Thermal Responses of Holstein Dairy Cows on Pastures with High Solar Loads and High Winds.

The purpose of this study is to determine if dairy cows exposed to full solar loads on pastures without access to shade under thermal conditions of moderate heat stress can maintain thermal homeostasis. A dairy with these conditions is found on the northern tip of the island of Hawaii. Unique to this dairy are persistent, strong winds which potentially increase convective and evaporative heat losses to offset the high solar heat gains during hot weather. Thermal homeostasis was monitored using vaginal temperature loggers to continuously measure core temperature. Environmental levels of solar radiation, wind speed, air temperature, and relative humidity were also measured. Thermal behavior of 5 predominately black and 5 predominately white Holsteins on the pasture of the dairy farm were observed from 9:00-16:00 for a three day period during August 2006. Solar loads averaged 850 watts/m2, THI averaged 77.3 and wind velocities averaged 8.2 m/s at midday. During the same period the average core temperatures of black Holsteins (39.2C) were 0.5C warmer (NS) than white Holsteins (38.7C). Previous studies have shown Holsteins seek cooler environments when their body temperatures reach 38.9C, suggesting the Holsteins in this study were not heat-stressed. These cows spent more time standing (60%) than lying (24%) which provides a thermal advantage because standing provides more surface area for cooling than lying. While standing they spent more time orienting their bodies parallel to the wind rather than perpendicular to the wind, which has been shown to enhance convective cooling under similar wind velocities and air temperatures.