Why does my horse eat mud?

It may be somewhat alarming to see your horse turned out in pristine pasture ignoring the readily available grass and instead tucking into a pile of dirt or even worse eating faecal material in their stable. There are several proposed reasons for this behaviour. In this article we look at why they may decide to eat mud or faeces. And if there is anything we can do to prevent this. 
Table of contents
Why do they do it?Mineral deficiency?BoredomLack of dietary fibreAgeWhat are the risks of pica in horses?Treatment or preventionProvide adequate forage Environmental enrichmentSupplementation

Pica is the term given to the desire to eat unusual substances with little or no nutritional value like dirt sand or tree bark. More specifically coprophagia and geophagia refer to the ingestion of faeces and soil respectively. Horses are not the only species observed to eat dirt geophagia has been observed in many others including humans and dogs as well as ruminants or in fact most animals reared outside. In humans pica can occur in association with pregnancy and as such females are overrepresented. No such gender bias has been observed in horses. Geophagia has been observed in both wild and domestic horses.
Why do they do it?
Mineral deficiency?
It has long been proven that sheep eat soil to ingest trace minerals such as copper and iron which helps to prevent nutritional deficiencies. Some evidence suggests that this may also be the case for horses. It has been demonstrated in a study involving a small number of horses that those who ate soil had lower serum iron and copper levels than those who did not eat soil. They also had slightly lower haemoglobin levels although still within normal values. These horses were not clinically anaemic and further larger studies are needed to investigate this more widely. However interestingly in addition to this soil in sites preferred by geophagic horses had higher concentrations of iron and copper than sites that were not actively chosen by such horses. So there is a potential link between mineral deficiencies and geophagia. 
Although it seems highly likely that horses seek out soil for its mineral content we cannot prove definitively that horses exhibit geophagia because they are deficient in copper and iron as we could question whether or not geophagia itself renders them deficient in these minerals. Wild horses have been observed returning to established lick sites repeatedly. As these sites have been shown