....I planted the RR Alfalfa this past May in West TN....got a great stand....was using it as a nurse/smother crop to attempt some heavy weed control. Success...??? Hay yield....would have been very nice in it's early/initial planting. I was trying to limit it somewhat, but did fertilize it according to soil test...almost. I wish I were in a position to utilize it fully for hay, but just not an option at this time. Weed control..???....ok so far....sprayed 2-3 times during the summer with Round-up. Grazing utilization....great so far....but I limit that amount.
.....I've got a few more acres to plant this spring.....will do at a lesser rate and wish I could put the extra in the barn.
Sorry,,,,not a good answer to your question, but soil fertility is going to give you the best answer. If you can do the max on that, you should do well with good rainfall.