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ENVIRONMENT |
Department of Animal Science, Hebrew University, Rehovot 76100, Israel
1 Corresponding author: berman{at}agri.huji.ac.il
Evaporative cooling of ambient air (EC) is a main path for heat stress relief in cattle kept in the shade of semi-confining structures. Evaporative cooling is particularly efficient in hot dry climates. We examined the potential of EC for heat stress relief in cattle in moderately warm and humid climates. The feasibility was examined by the reduction in ambient temperature (Tac) produced by EC as a function of ambient temperature (Ta) and humidity (RHa). A data set (n = 139) of temperature and relative humidity (RH) produced by EC over a range of air temperature (25 to 50°C) and humidity (10 to 70% RH) was analyzed by polynomial second order regressions. The analyses produced equations for the relations between ambient air temperature and ambient humidity and between respective conditions in air cooled by EC (Tc, RHc). Linear regressions were computed for a narrower temperature range (30 to 40°C). In all equations, R2 were >0.94 and regression terms were statistically significant. The Tac obtained by EC diminished by 0.3°C per °C rise in Ta, indicating a reduced efficiency of EC with rising Ta. The Tac obtained by EC also was markedly reduced by rising ambient humidity and increased by RHc. An attempt to sustain Tac at greater RHa by allowing a rise in RHc would only restore 2/3 of the reduction in Tac because the coefficient for the RHa effect on Tac is 1.5 larger than that of RHc. The Tac attained by EC partially depends on the humidity in the cooled environment. Elevated RHc may impede animal skin and respiratory evaporative heat loss and lead to moisture accumulation in bedding. If the upper desired limit for RHc is 70%, at RHa smaller than 45% (typical for hot-dry environments) the Tac is larger than 7.5°C, at RHa greater than 55% Tac is reduced to less than 5°C, and at RHa of 57.5 to 60% Tac is about 2.5°C. Coupling EC with forced air movement when Tac is small may partially assist in alleviation of heat stress by enhancing the smaller convective heat loss at ambient temperatures above 30°C. These indicate a limited role for EC in relief of heat stress in moderately warm and humid conditions when RHa is greater than 50 to 55%. Forced evaporation of water from the surface of the animal by sequential hair coat wetting coupled with forced air movement is an alternative little affected by ambient humidity.
Key Words: evaporative cooling • hair coat wetting • temperature humidity dependence
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