Planting brassicas that late up there last year was probably the reason they didn’t do well. Sometimes, if you get a good crop, it might take the deer a year or 2 to get used to them. Once they do, it’s game on.
For your area, right now would be the best time to plant both. Especially being as dry as it is this year.
What are you cutting it with, a brush hog type? If so, that should chew up the existing greens pretty good. And if it’s 3-4′ high and mostly wheat/rye you’ll end up with a lot of straw residue. If you throw your seed in, cut it with a brush hog type. Then roll it. If there’s not too much residue it’ll grow through it.
Burning it would be great but given the drought situation it might be iffy without starting the surrounding area on fire? If you can and it’s dry enough to burn, burn it, rough it up with the disk, plant it and roll it.
I have 2 plots that I cut a week ago. I’ll let it grow back to about a foot high, then spray it, wait a few days until I see it dying then spread my seeds right in it and then run a cultipacker over it. The crop residue covers it and keeps it moist. The seeds will grow right through it.
Another option for you but for your area, you’ll need to start soon whichever method you use.
You have a little extra time for the cereal grains but the brassicas should be done asap.