Spray preferably early in the am, when it is dead calm, so there is no drift.
Don't get any on your corn and you will be fine. If you get some on the corn, you'll just have a little bigger brassica plot.
Are you using a hand held sprayer? If so, you can use a gallon milk jug, with the bottom cut out, around the tip of the sprayer, to act as a guard, to keep from over spraying on your corn. Not sure how big your bare spots are and how tall your corn is?
If you're using an ag sprayer, you'll need to be more careful and just accept the fact you might have a bigger brassica plot.
You can spray right up to the corn and it won't hurt it, but don't get any on the corn or it's exposed roots.
Glyphosate goes inert when it hits dirt.