Could you drill some other type of food plot seed into all of your areas, including the switchgrass areas? My switchgrass from my CRP blend was planted with a mix of other grasses, clovers and an oat nurse crop. Eventually the switchgrass and a couple other grass types took over, but in the meantime the clover and oats fed the critters. I don't think the temporary competition from brassicas would hurt your switchgrass
Maybe you could drill in some brassicas or something similar so at least you have some food growing there this year and maybe you could frost seed some switchgrass in March in those same areas. I don't know the answer for the switchgrass issue, but you still have a lot of time left in the growing season to have an attractive food plot at least.