If time permits, I like to bushhog, wait a few weeks to let it get a rain or two on it and be actively growing again, and then spray it.
^This^. Plus, in the OP you didn't mention how tall the "grass/weeds" is you're wanting to spray. If it's pretty tall I'd definitely mow prior to spraying.
"The world is so dreadfully managed, one hardly knows to whom to complain."