To manage weeds preplant, meaning two to three weeks prior, or preemergence, an application of paraquat at 0.5 to 0.75 lb/A plus a residual herbicide is needed to remove existing weeds and maintain fields weed-free. If paraquat is applied preplant, a second application may need to be applied at planting to remove any remaining green vegetation.
If 2,4-D, dicamba, Elevore, and others may be applied, read the label because there are planting restrictions for cotton and soybean. However, there are no planting restrictions for Enlist Duo and Enlist One in Enlist crops or Engenia, FeXapan, Tavium, and XtendiMax in Xtend crops.
Choosing a residual herbicide, whether applied preplant and/or preemergence, depends on the crop to be planted and the weed spectrum. There are numerous choices of residual herbicides labeled preplant/preemergence in cotton, but research has shown that Cotoran at 2 pints/A is a good choice for control of numerous grass and broadleaf weeds. If glyphosate-resistant Palmer amaranth and waterhemp are a major concern, Brake plus Cotoran, both at 1 pint/A, is an excellent choice.
In soybean, there are numerous residual herbicide options. Many will provide control of glyphosate-resistant pigweeds; however, they differ in the other weeds they will control. For example, if pigweed, yellow nutsedge, and grasses are the targets, Boundary at 1.5 to 2 pints/A is a good choice. But, if morningglory or smellmelon are also an issue, herbicide formulations that contain sulfentrazone (Authority formulations, Sonic, BroadAxe, etc.) or Canopy DF at 4 to 6 oz/A plus S-metolachlor at 0.95 lb/A would provide control. Please contact your local LSU AgCenter agent to discuss your specific weed spectrum and residual herbicide options.
The length of maximum control provided by a residual herbicide is usually 3 to 4 weeks when properly activated. So, if applied 2 weeks prior to planting, one may only expect 1 to 2 weeks of residual control in-crop. In contrast, if the residual is applied preemergence, 3 to 4 weeks of control may be expected in-crop. So, the choice of preplant or preemergence residual herbicide application will influence when the first in-crop postemergence application should occur. Remember, seedling cotton and soybean must be protected from weed competition to help maximize yield potential, so plan accordingly.