As Pupdaddy said, I would have wanted to fall-kill that orchardgrass, let it lay over winter to break down the residue.  Fall-kill with glyphosate + 2,4-D would have killed existing dandelions, in my opinion, so they should have been taken care of. 

I would not see the need for RR alfalfa seed - as Pupdaddy suggested - but I would be concerned about trying to spring-kill orchardgrass and then plant back to alfalfa.