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Agronomic considerations regarding the use of corn as a cover crop include the following:
This is an excerpt from a formal letter Emerson Nafziger and I wrote on this issue. A link to the letter is included below.
1.      Neither seed nor grain produced from seed of transgenic (GMO) corn hybrids can be used as cover crop seed, due to patent prohibitions. Seed or grain grown from non-GMO, non-patented hybrids can probably be used for this purpose, but check seed labels and grower agreements to make sure.
2.      Non-silage corn, follow these practices designed to provide quick cover and to minimize or eliminate the potential for cover crop corn to produce viable seed (which cannot be harvested as grain) include: a) planting after July 15; b) planting in narrow rows—15 inches or less; and c) planting at least 70,000 seeds per acre, adjusted upward appropriately if the germination is less than 90%. Corn can be broadcast-seeded at higher seed rates but using a planter or drill will generally work better. If despite these measures plant pollinate and kernels begin to fill, the crop should be mowed down.
3.      Herbicides can be used on cover crop corn to manage weed growth. If cover crop corn (without grain) is to be grazed after September 1, herbicides need to allow this.
4.      The choice of cover crop should be based on what crop will follow in 2020, not on which crop was planned for 2019. Due to its potential to produce large amounts of high-carbon residue, corn as a cover crop will work best if the crop planned for 2020 is soybean.
Official University of Illinois Extension letter outlining the acceptable use of corn as a cover crop can be found here.