I'd mow and spray asap. Before seed is mature. I have not looked at mine lately, it's getting there, could be there, not sure. Don't waste time. Fry away! Be just fine if you mowed and sprayed in week or two after mowing - that would work just fine. If you are dealing with volunteer rye, which is probable or possible.... Couple sprayings possibly and really, likely you'll be spraying your brassicas about 2-3 weeks after germination with clethodim & crop oil to kill volunteer rye and weed grasses. Usually needed anyways. Not a lot of work, in total, a couple sprayings will really help in most cases. Round-up and mowing now and likely some clethodim later. Good luck. Doesn't change planting date but I'd burn er up. July 15-Aug 7 would be my target dates for planting brassicas in southern part of state. Generally July 20 is when I'm going full force, especially glad if I have a snag so I can wrap up before it's too late. good luck