Do you manage your grazing to allow adequate rest so your grass can increase its vigor? If not, don't waste money spraying weeds and brush -- they'll just keep on coming back. But if you do manage your grazing well, spraying established weeds and brush can hasten improvement of your pasture.
Right now in early June is the best time to control most perennials, annuals, and woody plants. It's better to spray too early than too late. Please note that there are grazing restrictions following the use of some of these chemicals, so read and follow all label directions.
Small annual broadleaf weeds are controlled well by either 2,4-D ester or Ally. Mixing some Banvel with the 2,4-D improves control if weeds have gotten larger. Be especially careful, though, when you use Banvel anywhere near sensitive crops, gardens, or trees because it can drift half a mile or more.
For tougher weeds, including most perennials like western ragweed, vervain, ironweed, and broom snakeweed, Tordon is quite effective and it provides some soil residual activity to limit new weed seedlings. Another good choice to control these tougher weeds is Grazon, which is a premix of Tordon and 2,4-D.
When woody plants are your main problem, Tordon, Spike, and Crossbow usually are your best choices in pasture and rangeland.
Herbicides can be useful on grasslands when used properly, but only when your grazing management helps the effects to last.