Quidding is a habit which some horses form due to mouth pain or other health problems. When a horse quids, it stores a bolus of food in the side of its mouth, or it drops food after a few bites. Sometimes horses will form balls of material while quidding and then spit them out; compacted masses of hay or grain covered in saliva around a stall or manger, for example, are a sure sign that a horse has been quidding.
The most common cause for quidding is bad teeth. Horse teeth grow continuously throughout life, because historically horses ate a range of forage which would wear down their teeth. Domestic horses often do not eat varied diets, and their diets tend to be low on abrasive minerals, so while their teeth keep growing, they are not worn down. As a result, the teeth may become pointed, sharp, or uneven, making it difficult to chew. Horses can also get cavities and gum infections, just like people, and these can both cause dental pain which makes it difficult or unpleasant to chew.
If a horse starts quidding, a vet visit should be scheduled immediately. The vet may need to float the horse's teeth, which means that the vet will use a power tool, rasp, or file to file down the sharp areas of the teeth and make them even again. This should ease the discomfort of the horse, resolving the quidding behavior. The vet can also check for signs of an infection in the teeth, gums, or jaw which might explain the quidding.
Horses who have experienced chronic neglect may retain quidding as a lifelong habit, because they are so accustomed to dental pain. Even with caring dental treatment, formerly neglected horses have difficulty abandoning the behavior. Quidding may also be accompanied with nervous habits like licking or chewing at the stall, sometimes indicating that the horse is frustrated. In these instances, you may need to feed your horse a special diet to ensure that he or she gets enough nutrition; a veterinarian can help with this.
In cases where quidding is a learned behavior, rather than a response to a medical problem, sometimes a horse will recover with time. An equine psychologist or behavioral consultant may be able to help with this, but it is still important to ensure that the horse gets supportive nutrition while he or she is treated for quidding.