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The Relative Importance of Atmospheric Humidity in Influencing the Grazing Performance of Dairy Cows During Warm Weather

Dairy Department, Louisiana Agricultural Experiment Station, Baton Rouge

ABSTRACT

In the study of the heat tolerance of dairy cows under natural conditions much of the work has been done without partitioning the weather into its component elements to determine their separate effects. Consequently, the effect of humidity on cow comfort and behavior has been to a large extent a moot question among cattlemen.

Owing to the use of relative humidity instead of vapor pressure, or absolute humidity, as a measure of the water vapor of the atmosphere, some of the research regarding the effects of humidity on an animal under natural conditions has been confused. In the early work of Seath and Miller (10) high humidity, when expressed as relative humidity, was shown to be statistically unimportant as a factor influencing cows' summer comfort, but as temperature was not held constant another variable, vapor pressure, was included. Lee (7) states that "for animal thermo-physiology, it is the absolute humidity, or rather the associated vapor pressure, which matters."