I am not an agronomist but I do know that N applied over the top needs time to change to an available form and also time to get to the roots.  Having the 2/3 side dressed applied by 30 days after emergence makes sure it is available and located so the plant can use it when needed.

N applied over the top in August would have a lot of trouble changing form and getting to the roots in most August weather conditions in the cornbelt that I have seen. Maybe Michigan is different.

Jim at Dawn