The glyphosate + 2,4-D should knock it out, especially if going with the 2 L rate of glyphosate.  I would not rely on the glyphosate alone, but the combination should work. Wait until the alfalfa is 8" or more to spray. If you are spraying too early (not enough topgrowth), that might be why you are having trouble killing it.

In season, Stinger will take out alflafa as well, and Stinger is labeled in small grains.  However, it is a more expensive way to go, and in small grains, you may not be able to get enough on, and stay within the labeled rate on small grains.