Explain the concepts of thermoneutral zone, lower critical temperature, and upper critical temperature, and how they relate to metabolic rate
The thermoneutral zone (TNZ) for healthy beef cattle is when the animal doesn't have to expend energy to maintain its normal body temperature. For healthy cattle, the thermoneutral zone is between 32° and 77°F.
Cold stress (lower critical temperature) occurs when an animal is exposed to weather conditions that put it below its lower critical temperature. To maintain core body temperature when it is cold, cattle shiver to maintain body temperature, and that requires energy. For cattle with a normal winter coat that is dry, the lower critical temperature is 32°F. If the coat is extra heavy, it drops to 18°F. If the normal coat is wet, however, the lower critical temperature may become 60°F.
Heat stress (upper critical temperature) is the opposite of cold stress and occurs when cattle have to expend energy to get rid of body heat to maintain core body temperature. When cattle get too hot, they begin to pant to get rid of heat, and that also requires energy.
Maintaining the thermoneutral zone can be influenced by several factors:
* the body condition of the animal;
* the type of hair coat (winter versus summer hair and all variations in between);
* whether the hair coat is wet, dry, or muddy; and
* pigmentation of the hide and hair.
Remember that outside temperature is influenced by temperature and wind speed. Also, relative humidity can play a factor in the summer.