My experience with old stands of alfalfa is 150-175 bu corn, no N added. (Need to keep the P & K up tho, and my started might have added 10 lbs of N!)
Test strips with NH3 added were no higher in yield.
My soils kinda peaks at 175, highly varriable hills/valleys keep me from ever hitting a good year of weather for _all_ the soil types in a round on my fields. Perhaps if you can regularly top 200bu you'll need extra N, but seems in my clays, the old alfalfa fields add enough N for a typical full corn yield.
Plowdowns, where the alfalfa/clover is a year or less old, don't supply so much.
As always, that is 'here'; different soils, climate, goals can lead to very different needs.