Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
An area of the field where the crop has drowned out gives weeds an opportunity to grow without crop competition, and potentially produce a tremendous amount of weed seeds. If a particularly troublesome species such as Texas panicum or Palmer amaranth is growing in these spots they could really cause problems for the next few years if they are allowed to produce seeds. So what should you do? Some things to consider are whether you can reach these spots with equipment such as mower or sprayers; what is the crop in the field; what do you intend to plant in the field after harvest; and what will effectively control or kill the weeds?
Mowing is an option, but in all likelihood the areas will need to be mowed at 10 to 14 day intervals to prevent seed production.
If considering a herbicide, first assess the situation. If you are treating areas of a field, and will be harvesting the crop around the bare areas, you are limited to herbicide options for the crop planted in the field. Furthermore, you are limited to the same herbicide rates and herbicide application timings. Use of a herbicide with residual control is going to be important.
Also, consider what will be planted in the field next and check your rotational intervals. Will you have enough time between herbicide application and planting the next crop? This is a situation where you will have to assess each field individually, but these drowned out areas may need special attention.