The normal respiratory rate for an adult horse at rest is 8-15 breaths per minute. When horses are exercised, their breathing rate can increase to 30 or more breaths per minute.
Respiration is an inhalation followed by an equally long exhalation. Differences in the length of inhalation and exhalation can signal some respiratory problems. Apart from exercise, an increase in the respiratory rate can be caused by heat, humidity, and pain.