As a rule of thumb, if a pasture has over 50% desirable plants, it’s the least cost to improve what’s already there by improving grazing strategies, controlling weeds, and overseeding. If less than 50% of the plants are desirable it’s probably more time and cost effective to “renovate” or start over by tilling up the existing vegetation and reseeding. If renovating, select portions of the pasture to renovate incrementally so that you always have some pasture that you can use while the new area is getting established. The best time to plant is in the fall.
The best long term strategy for weed prevention is to help the grass out-compete all other plants. Give the grass the fertility and pH that it needs by and don’t it abuse by over grazing. Mowing pastures after each grazing period, keeps uneaten weeds from going to seed and evens out the access to light for the shorter grazed grass. If using herbicides, consult with your herbicide supplier to know what herbicide is appropriate as well as timing of applications. Always read the labels and abide by warnings and instructions.
See the following links or fact sheets: