Heat Stress

Heat stress occurs when a dairy cow's heat load is greater than her capacity to lose heat, and is sometimes referred to as hyperthermia. Dairy cows in areas of New Zealand are affected by heat stress during summer.

Minimising heat stress

Insure against possible milk production losses with the following: 

Temperature Humidity Index (THI) calculator

The calculator below can be used to help manage heat stress in cows. It can also be downloaded in xls format.

Calculate Humidity

Temperature (°C)
Relative Humidity (%)

Temperature Humidity Index:

Note: This calculator uses the following formula:
THI = 0.8T+[RH × (T-14.4)]+46.4 (where T is daily maximum temperature (°c) and RH is mean daily percent relative humidity divided by 100.

Production Impacts

When THI reaches 68 for Friesians and 75 for Jerseys (equivalent to 21 and 25.5°C respectively at 75% relative humidity) New Zealand cows begin to experience the effects of heat stress. This is seen by a reduction in feed intake and a drop of around 10g milksolids per day per unit increase in THI (Bryant et al. 2007).

For example, an increase in THI from 68-78 would equate to a 100g drop in milksolids/cow/day. Note that a drop in fat/protein percentage occurs before a drop in yield or milksolids is visible (Bryant et al. 2007).

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