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.
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)