IFAS ExtensionUniversity of Florida
Login

Recognizing Heat Stress in Dairy Cows

Izabelle Toledo and Geoffrey Dahl

This publication is best viewed in pdf form.

The key to manage heat stress is to understand when heat stress begins!

What is heat stress?

Heat stress occurs when the heat produced by a dairy cow's biological processes and the heat the cow absorbs from the environment exceeds the cow's capacity to lose heat.

Figure 1. A common index used to evaluate heat stress in dairy cows is the Temperature Humidity Index or THI, which is calculated based on ambient temperature and relative humidity.
Figure 1.  A common index used to evaluate heat stress in dairy cows is the Temperature Humidity Index or THI, which is calculated based on ambient temperature and relative humidity.
Table 1. 

The relationship between THI, heat stress levels and body responses such as respiration rates and rectal temperature.

THI

Heat Stress Level

Respiration Rate (bpm)

Cow Body Temperature

68–71

Mild

> 60

101.3°F (38.5°C)

72–79

Mild to Moderate

> 75

102.2°F (39°C)

80–89

Moderate to Severe

> 85

104° F (40°C)

> 90

Severe

> 100

106° F (41°C)

Negative Effects of Heat Stress

In persistent hot, sunny, and humid conditions, the cow's cooling mechanisms are insufficient to dissipate all the heat accumulated and as a consequence, the cow's body temperature begins to rise, triggering a cascade of physiological changes to reduce this excessive heat load in the body.

  • Behavioral Changes
  • Health Issues
  • Impaired Reproduction and Immune Performances
  • Decreased Milk Production
  • Decreased Profitability

By the time physical indicators of heat stress are observed, production losses have already begun!

What are the visible signs and consequences of heat stress in dairy cows?

Figure 2. Visible signs and consequences of heat stress in dairy cows.
Figure 2.  Visible signs and consequences of heat stress in dairy cows.