Heat stress negatively influences cattle welfare, health and productivity. To cope with the forecasted increases in temperature and heat waves frequency, identifying high-producing animals that are tolerant to heat is of capital importance to maintain milk production. This study, based on the joint analysis of on-farm performance and weather data, had two objectives: (1) to determine the response in production performances (milk, fat and protein yields, fat and protein contents) and udder health (somatic cell score) to temperature-humidity index (THI) variations in Montbeliarde cows, and (2) to estimate the interactions between genotype and THI, to enable the identification of the most adapted animals for facing the expected increases in temperature. Test-day records from first and second lactations from 2016 to 2020 were associated with the average THI during the three days before the test-day record. In total, 446,717 test-day records from 55,650 cows in first lactation and 457,516 test-day records from 58,229 cows in second lactation were analysed. The optimal THI was below 55 (i.e. ~ 12–13 °C) for all traits. Individual responses to THI were estimated by random regression models, which also included individual responses to days in milk. Regardless of the stage of lactation, genetic correlations along the THI gradient were above 0.80, which suggests that genotype-by-THI interactions were weak for production and udder health traits. Nevertheless, a variability in the individual slope of decay could be highlighted at high THI. The genetic correlation between production level at moderate THI and the slope at high THI was negative, while for somatic cell score, it was positive, indicating that heat stress amplifies the susceptibility to mastitis. The optimal THI for French Montbeliarde cows is below 55 for production and udder health traits. Genetic-by-THI interactions are weak in French Montbeliarde cows for production and udder health traits, but not all animals react in the same way to high temperatures. Even if there is little room for improvement, using a heat tolerance index in cattle selection would be relevant to anticipate the expected increases in temperature. Further investigations are needed to interpret this variability on production traits. However, the current selection for mastitis resistance seems appropriate to adapt cattle to rising temperatures.

中文翻译：

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通过温湿度指数相互作用估计蒙贝利亚德奶牛产奶量和乳房健康特征的基因型

热应激会对牛的福利、健康和生产力产生负面影响。为了应对预测的温度升高和热浪频率，确定耐热的高产动物对于维持牛奶产量至关重要。本研究基于对农场性能和天气数据的联合分析，有两个目标：(1) 确定生产性能（牛奶、脂肪和蛋白质产量、脂肪和蛋白质含量）和乳房健康（体细胞分数）到 Montbeliarde 奶牛的温度 - 湿度指数（THI）变化，以及（2）估计基因型和 THI 之间的相互作用，以便能够识别最适应的动物以应对预期的温度升高。2016 年至 2020 年第一次和第二次泌乳的测试日记录与测试日记录前三天的平均 THI 相关联。总共分析了第一个泌乳期 55,650 头奶牛的 446,717 个测试日记录和第二个泌乳期 58,229 头奶牛的 457,516 个测试日记录。所有性状的最佳 THI 均低于 55（即 ~ 12–13 °C）。通过随机回归模型估计个体对 THI 的反应，其中还包括个体对牛奶天数的反应。无论泌乳阶段如何，沿 THI 梯度的遗传相关性均高于 0.80，这表明基因型与 THI 的相互作用对于生产和乳房健康性状而言较弱。然而，在高 THI 时可以突出显示单个衰减斜率的可变性。中等 THI 的生产水平与高 THI 的斜率之间的遗传相关性为负，而体细胞评分为正，表明热应激增加了对乳腺炎的易感性。法国 Montbeliarde 奶牛的最佳 THI 在生产和乳房健康特性方面低于 55。法国 Montbeliarde 奶牛在生产和乳房健康特性方面的遗传与 THI 相互作用较弱，但并非所有动物对高温的反应都相同。即使几乎没有改进的余地，在牛的选择中使用耐热指数也与预测预期的温度升高有关。需要进一步研究来解释这种生产性状的变异性。然而，目前针对乳腺炎抗性的选择似乎适合让牛适应不断升高的温度。