I was in your exact situation last year. Old pasture that I sprayed out middle of summer. I just broadcast brassicas and then rye a few weeks later and lucked out a bit with some timely rains. The plot turned out awesome. The benefit of having an old pasture is reduced weed seed. Get some food for the fall and then plant the alfalfa next spring. That's what I'm doing with a few of my new plots this year.