛肉生产系统势在必行。有效的策略将需要鉴定赋予婆罗门牛优异耐热性的基因。