I've had success with planting alfalfa after alfalfa as a hay grower for several years. As an FYI, I terminate alfalfa and ~8 weeks later replant alfalfa AND I fertilize pretty heavy (+525# per acre) every year.

Here's a cut/paste of an article I came across today (from Hay & Forage, April 2025).

In the past, Kim Cassida with Michigan State University has suggested that autotoxicity could be caused by a toxin in mature alfalfa. More recent research has led her team to believe this toxin may be produced and excreted into the soil when plants experience nutrient deficiencies, mainly phosphorus and potassium. However, the mystery continues.

My thoughts are maybe this has something to do with my success, my soil test(s) usually do not show shortage of either of these nutrients.  I figure that I send10 units of Phos. and 50 units of K down the road with every ton of hay sold. 

Just figured I pass this tid-bit of info on to those who didn't see it.