Waterhemp is an annual weed species in the pigweed family capable of producing greater than 1 million seeds/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 (1-12% survival after four 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 pre-emergence 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.
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 pre-emergence 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 LibertyLink soybean at maximum single-application rates of 1-3-in. 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 1-3 in. tall. 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, Camix, Harness/Surpass (acetochlor), Lumax, Prequel (ND only), SureStart/TripleFlex or Verdict pre-emergence. Mixtures of Balance Flexx, Callisto or Prequel with acetochlor will improve waterhemp control. Following the pre-emergence 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 1-3-in. waterhemp. To reactively manage glyphosate-resistant waterhemp in corn lacking herbicide resistance traits follow the pre-emergence and conventional postemergence management strategies outlined above. To reactively manage glyphosate-resistant waterhemp in Roundup Ready corn, apply a pre-emergence herbicide and mix one of the conventional postemergence herbicides mentioned above with glyphosate at 1.125 lb. acid equivalent (ae)/acre or apply Halex GT. To reactively manage glyphosate-resistant waterhemp in LibertyLink corn, apply a pre-emergence herbicide and follow with Ignite 280. Mixing atrazine at 0.38 lb. active ingredient (ai)/acre with Callisto, Halex GT, Ignite 280, Impact or Laudis should improve waterhemp control.
Follow label directions for herbicide rates according to soil type and weed size, use of products by geography and crop rotation. Consult the publication PRE and POST Herbicide Diversification Options  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.