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Early-season weed control in corn is essential for optimum yield potential. Early weed flushes can compete with corn for both water and nutrients, especially nitrogen. Sequential weed control programs, including preplant and preemergence (PRE) followed by postemergence (POST)herbicides, can provide the best weed control and greatest net returns. Profitable weed control can be all about the timing of applications in corn.
It is important to start the season clean with tillage, a burndown herbicide application, or both. 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.
Allowing weeds to be present at planting because of skipping or delaying the burndown herbicide application can give weeds a competitive advantage with the crop. Ideally, weeds should be controlled several weeks prior to planting in order to allow for decomposition of plant material. Planting into existing weeds or heavy weed residue that has not had time to decay can interfere with seed placement and reduce emergence due to poor seed-to-soil contact. If a burndown herbicide application is delayed, planters should be adjusted to compensate for the increased plant residue.
Weeds should be controlled when they are small and easier to control by cultivation or a burndown herbicide application. Roundup PowerMAX® 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. 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 residual herbicide can be applied in the preplant burndown application or PRE 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. A PRE herbicide works well with irrigation, which ensures moisture is available at the right time for good activation of the herbicide. Residual herbicides have planting interval or crop rotation restrictions and precautions that need to be considered. Consult individual product labels for precise instructions.
If you do not apply preplant or PRE residual herbicides, POST herbicides need to be applied as soon as possible. You want to extend the control of weeds that are still germinating with the use of residual herbicides. Use recommended rates, and do not cut rates. Apply follow-up treatments as needed to control escapes and late-germinating weeds.
In addition to starting with a clean field, removing weeds after planting when weeds are less than 4 inches tall is necessary to preserve yield potential. University research shows that an average yield potential of 3 bushels per day is at risk for every day that 3- to 4-inch weeds are left uncontrolled after the V3 to V4 growth stage of corn.1 At about 4- to 6-inch tall corn and weeds, the breakeven mark is typically past and yield potential is lost because of weed pressure. It is best to focus on weed control prior to the V2 growth stage of corn and maintain control throughout the season to obtain optimum yield potential.
A total POST program is the most risky weed control system, because no one can control the weather, and the timing of application may not always be optimal. It is better to apply sequential herbicide applications than a single POST treatment to provide more consistent weed control.
A PRE herbicide application preplant or at planting helps to keep later emerging weeds small and uniform enough in height to increase success when following up with a POST treatment. Using a PRE herbicide can also buy you more time to apply your POST herbicide for optimum weed control.
Timing is important for both PRE and POST applications. For PRE herbicide applications, try to time them closer to when you plant, especially if you have a weed spectrum in the field that can emerge later in the season. Weeds that re-infest after an initial herbicide application can also be very competitive, and you need to be vigilant about controlling these later emerging weeds while they are still small.
Including multiple sites-of-action in the weed management program is important to help prevent the development of weed resistance. Use of soil residual herbicides in the program can help to both start the crop off clean and reduce the potential development of herbicide-resistant weeds.
Finally, scout and monitor the field after each weed control practice to evaluate how well the treatment has worked and whether or not additional treatment might be needed. Remember that it is important to start with a clean field, use residual herbicides in the program, and control weeds when they are small with POST herbicide applications.
1Pocock, J. 2011. 5 tips for corn weed management. Corn and Soybean Digest. http://www.cornandsoybeandigest.com.
Wright, S., Canevari, W., and Munier, D. 2009. Integrated Weed Management. University of California Integrated Pest Management Program. http://ipm.ucanr.edu.
Pacific Northwest Weed Management Handbook. 2016. http://pnwhandbooks.org.
Web sites verified 3/26/2018. 180329201115