HEAT STRESS
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This page was last updated on: October 26, 2008
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WHAT IS HEAT STRESS?
The body temperature of sheep must remain within narrow limits otherwise they will show signs of distress. There has to be a balance between body heat production and body heat loss. If sheep are unable to achieve this balance and are unable to lose sufficient heat, their body temperature will rise and they will become heat stressed.

The main indicator of heat stress is continued panting, even when the animal is standing still. If the animal's body temperature continues to rise it will eventually collapse and die.

Under normal circumstances outdoors, sheep can maintain their body temperature within a safe range, known as the thermoneutral zone, without any problems. The fleece insulates the sheep's body, helping to maintain a constant body temperature by providing protection from the extremes of cold in winter and heat in summer.

The role of the fleece in the development of heat stress cannot be overstated. A sheep with a thick fleece is relatively immune to changes in ambient temperature due to the thick insulating layer surrounding its body. However, if body heat production is suddenly increased (for example, due to muscle activity when the animal is driven) it can have difficulty in losing sufficient heat to maintain a constant body temperature and may become heat stressed.

Conversely, a sheep which has been shorn is susceptible to extremes of climatic temperature and can easily become either cold stressed or heat stressed if exposed to extreme weather conditions.
Information from Defra