Most research indicates that there will be some changes in weed populations with a switch to direct seeding. Perennial weeds and winter annuals seem to flourish with less tillage. Annual weeds often become less of a problem. There are effective and economical ways to control weeds in direct seeding, but it is essential to develop a good system. Crop rotation, herbicide selection and timing of spraying are very important for achieving good weed control.
Dandelions are difficult to control in direct seeding.
However Roundup applied pre-harvest gives effective control if they are actively growing. As with any perennial you cannot rely on one treatment or method to eliminate your problem. The best strategy is a crop and herbicide rotation that keeps weeds at low infestation levels. The best time to control annual weeds with a non-residual herbicide is just before seeding. The weeds will be dying just as the crop emerges.