The urge to sample, and then regularly eat their own manure, is relatively common amongst stabled and confined mature horses. Manure eating is generally a harmless pastime, and does not increase risk of worm uptake, unless the manure is more than a week old. Some horses develop the dirty habit of stopping to eat fresh manure passed whilst feeding. Manure eating can also lead to a desire to eat sand and bedding.
The underlying cause is probably boredom in most horses. However, highly concentrated diets can increase the incidence due to low fiber intake. Irregular feeding times, with over 16 hours or longer from the night feed to breakfast, may trigger the habit in some horses if they become hungry between meals. Deficiencies of minerals, such as iron, zinc, and calcium have been blamed, but not proven.
Provide good quality hay between feeds, particularly overnight to keep the horse occupied. The additional fiber will dilute energy dense rations and help to improve digestive function. In my own experience, weekly injections of Vitamin B12 (5000mcg) for 2-3 weeks, seems to reduce the habit in some horses. Oral B-Complex vitamins twice weekly , may also be helpful.
Older remedies worth trying include adding 2 teaspoons of yellow sulfur powder to the evening meal, or alternatively 1/2 tablespoon meat meal in the morning and evening meals, increasing over seven days to 1 tablespoonful twice daily, and continuing for 7-10 days at this dose. Drenching with a single dose of two cups of cooking oil in a large syringe may make the manure smelly and less attractive. If all else fails, muzzle the horse between the main meals, or turn out in the paddock with grazing or access to good quality hay.