J. Dairy Sci. 89:3817–3825 © American Dairy Science Association, 2006. Extending the Potential of Evaporative Cooling for Heat-Stress Relief

Factorswere analyzed that limit the range of environ-mental conditions in which stress from heat may be relieved by evaporative cooling in shaded animals. Evaporative cooling reduces air temperature (Ta), but increases humidity. Equations were developed to pre-dict Ta reduction as a function of ambient temperature and humidity and of humidity in cooled air. Predictions indicated that a reduction of Ta becomes marginal at humidities beyond 45%. A reduction of Ta lessens with rising ambient Ta. The impact of increasing humidity on respiratory heat loss (Hre) was estimated from ex-isting data published on Holstein cattle. Respiratory heat loss is reduced by increased humidity up to 45%, but is not affected by higher humidity. Skin evaporative and sensible heat losses are determined not only by the humidity and temperature gradient, but also by air velocity close to the body surface. At higher Ta, the reduction in sensible heat loss is compensated for by an increased demand for Hre. High Hre may become a stressor when panting interferes with resting and rumination. Effects of temperature, humidity, air veloc-ity, and body surface exposure to free air on Hre were estimated by a thermal balancemodel for lactatingHol-stein cows yielding 35 kg/d. The predictions of the simu-lations were supported by respiratory rate observa-tions. The Hre was assumed to act as a stressor when exceeding 50 % of the maximal capacity. When the full body surface was exposed to a 1.5 m/s air velocity, hu-midity (15 to 75%) had no significant predicted effect on Hre. For an air velocity of 0.3 m/s, Hre at 50 % of themaximum ratewas predicted at 34, 32.5, and 31.5°C for relative humidities of 55, 65, and 75%, respectively. Similar results were predicted for an animal with two-thirds of its body surface exposed to 1.5 m/s air velocity. If air velocity was reduced for such animals to 0.3 m/ s, the rise inHrewas expected to occur at approximately 25°C and 50 % relative humidity. Maximal rates of Hre were estimated at 27 to 30°C when ambient humidity was 55 % relative humidity and higher. High humidit