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 This isnt answering your direct question at all, i know..

Ive said it before, and Im starting to sound like a  parrot...  But id suggest you investigate using Clopyralid as part of your spray mix to kill the alfalfa.    It will mix with Roundup just fine.

But you need to check your local information on it, and be mindful of  any plantback restrictions there might be for whatever crops you might want to grow.

Sorry...i dont know what "Rustler " is..