Allelopathy

Yesterday as the soybeans were coming out of our fields we discovered an issue.  The objective of planting soybeans this year was to "renovate" an old alfalfa stand.  The reason for renovating alfalfa is that as the years go by, alfalfa stands become weak and alfalfa plants die in the stand.  This leaves large gaps between plants which reduces the amount of crop produced per acre.  Also, as alfalfa plants get older the plant stems get thicker which creates hay that is stemmier rather than leafier.  With the nutrition being in the leaves, stemmy is not better.  But you may ask, why not just reseed?  The reason is that alfalfa use what is called allelopathy.   Allelopathy is a way for a species to control the distribution or concentration of its own or another species through the release of a chemical.  For whatever reason, alfalfa doesn't want itself or other alfalfa seeds to germinate near itself.  This prevents alfalfa from being reseeded and why alfalfa fields need to be renovated every four or five years as the stand wears out.  We tried to renovate the field by growing soybeans this year, giving the field time to lose its toxicity to alfalfa.  But if you look across our now harvested soybean fields, where there should be just dirt, there is a green hue of alfalfa.  So the questions are:  Do we spray something this fall to kill the alfalfa and hope that the allelopathic chemical is gone in the spring to allow germination, or do we plant something else like oats for hay and put alfalfa back in for the following year?  Maybe there is another that we haven't thought of yet.  Guess I'll also need to get more input on this.