Hallo Sakda
What we   do with CERES-Maize if we get such a great sensitivity to something
like planting date is to look at kernel weight and kernel number.  We
compare the two runs, if their are a big difference in kernel weight
between the two runs it is a indication that something like Nitrogen,
low temperatures or water stress have shortened the grainfilling
period, this will be confirmed by the difference in maturity dates.
 
If the difference is kernel number it is a indication of stress
during silking.
 
My suggestion will be to print out the two runs and compare then
from top to bottom and this  will give you some indication. Their are
some statistical methods  that you can use, and I could explain then
to you if you interested.
 
Regards
Andre du Toit