Most food animals in the United States are grown on highly concentrated factory farms, and the vast amounts of waste those animals produce poses a huge environmental and public health problem. Historically, farmers used animal manure as fertilizer, but factory farms produce far more manure than can be used responsibly on local fields. The over-application of manure leads to runoff from agricultural fields into waterways. The runoff dramatically alters the ecosystem, contributing to algae blooms and "dead zones" as well as impacting fishing and recreation economies and public health.