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The first step of your weed control program should be to start with a clean field using either tillage or a burndown herbicide application. Effective preplant weed control conserves soil moisture, facilitates planting by providing a clean seedbed, and helps provide critical weed control through planting and after crop emergence.
More weeds are likely to emerge prior to planting this year because of the wet conditions in many areas. Helping a newly emerged corn crop to grow for several weeks with little or no weed competition will be very important. Cultivation can cut or mechanically bury weeds in the seedling stage before they have a chance to become well established and competitive. Preplant tillage associated with normal seedbed preparation can control many annual weeds. No-till and minimum tillage cropping systems generally require the use of herbicides prior to planting for effective burndown of weeds.
Pre-irrigation or rainfall before planting corn can cause weed seeds to germinate. Weeds should be controlled when they are small and easier to control by cultivation or a postemergence herbicide. The presence of early emerging weeds generates several questions that are important for good weed management:
Including multiple sites-of-action in the weed management program is important to help prevent the development of weed resistance.
Roundup PowerMAX® (32 fl oz/A) can be effective for the control of emerged grass and broadleaf weeds in a burndown application prior to planting corn. A tank mix with dicamba or 2,4-D may be needed for difficult-to-control weeds, such as kochia, horseweed/marestail, or hairy fleabane. The choice between dicamba or 2,4-D will depend on the weed species present. Dicamba will be more effective on kochia and marestail, and 2,4-D tends to be more effective on hairy fleabane and winter annual mustards. In corn, dicamba is labeled for application from preplant to the 5th leaf stage or 8-inch corn, and you need to be cautious on soils with low organic matter or coarse texture. 2,4-D can be applied 7 to 14 days prior to planting or 3 days after planting and before corn emerges. Dicamba and 2,4-D may have use restrictions in certain geographies, and care must be taken as these herbicides can drift or volatilize onto sensitive crops and cause injury. Always read and follow label directions and precautions for the herbicide product.
A labeled residual herbicide, such as Harness® Xtra/Degree Xtra® or atrazine, can be applied in the preplant burndown application or preemergence at planting. Be aware that the residual herbicide component of the burndown tank mix, applied several weeks prior to planting, may not last long enough to provide adequate weed control into the growing season. Another residual herbicide application may be needed at planting or an earlier than normal early-post herbicide application may be required. The early-post application may need a residual component to control late-emerging weeds that can impact corn yield potential. Residual herbicides have planting interval or crop rotation restrictions and precautions that need to be considered. Consult individual product labels for precise instructions.
To avoid the selection for tough-to-control or weeds with tolerance to glyphosate, always use Roundup® brand agricultural herbicides in a program that includes herbicides with different sites-of-action. Residual herbicides and programs with multiple sites-of-action can be used to broaden and lengthen the weed control and help to reduce the risk of herbicide resistance development.
Wright, S.D., Canevari, W.M., and Munier, D.J. 2009. Corn pest management guidelines - integrated weed management. University of California IPM publication 3443. http://ipm.ucanr.edu. Whitesides, R. 2007. Weed control in corn and sorghum. Montana, Utrah, and Wyoming Cooperative Extension Services. 2006/2007 Weed Management Handbook. http:/www.uwyo.edu. Web sites verified 1/13/17. 170114113252