You are right about depending on the maturity of the corn but in our area most of the corn reached half milk line a week or 2 ago and we have had some heat since then. I would say that about 1/2 of mine is black layered and the rest needs less then 50 growing degree days to black layer. I would guess that most of the corn in central MN is about the same. I do know that most of the corn 50 miles south of me was black layered more then a week ago so there should not be any significant yield lose at this point.