Why does my horse eat mud?

to contain higher levels of iron and copper than others in the same pasture we can assume that they are not randomly chosen but how horses can determine the appropriate site remains a mystery. Horses tend not to try out different sites before choosing one so perhaps there is some olfactory mechanism at play either smelling or tasting the soil for its mineral content? Certainly an interesting area for research.
Boredom
There is a strong correlation between boredom and coprophagia or geophagia certainly in domesticated horses. Often we have to keep horses stabled for long periods of time due to management factors or health reasons such as injury. In general the longer horses spend indoors without access to forage even if it is just for several hours because their haynet has run out overnight the greater the risk of pica possibly due to a lack of mental stimulation. As pica is observed in wild horses though we must assume that boredom is not the only cause of this behaviour.
Lack of dietary fibre
In general we recommend a daily dietary intake of at least 1 52 bodyweight of dry matter or forage. Youngsters on high concentrate diets stabled for long periods of time are at an increased risk of coprophagia eating droppings. Fibre is critically important in the diet to maintain digestive health. Diets that are higher in starch rich concentrates are linked to an increased risk of stereotypical behaviours. 
Age
Coprophagia in foals is extremely common during the first few months of life. This behaviour rapidly disappears by five months of age. They usually eat the faeces of their own mother rarely that of others leading to the theory that a maternal pheromone may drive this behaviour. It may serve to populate the foals digestive tract with good bacteria that will help with digestion of forage as the diet adapts. Care should be taken to ensure that the mare has been treated with appropriate wormers though. This is because coprophagia can lead to parasitic infestation in the foal.
What are the risks of pica in horses?
Geophagia may be associated with an increased risk of sand colic when large volumes of sand accumulate in the digestive tract leading to diarrhoea or intestinal impactions. Sand is most commonly involved. But any large quantities of rough soil may irritate the mucosal lining of the digestive tract. It is not unusual for horses to ingest small quantities of soil when