Inquire from feed dealer or mills if a testing program is in place for horse feeds containing corn. Feed mills should have a testing program in place. While this doesn't absolutely guarantee that all feeds will be safe, you can have more confidence in suppliers with testing programs. Consider mixes which use other feedstuffs, or use grain mixes with small amounts of corn. Do not store feed products for more than two weeks.
Most molds require a grain mixes stored on farm.
Store grain mixes in proper storage facilities which protect the grains from moisture. Routine cleaning of storage facilities will decrease incidence of mold growth or contamination of newly stored products.