Horses chomping at each other is a behaviour known as ‘cribbing’ or ‘wind-sucking’ and can be caused by various factors. Cribbing is a learned behaviour, usually observed in horses that have been kept in captivity, such as those used for racing or dressage. It is thought to be an expression of boredom or frustration due to lack of stimulation and exercise. Other possible causes include dental issues, colic pain, acid reflux or even anxiety. However it should be noted that some horses simply engage in the behaviour out of habit; cribbing does not necessarily signify any underlying condition.