Conforto térmico, comportamento e aspectos reprodutivos de fêmeas bovinas a pasto

Abstract

Bovine heat stress responses have been increasingly studied to identify breeds and even individuals more adapted to certain regions. It is possible to identify more adapted individuals through their physiological, behavioral, and reproductive responses to the adverse environment. Thus, the objective of this study was to evaluate the indices of thermal comfort, behavior, and reproductive aspects of bovine females on pasture. For this, two experiments were carried out, using Nellore females in a tropical climate region. In the first experiment, 84 animals were used, divided into two groups: area with shading (n = 42) and area with little shading (n = 42). Among the groups, assessments were made of physiological parameters (heart rate-HR, respiratory rate-RR, rectal temperature-RT, surface skin temperature-ST, and vaginal temperature-Tvag), reproductive parameters (estrus incidence, conception rate, and pregnancy loss). There was a significant reduction in the body temperature of cows managed in shady paddocks. Such reduction was observed for TR (p=0.002), TS (p<0.001) in the evaluated points (flank region, forehead and shoulder), Tvag (p<0.001). A trend (p = 0.08) of higher estrus expression was observed in cows in the shading group, but no variation was observed in the rate of conception and pregnancy loss. In conclusion, the shading environment provided better thermal comfort conditions for Nelore animals in tropical regions but did not interfere with reproductive performance in the breeding season. In the second experiment, 10 cows were used, divided into two experimental groups, evaluated in the dry season (June to October) and wet season (November to April) in a tropical region. Evaluations were made between the groups of physiological parameters (heart rate-HR, respiratory rate-RR, rectal temperature-RT and temperature by thermographic imaging), reproductive parameters (oocyte viability rate), behavioral parameters (grazing time, rumination, idle and permanence in the shade), and biochemical analysis of blood serum and follicular fluid. The animals in the dry period showed an increase in RR (p=0.007), TR (p<0.001), and temperature of the ocular orbital region evaluated by thermography (p=0.05). In the wet period, females spent more time lying down during the daytime rumination, as well as the time spent in the shade was longer compared to the dry period. The oocyte viability rate was higher in the rainy period, when serum concentrations of nitric oxide and triglycerides were lower. The results of this study show that, despite the thermotolerance of Nelore females, the large variation in ambient temperature observed in the dry season represents a greater metabolic challenge for the animals, requiring the activation of thermoregulatory mechanisms and reducing oocyte quality.