Effects of seasonal chronic heat stress on body thermoregulation, cortisol release and uterine health in postpartum native Alentejana and Mertolenga beef cattle.

Selection towards resilience to heat is a main goal to mitigate Heat Stress (HS) effects. Resilient breeds entail a genetic pool to uncover novel thermoregulatory mechanisms. This study evaluated the response to seasonal chronic HS and mechanisms behind thermo-resistance in two heat-resilient native cattle breeds. Multiparous cows of Bos taurus native breeds Alentejana (ALT, n = 34) and Mertolenga (MERT, n = 55), maintained in continuous pasture, were examined at 40 ± 6 d postpartum at the end of Summer (S) and Winter (W). Cow-side temperature-humidity index (THI) and body temperatures were measured, blood was collected for BHB, leptin, total T3 and T4, neck hair was collected for cortisol measurement, and the genital tract was evaluated by ultrasonography, endometrial cytology and biopsy. In ALT cows, rectal temperature was similar in both seasons, whereas in MERT cows it was greater (p < 0.01) in S compared to W. Mean THI of previous 60 days attenuated the increment in body temperatures, revealing a significant (p < 0.01) acclimation mechanism, which was more efficient in ALT. In summer, T3 decrease was more accentuated in ALT than MERT cows (34% vs. 13%, p < 0.05). This highlights the breed's specific thermoregulatory traits. Hair cortisol concentrations were not affected by season, thereby failing as a reliable indicator of HS in native breeds. Endometrial cytology proportion of polymorphonuclear neutrophils (PMN) was reduced with increasing THI (p < 0.01), independent of cortisol secretion. In conclusion, this study identified chronic stress phenotypic markers of beneficial thermoregulatory mechanisms of heat resilient cattle breeds, relevant for selection programs.