With rapid planting progress and extended rainy periods over much of the region, some of our intended preemergence herbicide applications could not be completed prior to crop emergence. While the plans for use of preemergence herbicides were formed with the best weed management practices in mind, all is not lost when the crop emerges. There are many options in both corn and soybeans that will provide control of emerged weeds while still providing residual activity to handle later-emerging weed flushes.
We recommend making postemergence applications of approved residual herbicides sooner, rather than later. With moist soils and the prospect of warmer temperatures in the forecast, rapid germination and growth of weeds may soon take place. The sooner we address these flushes of weeds, the less likely they are to cause yield reductions to corn and soybeans. Also, the rain showers that reduced our ability to complete applications of the preemergence herbicides may be back when we attempt to spray our postemergence herbicides. Applying a postemergence residual herbicide as soon as conditions allow will help ensure timely management of emerged weeds.
For emerged CORN, here are some key "watch outs" when applying herbicides:
- Atrazine containing mixes are common used for residual in emerged corn. Make sure to observe best management practices concerning loading, mixing, and spraying of these products. Maximum use rate of atrazine in a single application is 2 lb ai/a, but check state and local regulations for alternative maximum use rates.
- In addition to atrazine, group 27 herbicides are commonly used to provide activity to emerged weeds as well as to provide additional residual. Examples of group 27 herbicides include Armezon®, Balance® Flexx, Callisto®, Impact®, and Laudis®. Group 2 and Group 15 herbicides are also commonly used for residual activity.
- Check your residual corn herbicide labels carefully. Many of the common products have either an 11 inch or 12 inch corn height restriction. Be sure you know which-is-which.
- Herbicide applications with UAN nitrogen solution as a carrier are not recommended when corn has emerged, since this will increase the risk of crop response.
For emerged SOYBEANS, here are some considerations when applying herbicides:
- Programs for control of emerged pigweeds will most likely include the use of PPO-inhibiting (Group 14) herbicides such as Cobra® or Flexstar® for glyphosate-resistant biotypes. In Roundup Ready 2 Xtend® Soybeans, XtendiMax™, Engenia™, and FeXapan™ will also be an option on pigweeds less than 4" tall.
- Residual herbicides for soybeans may include ALS-inhibitors (Group 2), or more commonly, Long-Chain Fatty Acid inhibitors (Group 15). Group 2 herbicides include Classic®, Raptor®, Pursuit®, and others. Group 15 herbicides include Dual II Magnum®, Zidua®, Outlook®, Warrant®, and others. Group 15 herbicides are the lead recommendation to provide residual waterhemp control in emerged soybeans.
- Consider adjuvant usage when applying herbicide tank-mixtures. PPO inhibitor herbicide labels will often recommend oil-based adjuvants, while the label of the residual herbicide in the mixture may not recommend oil based adjuvants. Many emulsifiable concentrate (EC) herbicide formulations can "heat up" contact herbicides when an oil-based adjuvant is used. In these instances, it is important to choose the correct adjuvant that will control the emerged weeds without overdoing it.
- Additional adjuvant restrictions are in place when using a new dicamba containing products. An example of this is the inability to use AMS with these products. Consult the respective manufacturer website prior to tank mixing dicamba with other herbicides or adjuvants.
Finally, prior to using any herbicide, always refer to the herbicide label to ensure proper application.
Contact your FS Crop Specialist for your agronomic information.
Image 1: Value of early postemergence herbicide application (front of picture) versus later application (back of picture).