There is a liguid sprayed chemical called "Glyphosate", 41% working chemical, the exact formula as Monsanto's Round-up at about 2/3rds. the cost of Round up. Use maximum 6 ounces per gallon. Any more is a waste. Go to your county Extention service and they can advise you on when to spray, probably at least twice, once in the spring and once in summer. Check it out.
This will kill everything in your pasture too. ... Well maybe not resistant Pigweed. If you have that too, you have a bigger problem.