Pica is defined as ingestion of items with no food value over a period of at least one month. This is actually normal in the young of all species. Horses rarely fit the description of chronic consumption except for devoted wood chewers but sporadic eating of dirt or manure is fairly common.
Bark stripping yields negligible nutritional benefits and could be considered a form of pica.
Most formal research has been done in humans and may not be directly relevant to the horse but at least they can provide feedback. Pica is seen in a number of developmental or psychiatric disorders. It is also commonly associated with anxiety, which may fit well with the equine picture.
In people, an association has been noted between pica and pregnancy and/or anemia. However, this has been difficult to tease out from other associated factors. For example, dirt eating during pregnancy is a cultural practice in some developing countries where dirt is actually sold by street vendors.
Detailed analysis of many studies has been unable to settle the chicken or egg question regarding a connection between mineral deficiencies and pica. One problem is that dirt eating often focuses on clay (same is true of animals) and clay can bind nutritionally beneficial minerals and/or be a source of toxic ones like lead or aluminum.
Another interesting association with pica reported by humans that we really can’t confirm or rule out in animals is that it can be caused by nausea. Clay is the obvious choice for relief of any GI upset and is a common ingredient in both human and animal products for both gastric upset and issues with diarrhea and gut toxins. Clay binds many fungal and bacterial toxins and is a major ingredient in Kaopectate. Horses are certainly no strangers to GI upset in many forms.
Eating feces (coprophagia) should be in a special category because it does have some nutritional value. It is so common in rats and rabbits that the practice is considered a normal behavior used to obtain protein and minerals that are not well absorbed from the large bowel and would otherwise be wasted. In fact, deficiencies can develop if they are prevented from eating feces. It is also a perfect prebiotic that is not only rich in live organisms but also precisely the types the animal needs.
Coprophagia is also normal in foals, piglets and even young ruminants like calves and sheep. Dogs are notorious for eating the fecal matter/manure of a variety of other species. Cats will eat their own feces out of hunger or boredom (reduces or stops if they are allowed free choice access to food) or when vomiting. It’s unclear if they are eating feces to induce vomiting (e.g. hairballs) or to attempt to recover nutrients.
So, what do we actually know about aberrant feeding behaviors in horses? Two studies in horses from the late 1970s identified low protein intake as a trigger for coprophagia which fits with data from other species. No solid connection with mineral deficiencies has been documented although by observation dirt eating may increase in ill horses and could be an instinctive but nonspecific drive for minerals. Horses may also eat dirt or lick metal when seeking salt. Pica as an outlet for anxiety, boredom or not having food available is likely an issue in many scenarios, as is dirt eating related to GI upset.
Fortunately, most of the possible causes are benign. Small net hay feeders, toys, plenty of exercise and a quality diet may help prevent pica. However, if anything seems “off” when your horse starts odd eating behavior it’s best to involve your veterinarian.
Eleanor Kellon, VMD