Waterhemp is an annual weed species in the pigweed family that is capable of producing greater than 1 million seeds per plant and due to a limited number of effective herbicides, especially in sugarbeet and soybean, is difficult to control compared to most weed species. In addition to the production of large quantities of seeds, continual germination throughout the growing season and an increased frequency of herbicide-resistant biotypes adds to the degree of difficulty in keeping this weed species under control. The good news is that the longevity of waterhemp seeds in the seedbank is relatively short compared to most species (1 to 12% survival after 4 years), meaning complete control (zero seed production) of all plants over a three to four year time period should significantly reduce the waterhemp seed bank densities, allowing the farmer to take control of this difficult weed problem.
Why is Proper Waterhemp Management Important?
Glyphosate-resistant waterhemp has recently been confirmed in a small area of Richland County, North Dakota. Glyphosate-resistant waterhemp was confirmed in southern Minnesota in 2007 and has continued to increase over time. In 2010, based on visual observations, a high percentage of fields in Renville and Swift Counties in Minnesota had some frequency (> 1 plant/field) of glyphosate-resistant waterhemp present at harvest. The frequency of glyphosate-resistant waterhemp has continued to increase due to the continued planting of Roundup Ready® crops and the exclusive use of glyphosate.
Where glyphosate-resistant biotypes are known to be present in fields, waterhemp must be properly managed using reactive management strategies. Proactive management strategies will be necessary in other fields in order to attempt to prevent glyphosate-resistant biotypes from developing. Often the level of weed control inputs will be lower in fields where proactive strategies are employed due to lower weed seed bank populations. Proper management requires managing waterhemp across the entire cropping system over time. Many growers have begun using preemergence residual herbicides. Increasing crop rotation diversity in the cropping system and focusing on the use of Roundup Ready crops in the rotation where the fewest alternative herbicides to glyphosate exist, will also reduce the selection pressure for glyphosate-resistant waterhemp. To illustrate, wheat, corn, and LibertyLink® corn and soybean provide more chemical weed control options and should be considered, especially when integrating Roundup Ready sugarbeet into your rotation.
Proper Management in Sugarbeet
To proactively manage waterhemp in Roundup Ready sugarbeet, apply Nortron® at 6 to 7.5 pt/A (use the highest rate allowable for the soil type) and follow with postemergence glyphosate at 1.125 lb ae/A in the first application to 1 inch waterhemp followed by glyphosate at 0.75 lb ae/A every 10 to 18 days as needed after the initial application. To reactively manage glyphosate-resistant waterhemp in Roundup Ready sugarbeet apply Nortron as mentioned above or Eptam® at 1.1 to 2.3 pt/A plus Ro-Neet 6EC or Ro-Neet SB at 2.7 to 3.3 pt/A (use the highest rate allowable of both products for the soil type). Eptam plus Ro-Neet must be incorporated to obtain waterhemp control and control is usually improved when Nortron is incorporated. In addition to the soil-applied treatment, apply Betamix® or Betanex® at 12 to 22 fl oz/A (use the highest rate based upon the size of sugarbeet plants at application) plus Nortron at 4 fl oz/A plus glyphosate plus a test-proven MSO-based HSOC to cotyledon to 1 leaf waterhemp or Betamix or Betanex at 3.0 to 7.5 pt/A (depending upon size of sugarbeet and size of waterhemp) plus Nortron at 4 fl oz/A plus glyphosate. Apply glyphosate at 1.125 lb ae/A to Roundup Ready sugarbeet in the first application and 0.75 lb ae/A in all sequential applications. All sequential applications containing these mixtures should be applied every 10 to 14 days as necessary, adjusting the rate of Betamix and Betanex for the size of waterhemp at the time of application. Follow label restrictions for maximum seasonal rates of all products used. Regardless of proactive or reactive waterhemp management in sugarbeet, scout fields to determine need for row cultivation and/or hand-weeding to stop seed production.
Proper Management in Soybean
To proactively manage waterhemp in Roundup Ready or LibertyLink soybean, apply Authority® Assist, Authority™ First, Authority® MTZ, Boundary®, Gangster®, Pursuit® Plus, Prefix®, Sencor® (metribuzin), Sharpen™ plus Outlook® (dimethenamid), Sonic®, Spartan®, Valor®, or Verdict™ plus Outlook preemergence or Valor or Sharpen plus Prowl® (pendimethalin) or Treflan® (trifluralin) preplant incorporated. Most premix products or mixtures of single products should provide more effective control than single-active ingredient products. Apply glyphosate to Roundup Ready soybean and Ignite 280 to Liberty Link soybean at maximum single-application rates of 1 to 3 inch waterhemp. Warrant™ or Dual Magnum® (S-metolachlor) can be mixed with glyphosate or Ignite 280 to provide additional residual control. To reactively manage glyphosate-resistant waterhemp in LibertyLink soybean follow proactive management recommendations. To reactively manage glyphosate-resistant waterhemp in Roundup Ready soybean apply one or more of the soil-applied products above and follow with Flexstar® GT 3.5 or Cobra® or Flexstar plus glyphosate when waterhemp is 1 to 3 inches in height. Please see the label for fomesafen (Flexstar, Prefix) rate restrictions for your geographic area.
Proper Management in Corn
To proactively manage waterhemp in all types of field corn apply Balance® Flexx (ND only), Callisto®, Capreno®, Camix®, Harness® / Surpass® (acetochlor), Lumax®, Prequel® (ND only), SureStart® / TripleFlex™, or Verdict preemergence. Mixtures of Balance Flexx, Callisto, or Prequel with acetochlor will improve waterhemp control. Following the preemergence applications, apply glyphosate to Roundup Ready corn and Ignite 280 to LibertyLink corn at maximum single-application rates. In corn lacking herbicide resistance traits consider conventional herbicides such as Callisto, Callisto Xtra, Clarity® (at maximum rates and/or in mixtures with other products), Impact®, Laudis®, Realm™ Q, or Status® applied to 1 to 3 inch waterhemp. To reactively manage glyphosate-resistant waterhemp in corn lacking herbicide resistance traits follow the preemergence and conventional postemergence management strategies outlined above. To reactively manage glyphosate-resistant waterhemp in Roundup Ready corn, apply a preemergence herbicide and mix one of the conventional postemergence herbicides mentioned above with glyphosate at 1.125 lb ae/A or apply Halex® GT. To reactively manage glyphosate-resistant waterhemp in LibertyLink corn, apply a preemergence herbicide and follow with Ignite 280. Mixing atrazine at 0.38 lbs ai/A with Callisto, Halex GT, Ignite 280, Impact, or Laudis should improve waterhemp control.
Proper Management in Wheat
Many herbicides are available for management of waterhemp in wheat and wheat competes well against waterhemp. Follow recommendations in the Minnesota and North Dakota Weed Control Guides. The most important recommendation in wheat is to make sure no waterhemp plants go to seed after wheat harvest.
Additional Comments and Summary
Follow label directions for herbicide rates according to soil type and weed size, use of products by geography, and crop rotation, especially for sugarbeet. Consult the publication PRE and POST Herbicide Diversification Options (http://z.umn.edu/herbicidediversification at U of MN or http://www.ag.ndsu.edu/weeds/sugarbeet-files/Herb_Divers.pdf at NDSU) for information regarding crop rotation and herbicide effectiveness. Scout fields following the first postemergence application to determine need for a second application. If planting no-tillage corn or soybean, apply the necessary preplant herbicides to control emerged waterhemp and other weeds prior to planting and use the appropriate adjuvant(s) and rate(s) for all products in the preplant treatment. In areas of low rainfall, incorporation of preemergence herbicides may improve waterhemp control but may increase crop-injury for some products.
Proper proactive and reactive management of waterhemp this growing season and into the future is necessary to reduce selection of herbicide resistant biotypes and maintain the effectiveness of Roundup Ready crops in the rotation. This is especially true for Roundup Ready sugarbeet in which few herbicides are available to effectively manage waterhemp.