Why Turnout Is Important for Your Horse
Boredom
Horses are intelligent herd animals and will grow restless and bored if confined individually indoors for long periods of time. A horse may entertain itself by chewing stall walls. Cribbing is a stereotypic behavior as a way to cope with stress in which a horse places its top incisors onto an upright object such as a fence post or stall door and arches its neck while inhaling.3 The horse may walk the perimeter of its enclosure, known as stall walking, an obsessive-compulsive habit that can lead to other health problems (if a horse is stall-walking constantly, it's not eating or resting).
While not all of these so-called "vices" are solely caused by stabling, keeping a horse indoors where it will be bored and unable to expend energy can exacerbate any bad habits the horse may have.
Companionship
Horses are herd animals. Keeping horses separated in stalls, where they are not able to see and interact with each other can be very stressful. Outdoor pasture, even if horses are kept in separate paddocks but can still see each other, is better for the mental health of your horses. Being allowed to run or trot with other horses is even better.
Digestive Health
Horses who live outside tend to have fewer episodes of colic than horses who are kept in a stall. A University of Nottingham study suggests that stalled horses may be more prone to colic and that the lack of movement slows the motility of the gut, leading to impaction colic, which is similar to low motility issues suffered by sedentary humans.4
Horses that are outside with room to roam tend to develop colic less often than stabled horses. The same slowed motility that can lead to impaction colic in stabled horses may contribute to equine gastric ulcer syndrome (EGUS), a painful gut condition. The mental stress of being stabled for long periods of time may also play a role.