A mature 1,000-to-1,200 pound horse will produce 40 to 45 pounds of manure per day, plus spilled water and urine. Manure is about 75 to 80 percent water, so the actual dry matter weight is 8 to 9 pounds.
Along with the manure, soiled bedding is usually removed from the horse stall. The total amount removed is around 75 pounds for a 1,000 pound horse and will vary depending on the bedding material, moisture level of the bedding material, and fastidiousness of the person cleaning out the stall.