Resumo
Background: Heat stress (HS) can compromise the female reproductive system, resulting in a massive reduction of reproductive performance. During the period of follicular growth, HS may compromise the oocyte, either because of a direct effect of elevated temperature on the gamete or because of changes in follicular function that would damage oocyte quality. Oocytes harvested from Holstein cows during summer have reduced ability to develop to the blastocyst stage than those harvested during winter. Nevertheless, although there are reports showing a clear effect of HS on oocyte competence to development after IVF, the exact mechanisms by which the oocyte is compromised remain unknown. Review: Resistance to HS is dependent on cow's genotype. Bos indicus breeds (i.e. Gir) are more resistant to elevated temperature and humidity than breeds (i.e. Holstein) that evolved in a temperate climate. This resistant is, among other factors, due to the superior ability of certain breeds to regulate their body temperature. Thus, breed selection is present as a possible tool to minimize the effects of temperature and humidity on milk production and reproduction. On the other hand, Holstein cows are the most common breed used, even in tropical countries (i.e. Brazil), because of their yield in milk production. As a result, there is a considerable effect upon milk production and reproductive performance through the year in tropical and subtropical countries. A possible way to minimize this effect is the employment of reproductive biotechnologies such as fixed-time artificial insemination (FTAI) and embryo transfer (ET). FTAI has been shown to eliminate problems of estrus detection caused by HS. Nevertheless, it is not enough to complete restore herd pregnancy rates because oocytes and early embryos have already been damaged by HS. In contrast, embryos three days after conception orolder are less sensitive to HS. Thus, ET has been used to improve cow's reproductive performance during the hot months of the year (i.e. spring and summer). This is carried out by producing embryos during the fresh months but using them for ET during the HS period. Regarding fertility problems, it is recognized that repeat-breeder (RB) cows cause huge economic problems to the farmer. These cows are characterized by poor fertilization rates and/or early embryonic loss. It is still not certainly known why these cows become RB, when it is the critical period of the year for them, and what should be done to avoid this problem. We have observed that the conception rates in RB Holstein cows were greater after ET (41.7%) than after AI (17.9%). This indicates that ET may be an effective alternative to achieve satisfactory conception rates throughout the year in RB cows, especially during periods of HS. Moreover, when three categories of Holstein cows (heifers, high-producing cows in peak lactation and RB) were subjected to ovum pick up during the summer and the winter, blastocyst rates and quality were significantly affected by both, category and period, being RB cows mostly affected during the summer. Moreover, lower amounts of mitochondrial DNA were found in RB oocytes, suggesting that the low reproductive performance observed in these cows is related to some injure in oocyte quality. Conclusion: HS negatively affects the physiology and fertility of dairy cows and heifers. Concerning about the reproductive field, the effects of HS were detected in vitro and in vivo on oocytes and embryos and also on conception rates. Moreover, RB Holstein cows seem to be even more sensitive to HS and this can probably be related to their oocyte potential into develop in blastocysts. Therefore, efforts should be taken in order to improve the environmental conditions and heat resistance of dairy cows in tropical and subtropical areas. Studies are been conducted to understand the molecular level by which HS disrupt reproduction aiming to develop methods to attenuate or reverse HS consequences upon fertility of dairy cows.