Farmers in much of Kansas are planting at a rapid pace following the recent rain. However, the speed of planting coupled with unusually windy conditions can potentially interfere with pre-emergence herbicide applications (Fig. 1). There are several things to consider when the planter ‘gets too far ahead’ of the sprayer and corn is emerged before pre-emergence herbicides can be applied to corn.

Fig. 1 Because of weather conditions this spring, planting may have gotten ahead of preemergence herbicide applications in corn. Photo by Sarah Lancaster, K-State Research and Extension.

Residual herbicides that prevent weed emergence are the foundation of any weed management program. Fortunately, many of the residual products that might be applied pre-emergence to corn can also be applied to small corn. These include:

• Atrazine
• Group 15 herbicides such as acetochlor (Harness, Warrant, others), dimethenamid-P (Outlook, others), pyroxasulfone (Zidua, others) and S-metolachlor (Dual, others)
• Group 27 herbicides such as isoxaflutole (Balance Flexx, others) and mesotrione (Callisto, others); and
• Premixes that contain various combinations of these products, such as Acuron, Anthem, Armezon PRO, Corvus, Lumax or Lexar, Resicore and SureStart II.

Do not apply products that contain flumioxazin (Valor, others) to emerged corn. Table 1 includes key information about some of these products, including the maximum growth stage at which the products can be applied and the amount of rainfall required for activation. Activation requirements could be especially important this year, because of the concerns that drier-than-normal conditions may continue throughout the summer.

Herbicides that control emerged weeds should also be considered. Many corn hybrids are resistant to glyphosate and/or glufosinate (Liberty, others), so these products can remain in the tank mix. Dicamba can also be applied to emerged corn. The rates of dicamba that can be applied to emerged corn depends on the size of the corn. For corn from the spike stage through 8 inches, apply up to the 0.5 pound per acre rate (reduce this to 0.25 pound per acre on coarse-textured soils). Apply up to the 0.25 pound per acre rate until the corn is 36 inches or until 15 days before corn tassels emerge, whichever is earlier.  

However, other products that are often included in pre-emergence applications to control emerged weeds cannot be safely applied to emerged corn. These products include paraquat (Gramoxone, others), saflufencail (Sharpen) and tiafenacil (Reviton).

Be sure to include appropriate adjuvants when applying premixed herbicides to emerged weeds. Consider using more "aggressive" adjuvants, such as COC or MSO, in dry conditions because emerged weeds will likely be more difficult to control. However, be aware that this decision also increases the risk of crop injury. Recommended adjuvants are listed in Table 1. When planning tank mixes, always check the herbicide label to clarify tank mix partners, other crop size restrictions and adjuvants for the tank mix.

For more detailed information, see the “2022 Chemical Weed Control for Field Crops, Pastures and Noncropland” guide available online at https://www.bookstore.ksre.ksu.edu/pubs/CHEMWEEDGUIDE.pdf or check with your local K-State Research and Extension office for a paper copy. The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.