I think I would skip the herbicides.  Its almost an impossible situation from that angle.  Its a biennial, perennial, that starts from seed or roots (maybe rhizomes?) in the ground.  I was fascinated to see the stuff will germinate from seed anytime the soil temperature gets to 65-degrees, fall or spring - or winter in the deep south.  So, if you going to use herbicides timing is a difficult proposition.
I've never had a problem with it. Like M-man said, just mow it and get your crop in a position to out-compete the dog fennel.