The current review is designed with aims to highlight the impact of heat stress (HS) on calves and heifers and to suggest methods for HS alleviation. HS occurs in animals when heat gain from environment and metabolism surpasses heat loss by radiation, convection, evaporation and conduction. Although calves and heifers are comparatively heat resistant due to less production of metabolic heat and more heat dissipation efficiency, they still suffer from HS to some degree. Dry matter intake and growth performance of calves and heifers are reduced during HS because of redistributing energy to heat regulation through a series of physiological and metabolic responses, such as elevated blood insulin and protein catabolism. Enhanced respiration rate and panting during HS accelerate the loss of CO2, resulting in altered blood acid-base chemistry and respiratory alkalosis. HS-induced alteration in rumen motility and microbiota affects the feed digestibility and rumen fermentation. Decreased luteinizing hormone, estradiol and gonadotrophins due to HS disturb the normal estrus cyclicity, depress follicular development, hence the drop in conception rate. Prenatal HS not only suppresses the embryonic development by the impaired placenta, which results in hypoxia and malnutrition, but also retards the growth, immunity and future milk production of newborn calves. Based on the above challenges, we attempted to describe the possible impacts of HS on growth, health, digestibility and reproduction of calves and heifers. Likewise, we also proposed three primary strategies for ameliorating HS consequences. Genetic development and reproductive measures, such as gene selection and embryo transfers, are more likely long-term approaches to enhance heat tolerance. While physical modification of the environment, such as shades and sprinkle systems, is the most common and easily implemented measure to alleviate HS. Additionally, nutritional management is another key approach which could help calves and heifers maintain homeostasis and prevent nutrient deficiencies because of HS.

中文翻译：

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犊牛和小母牛的热应激：综述。

当前的审查旨在突出强调热应激（HS）对犊牛和小母牛的影响，并提出减轻HS的方法。当来自环境和新陈代谢的热量因辐射，对流，蒸发和传导而超过热量损失时，HS就会在动物体内发生。尽管小牛和小母牛由于代谢热的产生较少且散热效率更高而相对耐热，但它们在一定程度上仍遭受HS的折磨。在HS期间，小牛和小母牛的干物质摄入量和生长性能下降，因为通过一系列生理和代谢反应（例如血液胰岛素和蛋白质分解代谢升高）将能量重新分配给热量调节。HS期间呼吸频率的提高和气喘加速了CO2的流失，导致血液酸碱化学改变和呼吸性碱中毒。HS引起的瘤胃动力和微生物群的改变影响饲料的消化率和瘤胃发酵。黄体生成激素，促黄体生成激素，雌二醇和促性腺激素的减少会干扰正常的发情周期，抑制卵泡发育，从而降低受孕率。产前HS不仅会抑制胎盘受损导致的胚胎发育，从而导致缺氧和营养不良，而且会阻碍新生小牛的生长，免疫力和未来的产奶量。基于上述挑战，我们试图描述HS对小牛和小母牛生长，健康，消化率和繁殖的可能影响。同样，我们还提出了三种改善HS后果的主要策略。遗传发展和生殖措施，例如基因选择和胚胎移植，是提高耐热性的长期方法。虽然对环境进行物理修改（例如阴影和洒水系统）是减轻HS的最常见且最容易实现的措施。另外，营养管理是另一种关键方法，可以帮助牛犊和小母牛保持体内平衡，并防止由于HS造成的营养缺乏。