Why do horses produce frothy saliva when they are being ridden?
I have heard that it is considered a virtue in the dressage horse. The drooling of frothy saliva at exercise is neither a virtue nor a vice; it is the physiological result of placing one or more foreign bodies (bits) in the mouth. Salivation is only one of a number of reflex responses that can be expected from such a step. The bit also breaks the otherwise airtight seal of the lips, admitting air into the oral cavity and, in the absence of food, allows the foamy saliva to escape. Apart from reflex salivation, other responses include movement of the lips, jaw, and tongue. Often the bit results in a mouth that is frankly open and a horse that makes occasional swallowing movements. All of these are normal digestive system responses. They are entirely appropriate in a horse that is feeding. But if a horse is exercising, none of these responses are appropriate. For the deep breathing of exercise, an entirely opposite set of responses is required. The mouth should be shut and the lips sealed
