A "herbicide" license can be handy for RUP (Restricted Use Pesticides) if you need them.
Mississippi issues a Private applicator certification that expires after 5 years. You have to complete an online course (cost $20 last year I think) that took me about 20 hours of Laz-Boy surfing.
(I probably could of done it in 5 hours if I had set up two monitors so I could have looked up things while taking the tests
) Basically the course can be summarized as read and follow directions on the label and keep record of using RUP.
Other states handle it differently but may have reciprocity.
Link to MSU course