Evaporative cooling and cutaneous surface temperature of Holstein cows in tropical conditions

The effects of skin temperature (T-S) on the rate of heat loss by cutaneous evaporation (E-S) in Holstein cows chronically exposed to sun, considering hair coat colour were studied. Sixteen purebred cows were measured for E-S and T-S at 01:00 p.m. after 6 hours of exposure to sun, on three body regions (flank, neck and gluteus) and considering dark and white spots separately. Sweating rate (S) and E-S were measured by means of a ventilated capsule. Black skin areas presented mean S (138.9 +/- 8.5 gm(-2) h(-1)), E-S (93.3 +/- 5.7 Wm(-2)), and T-S (33.1 +/- 0.2 degrees C) higher than those in the white areas (109.5 +/- 9.7 gm(-2)h(-1)), 73.6 +/- 6.5 Wm(-2) and 32.6 +/- 0.2 degrees C, respectively). There is an exponential relationship among cutaneous temperature and cutaneous evaporation, which can be represented by the equation: E-S = 31.5 + exp{(TS - 27.9)/2.19115}, with coefficient of determination r(2) = 0.68. Cutaneous evaporative heat loss remains almost constant around 48 Wm(-2) until T-S reaches nearly 31 degrees C.