If your horse exhibits “pica” – the general term for the consumption of non-nutritive substances such as mud, wood, cloth, bedding, or manure – you need to look at both his diet and his environment for answers. If the horse’s diet is well-balanced nutritionally, it has plenty of access to quality forage, and a veterinary exam has ruled out the possibility of parasite infestation, then other factors such as boredom or loneliness might be the trigger. In the latter case, something as simple as increased exercise, a companion, or being stabled with a view of other horses may help.
Geophagia, or purposefully consuming dirt (not accidentally swallowing it along with feed off the ground), is actually not uncommon, especially in feral horses. Originally thought to be caused by salt, phosphorous, or potassium deficiency, this theory has been dispelled somewhat in the case of domesticated horses, as horses fed a healthy, balanced diet will still eat dirt. While the practice is generally harmless, consuming sand can cause colic and other intestinal troubles and should be discouraged by not feeding hay off the ground or avoiding turning the horse out in sandy paddocks if possible.
Lignophagia is the eating of wood, including stall walls and shavings, which may indicate a dearth of roughage in the diet caused by lack of grazing, or boredom from lengthy periods of stall confinement. Feeding smaller meals more often, switching to a different type of bedding, and providing more turnout, exercise, and interaction with other horses may alleviate the problem.