The prolonged wet conditions this spring have created additional challenges in regards to weed control. The following are possible scenarios that corn farmers may encounter and factors to consider as we progress through the next several days.
1) Corn Not Planted And Herbicide Was Not Applied: Growers will have flexibility in the choice of burndown options; however, weeds may be larger than typically observed when corn is planted earlier in the spring. Glyphosate may be the best choice for control of larger emerged weeds, but higher rates may be required. Whereas, paraquat may be preferred over glyphosate in cases where rapid burndown control of weedy vegetation is needed for a more favorable environment for corn to emerge. Some disadvantages with paraquat are: a) it may be more expensive, b) usually requires a greater spray volume for best results, and c) is limited in its ability to control large plants relative to glyphosate. The addition of dicamba (eg. Clarity, etc.) or 2,4-D with glyphosate or paraquat may be warranted to aid in control of larger broadleaf weeds. A soil-residual herbicide treatment will extend control during the early part of the season and will limit the need for multiple postemergence herbicide treatments. However, warm-season weeds are likely to emerge quickly and grow rapidly as temperatures increase; therefore, growers should monitor fields regularly and be timely with postemergence herbicide treatments when needed for managing weed escapes.
2) Corn Not Planted But Preplant Herbicide Was Applied: A significant portion of soil-residual herbicides applied earlier in the spring have probably dissipated from the soil surface due to the large volume and intensity of rains that occurred since the treatments were sprayed. The need to spray an additional herbicide treatment will depend on specific field situations. If fields are currently weed-free and do not have a history of weed problems, growers may prefer to go ahead and plant the field, and then monitor fields closely and apply additional herbicide treatments as needed in a timely manner. However, if weeds are emerging, then it would be prudent to apply a burndown treatment that also includes a soil-residual herbicide. The maximum cumulative rate of the soil-residual herbicide (i.e. amount initially applied plus what will be applied) can determine the rate and specific herbicide to apply. For example, if the previous amount of atrazine applied was 1.5 lb ai/A, then an additional amount of 1.0 lb ai/A can be applied in order to comply with the maximum total of 2.5 lb ai/A of atrazine allowed per season. When using premixed products, consider the cumulative level of all active ingredients for determining the rate of a specific product. In cases where growers plan to switch from corn to soybeans, they need to consider the risk of soybean injury from corn herbicides applied earlier in the spring. Soil-applied herbicides containing atrazine are a primary concern when switching from corn to soybeans. For additional information on this matter consult Corn & Soybean News April 2007, Vol 7 Issue 5.
3) Corn Emerged But No Herbicide Was Applied: Growers who planted Roundup Ready (RR) corn hybrids will have the opportunity to use glyphosate for managing emerged weeds after corn has also emerged. It may be more difficult to salvage fields where corn does not have the RR herbicide tolerant trait. Several cool-season weeds are beginning to mature and dieback and should not pose a problem; whereas, such weeds as Italian ryegrass, marestail, ragweeds will be a challenge to control, particularly if nitrogen was applied preplant. Applying a soil-residual herbicide is a good strategy for extending weed control during the early part of the season. Many soil-residual herbicides can be tank mixed with postemergence glyphosate applications in RR-corn. Consult the labels of soil-residual herbicides for timing relative to maximum size or growth stage of corn. Do not use liquid fertilizer as a carrier for applying herbicide treatments after corn has emerged. If initial corn stands are poor, consider a burndown program that will control surviving plants and then replant. For options to manage unwanted corn to replant see UK Extension bulletin AGR-6 (page 23).
4) Corn Emerged and Herbicide Was Applied: Many comments in scenario 2 may also be applicable for this situation. An additional soil-residual herbicide that is registered to be applied over-the-top of emerged corn may be warranted in order to extend early -season weed control. Corn plants stressed due to saturated soil conditions may be prone to herbicide injury compared with corn growing in normal soil conditions. If corn stands are not acceptable, consider a burndown program that will control the unwanted corn and provide soil-residual control of weeds.
5) Corn Emerged But Field Has Scattered Flooded Low Lying Areas With No Corn: Assuming that corn stands in the majority of the field is acceptable, growers may replant the low lying areas. Keep in mind corn growth in the replanted areas will be delayed relative to the remainder of the field. This may impact how the areas are managed in regards to weed control particularly in regards to applying postemergence herbicides.