This work aims to study the relationship between variations of the Temperature‐Humidity Index (THI) and the parameters of reproduction especially the first conception rate (FCR) and to determine the threshold THI value where cows’ fertility rate dropped in 12 Holstein dairy herds raised in the arid climatic conditions of Tunisia. THI values were calculated over 22 years (1996–2018), and the mean monthly temperature and relative humidity data were obtained from the Meteorological Institute of Tunisia. A total of 20,396 individual records (Insemination and calving dates) were extracted from the Livestock and Pasturing Office (OEP, Tunisia) with regard to the highest THI before breeding, on the breeding day, and after breeding. Statistical analysis was performed using the GLM procedure of SAS software. Results point to the fact that a summer heat stress exists in southeast Tunisia and lasts for 4 months starting from June until September with THI values fluctuating between 73 ± 2.38 and 79 ± 3.01 exceeding, therefore, THI threshold of 72. Increased THI from ≤70 to ≥80 units was associated with drops in conception rate (CR) and fertility rate (FR) of 49% and 45% giving a correlation with the THI of (r = −.72, p < .05) and (r = −.74, p < .05), respectively. When cows were inseminated on extremely hot days (THI ≥ 80) preceded by cooler temperatures, pregnancy by service (P/AI) was 7% points higher than for other cows that were exposed to high temperatures before breeding. The average number of insemination was higher (p < .05) from THI ≤ 70 (2.01) compared to THI ≥ 80 (3.41). Cows calving during an absence of heat stress (THI ≤ 70) have the shortest average calving intervals (CI: 420 ± 15.1 days). Contrastly, calving in the condition of heat stress (THI ≥ 80) has the longest CI (487 ± 12.8 days). For each point increase in the THI value above 67, there is a decrease in the first conception rate by 1.39%. In this particular arid environment, high‐yielding Holstein cows’ breeding success is strongly affected by heat stress that takes place just before or after breeding.

中文翻译：

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热应激对北非干旱地区荷斯坦奶牛繁殖效率的影响。

这项工作旨在研究温度-湿度指数（THI）的变化与繁殖参数（尤其是第一个受胎率（FCR））之间的关系，并确定在12头荷斯坦奶牛场中，母牛的受精率下降的THI阈值在突尼斯的干旱气候条件下。THI值是在22年（1996-2018年）内计算得出的，平均每月温度和相对湿度数据是从突尼斯气象学院获得的。从育种前，育种日和育种后的最高THI值方面，总共从畜牧和畜牧处（突尼斯OEP）提取了20,396条个人记录（授精和产犊日期）。使用SAS软件的GLM程序进行统计分析。r  = -.72，p  <.05）和（r  = -.74，p  <.05）。当母牛在极热的日子（THI≥80）且温度较低时进行授精时，按服务怀孕（P / AI）比其他在繁殖前暴露于高温的母牛要高7％。平均受精次数更高（p THI≤70（2.01），而THI≥80（3.41）则为<0.05。在没有热应激（THI≤70）的情况下产犊的平均产犊间隔最短（CI：420±15.1天）。相反，在热应激条件下（THI≥80）产犊的CI最长（487±12.8天）。对于高于67的THI值每增加一点，第一次受孕率就会降低1.39％。在这种特殊的干旱环境中，高产荷斯坦奶牛的繁殖成功受到繁殖前后热应激的强烈影响。