I'm wondering if it could be sweet clover, e.g. Melilotus alba. It looks quite tall for alfalfa and the leaf margins are serrated most of the way around the leaflets, I don't recall alfalfa leaflets being serrated that much but I could be wrong. Apparently one way to tell is to look at the undersides of the leaves - alfalfa has short hairs on the underside whereas sweet clover doesn't. Of course the flowers would answer the question eventually
