As someone who does this on a regular basis, let me offer some helpful input. First, do as Eddie advised. Get the soil tested. Second, no herbicides, ever. A lot of grasses are not immune to herbicides and it will kill it also. Roatate livestock. A pasture should be dormant for 28 days after 7-10 days of grazing. Fertilize regularly, I use a mixture of manure and triple 13 for my bahia and the same mixture for my rye with ammonia (urea) for my rye. Also, using a harrow or root rake, break up the manure monthly and scratch the surface, allowing nutrients to get in. In the spring, I set my discs on the least aggressive setting, pass over the pasture and fertilize. I wait for the grass to get shin heighth before the first graze. Pretty soon you will find the grass will choke out most if not all your weeds.