Acid-base equilibrium of Brazilian locally adapted sheep under heat stress in a semi-arid region

The Morada Nova is a Brazilian non-wool sheep, used for meat production that can present three coat colors: the black, the white and the red coat. However, black coated and white coated animals are in risk of extinction, once both are considered less adapted to warm conditions. The aim of this work was to evaluate thermoregulatories responses and their influence on acid-base equilibrium in red, black and white varieties of Morada Nova sheep, before, during and after the exposure to heat stress. Rectal temperature (RT, ºC), respiratory rate (RR, breaths.minute^-1), skin temperature on the neck (STN, ºC), skin temperature on the flank (STF, ºC), cutaneous evaporation on the neck (NEv, W.m^-2) and on the flank (FEv, W.m^-2), were collected once a week, from 5 to 6 AM, from 12 to 1 PM and from 5 to 6 PM. The hemogasometric tests consisted in: blood pH (pH), carbon dioxide pressure (PCO2), Oxygen pressure (PO₂), base excess (BECEF), concentration of bicarbonate ions (HCO₃^-), Oxygen saturation (SO₂). Meteorological data were registered, and Black Globe Humidity Index and Radiant heat Load were estimated. The lower means of rectal temperature and respiratory rate were registered before the exposure to heat stress and important increases of RT and RR were registered during heat stress. The three varieties used intensively respiratory thermolysis during heat stress. The black animals showed most intensive thermoregulatories responses, followed by the red ones, however they did not keep the homeothermy. The white ones exhibited lower warm and kept normal temperature, what is advantageous for animals raised under high radiation. No differences in cutaneous evaporation were verified among black, red and white coated animals. During heat stress, the pH increased and the PCO₂ and HCO₃^- decreased in white and in red-coated ewes, characterizing a respiratory alkalosis, whereas the black animals presented also an increase of HCO₃^-, which suggests respiratory and metabolic alkalosis. Four hours after heat stress peak the animals recovered homeothermic and normal blood gases patterns. We concluded that respiratory thermolysis is the most important mechanism used for sheep; however, the excessive respiratory effort during heat stress can cause acid-base disequilibrium.