Heat together with high humidity is one of the causes of reductions in milk production and cattle fertility, especially in summer or in tropical and semitropical regions. The forecast of global warming may worsen such problem. In the warmer regions of Brazil, there is a preference for crossbreed cattle of the Holstein (H) and Zebu (Z) breeds, mainly Gir (G), which form the synthetic Girolando breed whose composition is ⅝ GxH. Animals of these breeds are more tolerant to heat than breeds of European origin, such as the Holstein. However, even these animals are sensitive to high heat, with great variability in thermotolerance, as previously demonstrated. To address the effects of excessive heat on the fertility and consequent productivity of these animals, which can be worsened by possible climate changes, this project evaluated Girolando females (heifers and cows) with blood levels of ¼ to ⅞ under conditions of high temperature and humidity. Animals were characterized phenotypically for thermotolerance for genome-wide association studies (GWAS). Four genes with biological functions in adaptive thermogenesis were identified and should be targets for future projects looking for alleles associated with thermotolerance. Animals with different genetic values (GV) for thermotolerance were challenged in a climatic chamber and parameters associated with the morphological quality of the oocytes and expression of proteins analyzed by immunofluorescence in the oocytes were evaluated. Although there was no difference in the viability of oocytes between animals with higher and lower GV subjected to stress in a climatic chamber, it was observed that oocytes from animals with lower GV (lower vaginal temperatures) showed greater expression of the capase 3 protein, an enzyme that is part of the process of cellular apoptosis, than oocytes from animals with higher GV when subjected to thermal stress. It was found that supplementing the bovine diet with chromium minimizes the effects of high ambient temperature on vaginal temperature and improves glucose metabolism. The information on genes associated with adaptive thermogenesis found in this project should be used in new studies to identify candidate alleles associated with heat stress. Furthermore, thermal stress on the oocyte influences the chromatin organization of embryos generated from fertilization, which must be related to lower embryonic viability and consequent lower reproductive efficiency under stress conditions.

The information presented in this document contributes to achieving Sustainable Development Goals (SDGs) number 1 (No poverty: End poverty in all its forms, everywhere), 2 (Eradication of hunger: End hunger , achieving food security and improved nutrition and promoting sustainable agriculture) and 13 (Combating Climate Change: Take urgent action to combat climate change and its impacts).