Grazing and haying restrictions. The number one tool for controlling weeds in pastures is good grazing management. Don't grub the grass into the ground and keep the fertitity up. The second best tool is clipping pastures. This will keep brush and trees to a minimum and prevent annual weeds from going to seed.
Herbicides are another alternative.
I'ts important, though, to compare costs of treating with chemicals to clipping or increasing the fertility. If overgrazing of poor fertility isn't corrected, the weeds will probably return. Also, make sure to read the label. Many of these products have grazing or slaughter restrictions. These are just general guidlines.