The effects of environmental heat-stress on production and reproduction in dairy cows have been intensively studied throughout the past few decades. In light of climate changes and global warming, this issue has gained attention worldwide. So far, most of the documentations are related to warmer-climate regions, however, environmental thermal stress has recently been reported in cooler regions, such as Europe. The review attempts to present the experiences from the past years and lessons for the present. The review highlights some of the environmental characterizations and provides some practical approaches to estimate the level of heat load on farms. For instance, the intensity of heat stress can be evaluated by the temperature humidity index (THI). Other environmental parameters, such as the increased number of consecutive hot days or the increased frequency of extremely hot days, can be also used to estimate the level of heat load on farms. Exposure of dairy cows to environmental thermal stress results in multiple behavioral changes, physiological responses and endocrinological alterations, which in sequence, lead to reduced reproductive performance. Multiple in-vitro studies have been performed for better understanding the mechanism by which heat stress impairs reproductive processes. However, the current review focuses mainly on animal reactions and on the limitations of physiological and behavioral responses in maintaining normothermia, without human intervention. The review provides evidence that thermal stress induces alterations in the hypothalamus-pituitary-ovarian axis. For instance, impaired gonadotropin secretion, attenuation of follicular development, reduced steroid production and progesterone concentration in the plasma. These were found to be associated with impaired estrus behavior, reduced oocyte developmental competence and embryo survival. Heat stress also has direct and indirect effects on the preimplantation embryo. The review summarizes the thermo-sensitivity of the embryo and the acquisition of its thermotolerance through early developmental stages. Understanding the effects of environmentally elevated temperature on the reproductive physiology of lactating cows is extremely important for the development of new strategies in order to mitigate the effects of heat stress on farms. The review also provides various types of management and practical tools, in order to alleviate the effects of thermal stress. It introduces some approaches that have been developed during recent years, ones that have been practically used to alleviate the effect of the environmental heat load and suggested to be implanted. Cooling is the predominant strategy used nowadays in order to alleviate the effects of heat stress. It includes indirect cooling of the environment surrounding the animal, by providing shed and ventilation (with or without water) or direct evaporative cooling of the cow with water and fans. Using an efficient cooling system can improve milk production during the hot season, but it cannot eliminate the decline in reproduction. The review also discusses some additional approaches such as timed artificial insemination, hormonal treatment and embryo transfer, which have already been developed. These are suggested to be examined, adapted and implemented in dairy farms located in new regions that have recently suffered from environmental heat stress. The review also discusses unclear points and open questions some of which might lead new research directions.

中文翻译：

---

暴露于环境热应激的奶牛的生殖生理学和内分泌学反应——过去的经验和现在的教训

在过去的几十年中，环境热应激对奶牛生产和繁殖的影响得到了深入研究。鉴于气候变化和全球变暖，这一问题已引起全球关注。到目前为止，大多数文献都与气候温暖的地区有关，但是，最近在欧洲等较冷的地区报道了环境热应力。该评论试图介绍过去几年的经验和现在的教训。该评论重点介绍了一些环境特征，并提供了一些估算农场热负荷水平的实用方法。例如，热应激的强度可以通过温湿度指数 (THI) 来评估。其他环境参数，例如，连续炎热天数的增加或极热天数的增加，也可用于估计农场的热负荷水平。奶牛暴露于环境热应激会导致多种行为变化、生理反应和内分泌改变，进而导致繁殖性能下降。为了更好地了解热应激损害生殖过程的机制，已经进行了多项体外研究。然而，目前的审查主要集中在动物反应以及在没有人为干预的情况下维持常温的生理和行为反应的局限性。该评价提供了证据表明热应激会引起下丘脑-垂体-卵巢轴的改变。例如，促性腺激素分泌受损，卵泡发育减弱，血浆中类固醇生成和孕酮浓度降低。发现这些与发情行为受损、卵母细胞发育能力降低和胚胎存活有关。热应激对植入前胚胎也有直接和间接的影响。该综述总结了胚胎的热敏感性及其通过早期发育阶段获得的耐热性。了解环境高温对泌乳奶牛生殖生理的影响对于制定新策略以减轻热应激对农场的影响极为重要。该评论还提供了各种类型的管理和实用工具，以减轻热应力的影响。它介绍了近年来发展起来的一些方法，这些方法已被实际用于减轻环境热负荷的影响并建议植入。冷却是当今使用的主要策略，以减轻热应激的影响。它包括通过提供棚屋和通风（有水或无水）或用水和风扇直接蒸发冷却奶牛，间接冷却动物周围的环境。使用高效的冷却系统可以在炎热季节提高产奶量，但不能消除繁殖率下降。该评论还讨论了一些额外的方法，例如定时人工授精、激素治疗和胚胎移植，这些方法已经开发出来。建议检查这些，在位于最近遭受环境热应激的新地区的奶牛场中进行了调整和实施。该评论还讨论了不明确的点和开放性问题，其中一些可能会导致新的研究方向。