Sweating Rates of Dairy Cows and Beef Heifers in Hot Conditions

摘要

Sweating rates from heat-stressed dairy cows and beef heifers were measured using a "Portable Calorimeter" and a "Bovine Evaporation Meter" designed and fabricated for the studies reported herein. Measurements were taken when cows were in their natural habitat. The focus of the study was to compare sweating rates measured from different breeds of dairy cows and beef heifers, and determine the level of influence of environmental factors (air temperature, relative humidity, solar load, and air velocity) and hair-coat color on sweating rate. The cows were exposed to solar radiation greater than 500 W/m super(2) (average 833 plus or minus 132 W/m super(2)), average THI was 82.7 plus or minus 1.64 for all studies except for the Nebraska data where the THI was 77.4 plus or minus 4. Air velocity in the sample area was between 0.8 and 1.2 m/s, and body (rectal) temperature was greater than 38.8 degree C (threshold for heat stress). Sweating rates ranged between 189 plus or minus 84.6 and 522 plus or minus 127.7 g/m super(2)-h. Body temperature ranged between 39.3 plus or minus 0.53 degree C and 41.7 plus or minus 0.19 degree C. Differences in sweating rates were statistically significant at P < 0.05 between breeds, between black and white hair coats, and changes in solar load, relative humidity, and air velocity. Wetting the skin surface and increasing air velocity profoundly increased evaporation rate by converting sensible heat to latent heat.