If you don't mind a bald spot for awhile use Round Up. If not try Grazon. Unless you have a license to use herbicides like Grazon you may have to get someone to spray it for you.
> Does anyone have any ideas on what
> to use to spray out Blackberry
> Briars? Have used hi-dep with
> little success.
Try some Remedy. It's expensive (around $90 per gallon) but for foliar treatmet I think you use a 2% solution with water plus a surfacant, so you could spray a very large area with one gallon of the herbicide. It's also very good for "stem treatment" (when mixed with diesel) to kill many types of brush, trees, etc.
2-4-D Amine will kill it if you spray it when they are blooming. I mix 2 quarts to 20 gallons of water. Also have heard of mixing diesel in with this. Like Grazon though, you need an applicator's liscense to get it.