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United States Department of Agriculture

Agricultural Research Service

Title: Heat Stress

Author
item Lay, Jr, Donald

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: September 3, 2004
Publication Date: N/A

Technical Abstract: When the body is challenged with elevated external thermal temperature which begins to elevate body temperature beyond the physiologic homeostatic body temperature range the animal is considered to be exposed to thermal stress. Thermal stress is a serious consideration for animal welfare because the inability to cope with this stress can rapidly lead to suffering and death. Animals have a physiologic thermal zone, termed the thermal comfort zone, which is a range of temperatures in which the body is able to effectively operate without invoking extra-ordinary measures. If the body temperature begins to exceed the upper critical temperature then physiologic and behavioral mechanisms are initiated that help to dissipate heat and bring the body back into its thermal neutral zone. To increase temperature loss the animal's body will alter its vasculature to increase vasodilation to peripheral areas such as the skin to allow heat loss through convection, radiation, and conduction. In addition, the animal will increase respiration and perspiration if possible to dissipate heat through evaporation. Cardiac output including increased blood volume will occur to enhance heat transfer from the body core to the periphery. The animal will also invoke behavioral responses which aid in decreasing core body temperature. Many animals will seek shade and water to help cool the body. Swine, unable to sweat sufficiently, will enter water or a mud wallow to cool the body using evaporation. If the animal is in a windy environment it may orient its body broadside to the wind to increase the surface area which is cooled. In all species, appetite is typically depressed which prevents further creation of heat from digestion. In modern livestock production, producers can incorporate artificial shade and/or misters in their facilities to help keep animals cool. Livestock rapidly learn to use shade and misters during periods of elevated temperature. Livestock are often able to withstand relatively hot conditions if given the chance to slowly become accustomed to the heat. Death losses of livestock in summer months typically occur from spikes in environmental temperatures rather than constantly increasing temperatures. Swine are especially susceptible to heat stress due to their typically high body fat content and their inability to sweat sufficiently. All livestock and poultry are at risk to heat stress when being transported. Transportation of these species typically involves moving a large number of animals into a limited amount of space for the trip. In an environment which is not overly hot, these livestock can still succumb to heat stress and even death without proper ventilation. When environmental temperatures start to raise even a few degrees the temperature inside the transportation vehicles can reach critical values at an alarmingly fast rate. Therefore during hot temperatures, all livestock and poultry should be transported during the night time.

Last Modified: 10/25/2014
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