Emphatically indicate that you only want certified weed free seed. Be sure to inspect all the seed bags and double check you re getting what you ordered. Check to see if the contractor applied the right amount of seed per acre that he had specified. Move the animals around to different pastures periodically to prevent this. Probably the most important step of prevention is early detection.
Weeds are far easier to control in the early stages of development.
If the weeds do get a strong foothold, they will grow extremely deep roots that make it very hard to control and remove. There are several different herbicides that are specific to different weeds. Make sure to correctly identify the weed, and then purchase the correct herbicide. The herbicide is applied a few weeks before seed germination. For summer annuals apply in the spring, and for winter annuals apply in the fall.