Dormant season seedings are usually used for warm-season or native grasses mixes. One hundred percent "weed-free" fields are rare to find because even if the weeds were controlled this past growing season, there are likely weed seeds in the soil, or they will blow into the field over the winter. If the field was weedy during the past growing season, it is just expected that weeds will be a problem again next year. It is probably still worthwhile to do the dormant seeding, but be prepared for some weed control next growing season. This can be either through mowing or shredding or using the appropriately labeled herbicide at the correct time. The rainfall patterns will also have a significant effect on the weeds as well as the establishment of the seeded species.