Thermoregulation in Sheep

Michele Jones
Braden Kemmerling
Amy Neal

Introduction:

To maintain a healthy, productive sheep flock, managers must consider many techniques of thermoregulation in order to keep their animals comfortable. The normal body temperature of a sheep hovers around 102.3 °F and ranges from 100.9-103.8 °F. This body temperature (Thermal Neutral Zone) is vital to maintain. If body temperatures rise above the normal range, metabolism begins to increase due to panting. Panting will increase heat production due to contracting muscles. If these body temperatures continue to rise uncontrollably, enzymes in the animal's body will cease to function and ultimately lead to death. On the other hand, body temperatures falling below the Thermal Neutral Zone (also called Comfort Zone) will cause the animal to increase metabolism to produce heat. Now the feed the manager provides is going into heat production rather than building muscle and gaining weight. Also, if the body temperature continues to drop, enzymes will again cease to function, and the animal will die.

The following information will provide a beginning sheep farmer with some basic aspects of thermoregulation in sheep. These techniques must be considered for a successful operation. First, we will mention some heat transfer mechanisms used by all animals. We will also discuss requirements of sheep at different production phases and at different fleece lengths. Further, we will inform managers of behavioral and non-behavioral methods used by the animals in their flock. Lastly, we will discuss ways managers can provide the most comfortable environment for their animals.


Photo courtesy Sheep! Magazine.

Heat Transfer Mechanisms:

  1. Conduction: This mechanism can be described as heat flow between two solid objects. An example would be an animal lying on its side on a cool or warm surface to gain or lose heat. Heat is transferred from the warmer solid to the cooler solid. Conduction is dependent on heat differential. The greater the differences in temperature between the objects, the more quickly heat will be exchanged. It also depends on effective surface area. A sheep lying on its side will exchange more heat from the ground to itself than a sheep standing on all fours. Another factor affecting conduction is the quality of the conductor. Good conductors allow heat to flow. Concrete is a good conductor, whereas feathers, fat, and hair are bad conductors.
  2. Convection: This mechanism can be described as heat flow as a result of a flowing medium such as air or water. A cool breeze is a good example of heat loss due to convection. Again, this mechanism is reliant on effective surface area (how much area is exposed to the breeze). Air temperature is also a factor. Cooler air or water will result in more heat loss from the animal. Last, wind speed also determines how much heat is lost. Wind chill index takes into account the air temperature along with wind speed. It is a measure of the rate of heat loss.
  3. Radiation: This mechanism can be described as heat transfer through electromagnetic waves from the emitter to the absorber. A good example of radiation is sunlight. Color is a factor in how much heat energy is transferred. Black surfaces will absorb more heat from radiation during the day, but will lose more heat during dark hours. Again, surface area will play a big role in how much heat is transferred.
  4. Evaporation: This mechanism is perhaps the most important form of heat loss in heat-stressed animals. This mechanism uses the evaporation of water to the air. The most common example of this is panting. Water evaporates from the mouth, and they lose 600 calories/ gram of water lost. Effective surface area and humidity are factors affecting evaporation. In levels of high humidity, it is harder for water to evaporate; thus, it is harder for the sheep to lose heat.
  5. Condensation: This mechanism is very rarely considered. Condensation regains the water from the air. The animal then regains the 600 calories/ gram of water. However, this rarely is a factor in thermoregulation and is never truly considered.

Heat Increment:

The heat increment is the amount of heat that will be produced from 100 kcal of metabolizable energy (ME). The numbers in the table below are measured in kcal/100 kcal ME. Sheep and cattle produce much more heat during digestion than monogastric animals because of the microorganism activity in their rumen.

 

Fat

Carbohydrates

Protein

Pig

9

17

26

Cattle

35

37

52

Sheep

32

32

54

Lower Critical Temperatures:

An animal's lower critical temperature, the lowest environmental temperature that will support normal body function, depends on its phase of production and fleece length. If a sheep's body temperature drops below the lower critical temperature, it must increase its own heat production, therefore putting energy towards generating heat that would be normally used for growth and production. As shown in the table below, a sheep that is eating more (at a higher production phase) will have a decreased lower critical temperature, and a sheep that has longer fleece will have a decreased lower critical temperature.

 

Lower Critical Temperature (ºC)

5-mm fleece (fixed)

 

    Fasting

31

    Maintenance

25

    Full-fed

18

Maintenance

 

    1-mm fleece

28

    10-mm fleece

22

    50-mm fleece

9

    100-mm fleece

-3

Techniques Used by Sheep to Maintain Body Temperature

>Non-Behavioral
 Non-behavioral mechanisms include the carotid rete, countercurrent heat transfer, utilization of brown fat in neonates, panting, shivering and non-shivering thermogenesis, sweating, and insulation/building a wool coat.

Skull

 

The carotid rete is a network of small blood vessels located under the hypophysis of the sheep. It serves as a mechanism for cooling blood before entering the brain. The carotid artery carries “hot” blood from the heart and branches many times throughout the rete. Cooled blood leaving the brain and sinus cavity also passes through the rete. Heat is transferred from the hot blood to the cool blood. This transfer ensures proper blood temperature before entering the brain.


 

>Behavioral
Behavioral mechanisms include huddling or scattering, seeking shade, seeking water, seeking shelter, decreasing/increasing feed consumption, seeking cool/warm surfaces, and increasing or decreasing activity level.

  sheep in shelter     sheep in shelter
(photos courtesy of Susan Schoenian , Sheep 201: A beginners guide to raising sheep)

Management Techniques:

Keeping an ambient thermoenvironment for sheep is easy to do if the proper management techniques are followed. Heat lost to conduction can be decreased by bedding the barn with straw, increasing the number of animals in the pen, not shearing the sheep, and increasing bedding thickness. Heat lost to convection can be decreased by housing sheep in a barn, increasing the number of animals in a pen, and using a wind break in open areas.

General considerations for cold weather include not shearing, using bedding, having adequate feed available, and keeping a layer of manure packed down in the barn. For hot weather, some techniques include shearing sheep, providing shade, providing adequate water, and giving an alcohol bath.

sheep in straw bedding
Sheep in straw bedding courtesy of Michele Jones

These guidelines should help producers understand some of the basics of sheep thermoregulation. Following these simple management procedures will keep your sheep comfortable and healthy in any environment.

Sources:

Allrich, Rod. Lecturer. ANSC 332 - Environmental Physiology of Domestic Animals. Lilly Hall of Life Sciences, West Lafayette, IN, Purdue University. Jan – Mar 2007.

Gomes da Silva, Roberto, Newton LaScala, Alvaro Lima Filho, and Marcelo Catharin. “Respiratory Heat Loss in the Sheep: A Comprehensive Model.” Internacional Journal of Biometeorology. Vol. 46.3 (2002): 136-140.

Johnson, K.G. “Shading Behavior of Sheep: Preliminary Studies of its Relation to Thermoregulation, Feed and Water Intakes, and Metabolic Rates.” Australian Journal of Agricultural Research. Vol. 38.3 (1987): 587-596.

Ball-Gisch, Laurie. "Heat Stress & Sheep." Sheep! Magazine July/ August 2006

Schoenian, Susan. "Housing." Sheep 201: A Beginner's Guide to Raising Sheep. 12/27/2005. 7 Mar 2007 <http://www.sheep101.info/201/housing.html>.