The objective of this study was to evaluate the influence of thermal stress indices and minimum temperature on the semen quality of dairy Gyr bulls and to determine whether the minimum night temperature is able to balance the thermal stress that occurred during the day. A total of 922 semen samples were collected from 391 bulls aged between 15 and 49 months, which participated in progeny pretest trials, from December 2011 to April 2017. Semen characteristics (ejaculate volume, gross motility, sperm motility, sperm vigor, sperm concentration, and percentage of total defects) and scrotal circumference were analyzed. Meteorological data were recorded daily by the National Institute of Meteorology. The analyses were carried out using mixed models. The following fixed effects were included in the model: test, month of sample collection, scrotal circumference, the age of the bull at sampling as covariate (linear regression), and the climatic variables (equivalent temperature index [ETI], temperature and humidity index [THI], or minimum temperature). The effect of the bull nested within the test was included in the model as a random effect. Two periods were considered to evaluate the effect of the climatic variables on sperm quality: 7 days before collection (period 1) and the day of collection (period 2). There was an effect of ETI on sperm vigor and concentration in both periods and on volume and percentage of total defects only in the second period. The THI exerted a significant effect on volume in both periods and on sperm motility and concentration only in the second period. There was an effect of the minimum temperature on ejaculate volume, sperm motility, sperm concentration, and percentage of total defects only in the second period, which corresponds to the day of semen collection. Taken together, the results show that there was sufficient thermal stress to negatively affect semen quality. However, the minimum temperature during the night was a significant factor that balanced these negative effects of THI and ETI on semen traits of Gyr bulls.

中文翻译：

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热应激对公牛基本精液特性的影响

本研究的目的是评估热应激指数和最低温度对奶牛 Gyr 公牛精液质量的影响，并确定夜间最低温度是否能够平衡白天发生的热应激。2011年12月至2017年4月，参加子代预检试验的391头15～49月龄公牛共收集922份精液样本。 精液特征（射精量、总活力、精子活力、精子活力、精子浓度、和总缺陷的百分比）和阴囊周长进行了分析。气象数据由国家气象研究所每天记录。使用混合模型进行分析。模型中包含以下固定效应：测试、样本收集月份、阴囊周长、采样时公牛的年龄作为协变量（线性回归）和气候变量（等效温度指数 [ETI]、温湿度指数 [THI] 或最低温度）。嵌套在测试中的公牛效应作为随机效应包含在模型中。考虑了两个时期来评估气候变量对精子质量的影响：收集前 7 天（时期 1）和收集日（时期 2）。ETI 对两个时期的精子活力和浓度都有影响，仅在第二个时期对总缺陷的体积和百分比有影响。THI 对两个时期的体积产生显着影响，仅在第二个时期对精子活力和浓度产生显着影响。最低温度对射精量、精子活力、精子浓度和总缺陷百分比仅在第二阶段，即与精液收集日相对应。总之，结果表明存在足够的热应力对精液质量产生负面影响。然而，夜间的最低温度是平衡 THI 和 ETI 对 Gyr 公牛精液性状的这些负面影响的重要因素。