Mo. Rev. Stat. § 644.059
Agricultural storm water discharges and return flows from irrigated agriculture shall be exempt from permitting requirements set forth in sections 644.006 to 644.141. Agricultural storm water discharges and return flows from irrigated agriculture shall not be considered unlawful under subdivisions (1) and (2) of subsection 1 of section 644.051 unless such discharges or return flows have entered waters of the state and have rendered such waters harmful, detrimental, or injurious to public health, safety, or welfare, or to industrial or agricultural uses, or to wild animals, birds, or fish. For the purposes of this section, agricultural storm water discharges and return flows from irrigated agriculture shall include storm water and snow melt runoff, drainage, and infiltration, including water that leaves land as a result of the application of irrigation water, both surface and subsurface, from standard farming industry practices. This shall include but not be limited to cultivation and tillage of soil, and production, growing, raising, and harvesting of agricultural commodities and livestock. Nothing in this section shall be construed to effect, limit, or supersede sections 640.700 to 640.755 or any other law or regulation of concentrated animal feeding operations.
§ 644.059, RSMo