The Environmental Protection Agency defines an Animal Feeding Operation (AFO) in the Code of Federal Regulations (CFR, Federal Register, V. 68 No. 1, page 7265) as a lot or facility (other than an aquatic animal production facility) where the following conditions are met:

• Animals (other than aquatic animals) have been, are, or will be stabled or confined and fed or maintained for a total of 45 days or more in any 12-month period, and
• Crops, vegetation, forage growth, or post-harvest residues are not sustained in the normal growing season over any portion of the facility.