AN356/AN356: Recognizing Heat Stress in Dairy Cows

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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)

View Table

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.