Reproductive physiology and endocrinology responses of cows exposed to environmental heat stress - Experiences from the past and lessons for the present.

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.