Adaptative Responses of Brazilian Brahman Bulls in Different Seasons

Environmental high temperatures are detrimental to the productivity of livestock industry, because animals of better genotype normally have a higher endogenous heat, due to their metabolic activity. Heat stress is one of the most important factors that affect animal performance in tropical regions, meanly beef cattle managed in extensive field conditions. With the aim of develop efficient livestock systems in brazilian tropical regions, research about heat tolerant genotypes is increasing in the last years because of their ability to adjustments to climatic changes. The capability of animals to maintain the equilibrium with environment is directly associated with their ability to trigger thermoregulatory mechanisms, which depends on a pool of morphological and physiological traits acquired during the evolutionary process, in order to guarantee the efficiency of thermoregulation. Brahman cattle have strong skin pigmentation, which filters the intense short wave radiation as well as keeps the breed free of cancer. A special feature of Brahman breed is its higher ability to sweat freely in comparison with other breeds. This fact contributes to their heat tolerance greatly. Other adaptive advantages become Brahman well suited to many environmental conditions include the ability to utilize lower-quality food, to travel longer distances for feed and water, to resist insects and external parasites and the ability to reproduce on a regular basis in a stressful environment. A factor that contributes to the Brahman outstanding ability to withstand temperature extremes is a short, thick, glossy hair coat that reflects much of the sunrays, allowing them to graze at noon without suffering. The goal of this work was to evaluate the adaptive responses of young Brahman Bulls, based on thermoregulatory, morfophysiological and blood biochemistry parameters, during the dry and wet seasons. The study was performed in a commercial farm located in Uberlândia-MG, Brazil, at 18°55'S, in the Cerrado Bioma. 63 young Brahman bulls were sampled, aging about 233 to 264 days. The trial begun at the weaning and the assessment was conducted during seven months, including dry and wet season. Sampling were carried out once a month and the data were collected at 8:00 AM, under natural field conditions. The included environmental variables were wind speed (WS, m/s), air temperature (AT, °C), wet bulb temperature (WBT,°C) and black globe temperature (BGT, °C), utilized to estimate the radiant heat load (RHL, W/m²) and Black Globe Humidity Index (BGHI). The animal assessment based on rectal temperature (RT,°C); respiratory rate (RR, breaths for minute) and sweating rate (g/cm2/min). Blood parameters were hematologic analysis, thyroid hormones (T3 and T4) and Cortisol concentrations. The morphologic evaluations were coat density (CD, hair/cm2); coat thickness (CT, mm), hair length (HL, mm) and mean hair diameter (HD, mm); To test our hypothesis, were considered as fixed factors the collect and the animals. We did not detect significant differences in blood parameters, such as hematologic and thyroid hormones values. On the other hand, we observed significant effect of the month of sampling. In December and January, the animals exhibited highest values of rectal and skin surface temperature, followed by highest means of respiratory rate and sweating rate, as shown in Table 1. These results indicated the use of cutaneous and respiratory evaporative losses as an efficient way to dissipate the excessive heat. Principal Component Analysis (PCA) of physiological and morphological variables average showed that respiratory rate, sweating rate, coat thickness and hair length are the most relevant variables of bull distribution in axis 1 (29,8% of explained variance) and coat thickness, hair length and mean hair diameter in axes 2 (17% of explained variance). Therefore, bulls that showed high coat thickness and hair length used respiratory rate instead of sweating in order to maintain normal body temperature as axis 1 showed. Less relevant is the fact that some bulls characterized by high hair length, mean diameter and coat thickness did not modify physiological variables (as show axis 2). According to PCA results, most relevant variables were used to k-means clustering analysis. Bulls were classified in three groups basing on PCA most relevant variables (CT, HL, MD, respiratory rate and sweating rate): high, mean and low adaptive capacity. The high adaptive considered group showed lower CT, HL, RR and SR. Additionally, PCA of physiological and morphological variables standard deviation showed that bulls distribution depends on uneven level response of respiratory rate, sweating rate and hair diameter, as axis 1 showed (22,8% of explained variance). PCA axis 1 point out that those animals that tend to maintain mean diameter among samples showed uneven evaporative thermolysis, indicated by higher respiratory and sweeping rate standard deviation. Among animals characterized by stable respiratory and sweating rate (low value of PCA axis 1), PCA axis 2 shows that animals tend to maintain body temperature when they are characterized by uneven hair length and coat thickness among samples. The animals that exhibited higher phenotypical plasticity for hair coat traits were able to keep normal and stable physiological responses. We concluded that Brahman bulls were able to keep homoeothermic features under Brazilian cerrado conditions. For that reason this breed can be indicated to meat production in extensive system, however special attention must be pay to heat stress management during the hottest and the most humid months, since heat stress response increased even in more adapted animals. The authors acknowledge the Uberbrahman Group, specially Dr. Thiago Valente and Dr. Aldo Valente.