I would agree that if oats are harvested in boot stage as forage, it will be easier on the alfalfa.  However, my question is.....  if you don't want the oats, why are you planting them?  Most of the alfalfa I see around here is seeded without an oats companion crop.  Some is seeded with orchardgrass; some is seeded alone; very little has oats in it.  So, why would you want the oats in the first place?