Equivalent temperature index for cows

The effects of temperature, humidity and wind speed on the thermal balance of dairy cows were combined by Baeta et al.(1987) in an equation termed the equivalent temperature index (ETI, °C):

 is the air temperature (°C)
R is the relative humidity (%)
v is the wind speed .
This index was calibrated from data with high producing Holstein cows exposed to changing environmental conditions
(temperatures ranging between 16 °C and 41 °C). Results indicated that an increase in the value of ETI up to 41 °C from thermo-neutral conditions can be associated with a decrease of 38 % in milk production, and elevated rectal temperature to 40.8 °C. To this end, the following ETI values are considered as representing a damaging risk to animals:
No problems: 18 °C to <27 °C
Caution: 27 °C to <32 °C
Extreme caution: 32 °C to <38 °C
Danger: 38 °C to <44 °C
Extreme danger: >=44 °C


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