Heat stress (HS) has a pronounced deleterious effect on fertility in dairy herds throughout the world, especially in hot and humid summer months in tropical and subtropical areas. Summer HS reduces feed intake and increases negative energy balance, induces changes in ovarian follicular dynamics, reduces estrus detection rates and alters oviductal function leading to fertilization failure and early embryonic death. Furthermore, oocytes harvested from lactating cows during summer HS have a decreased ability to develop to the blastocyst stage after in vitro fertilization when compared with oocytes harvested during winter. The present manuscript describes the detrimental effect of HS on reproduction, with emphasis on preovulatory oocytes and carry over effects of HS on embryo development and P/AI. Embryo transfer (ET) has been an effective tool to reestablish fertility during HS because it bypasses the damage to the oocyte and early embryo caused by hyperthermia. Therefore, a management strategy to maintain increased fertility throughout the year would be to produce embryos during the cooler months, when oocyte quality is greater, and use them to produce pregnancies during the periods of HS, when oocyte quality is compromised. However, this strategy only can be implemented using cryopreserved embryos, what is still limiting. During the warmer months, the use of heifers or non-lactating cows as oocyte or embryo donors may facilitate embryo production, mainly because of the lesser deleterious effects of HS comparing to lactating cows. Also, genetic selection of donors for thermoregulation ability is one potential strategy to mitigate effects of HS and increase embryo production during the warmer months. These alternatives enable the transference of fresh embryos with more efficiency during HS periods. Additionally, the application of timed ET protocols, which avoid the need for estrus detection in recipients, has facilitated management and improved the efficiency of ET programs during HS.

中文翻译：

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使用胚胎移植减轻热应激引起的不孕症

热应激 (HS) 对全世界奶牛群的生育能力具有明显的有害影响，尤其是在热带和亚热带地区炎热潮湿的夏季。夏季 HS 会减少采食量并增加负能量平衡，引起卵巢卵泡动力学的变化，降低发情检测率并改变输卵管功能，导致受精失败和早期胚胎死亡。此外，与冬季收获的卵母细胞相比，在夏季 HS 期间从泌乳牛身上收获的卵母细胞在体外受精后发育到囊胚阶段的能力降低。本手稿描述了 HS 对生殖的不利影响，重点是排卵前的卵母细胞和 HS 对胚胎发育和 P/AI 的影响。胚胎移植 (ET) 已成为 HS 期间重建生育能力的有效工具，因为它绕过了因体温过高而对卵母细胞和早期胚胎造成的损害。因此，保持全年生育率增加的管理策略是在较冷的月份产生胚胎，此时卵母细胞质量更高，并在卵母细胞质量受损的 HS 期间使用它们进行妊娠。然而，这种策略只能使用冷冻保存的胚胎来实施，这仍然是有限的。在温暖的月份，使用小母牛或非泌乳牛作为卵母细胞或胚胎供体可能会促进胚胎生产，主要是因为与泌乳牛相比，HS 的有害影响较小。还，供体的体温调节能力的遗传选择是减轻 HS 影响和在温暖的月份增加胚胎产量的一种潜在策略。这些替代方案能够在 HS 期间以更高的效率转移新鲜胚胎。此外，定时 ET 协议的应用避免了接受者发情检测的需要，促进了管理并提高了 HS 期间 ET 程序的效率。