spraying alfalfa grass mix

I have been told you can spray alfalfa grass mix and kill weeds with 2,4d after one or two good frosts in the fall. The person told me the alfalfa will be dormant enough where it wont kill it. Anyone do this. The weed I am mostly after killing is spotted knapweed. I believe he said spray Amine. I am in northern MI.

spraying alfalfa grass mix