Last year I tried planting corn into an established pasture which was mainly grass with a healthy number of wild flowers. I mowed it really low and poked the corn down 2" or so. Many of them sprouted but none ever got over 8". I think the competition was just too fierce.
If they were planted at the same time as the grasses and flowers, maybe they'd've done just fine.
If I were to try it again, I'd prepare the autumn before by laying down a stripe of cardboard where each row of corn will be the next spring. Cover that with lasagna mulch, dirt, compost, etc and cover it with mulch. That will smother the existing grass. Then in the spring, plant away. If you're in a warmer climate (or maybe even in a colder one) you could try a cover crop in the meantime that fixes some nitrogen for the upcoming corn crop.