These are planted, allowed to grow, then either mown or killed with a herbicide such as Roundup. The crop is then planted in the residue.
Cover crops can also help control soil erosion.
In fact herbicides are now the most widely used group of pesticides. Clearly there are alternatives to using herbicides for weed control. The reasons why these alternatives have not been developed further, or been more widely implemented, are complex and may not necessarily reflect the effectiveness of these methods. One reason for this may be that you can sell a herbicide every year and make a profit from these sales. However, it is much more difficult to make a profit from selling information on how to use cultural practices to control weeds.