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A 1000 pound horse produces about 45 pounds or 0.75 cubic feet of manure per day. 50% moisture, manure, urine, bedding or other material such as hay, grass, straw, etc.

If the manure is not properly processed it may contaminate ground water. This may be by spreading on pastures or by being washed into water ways from being stored in large piles too close to water sources. You may also run the risk of contaminating the water if the manure is left in paddocks.

Horse manure with wood products that is spread on crops or pastures without being properly process can run the risk of stunting what is being grown on the land. (taken from the Ohio State university Extension Service, http://ohioline.osu.edu/agf-fact/0212.html)