Glyphosate-resistant waterhemp is now widespread in Minnesota and the Midwest. Some growers use preemergence residual herbicides to control waterhemp. Others use crop rotation to control waterhemp.
Waterhemp is an annual weed species in the pigweed family that is capable of producing more than 1 million seeds per plant. Due to a limited number of effective herbicides, especially in sugarbeet and soybean, waterhemp is difficult to control compared to most weed species.
In 2007, glyphosate-resistant waterhemp was confirmed in southern Minnesota and is now present in southern and west-central Minnesota. It continues to increase in frequency due to the continued planting of Monsanto's Roundup Ready® crops and the exclusive use of glyphosate. University of Minnesota Extension and North Dakota State University (NDSU) Extension have been working together to develop research-based management recommendations for growers.
In addition to the production of large quantities of seeds, continual germination throughout the growing season and an increased frequency of herbicide-resistant plants add 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 (1 to 12 percent survival after four years) compared to most species, meaning complete control (zero seed production) of all plants over a three- to four-year time period should significantly reduce the waterhemp seed bank, allowing the farmer to take control of this difficult weed problem.
In fields with low populations of waterhemp, it is important to be proactive and prevent waterhemp from establishing by trying to prevent even a single plant from going to seed in your field. Often the level of necessary weed control inputs will be lower in fields where proactive strategies are employed.
Proper management requires managing waterhemp across the entire cropping system over time. Many growers have begun using preemergence residual herbicides to manage waterhemp. Increasing crop rotation diversity and using Roundup Ready crops in the rotation where the fewest alternative herbicides to glyphosate exist will also reduce the likelihood of selecting for glyphosate-resistant waterhemp. Wheat, corn, and LibertyLink® corn and soybean provide more chemical weed control options and should be strongly considered, especially when integrating Roundup Ready sugarbeet into your rotation.
Where glyphosate-resistant plants are known to be present in fields, waterhemp must be managed diligently to reduce the amount of waterhemp in the soil seed bank; this will require an increase in weed management inputs.
Active management of waterhemp this growing season and into the future is necessary to reduce selection of herbicide-resistant plants and maintain the effectiveness of Roundup Ready crops in the rotation. This is especially true for Roundup Ready sugarbeet for which few herbicides are available to effectively manage waterhemp.
For more details on proper waterhemp management practices for corn, soybean, sugarbeet and wheat, visit University of Minnesota Extension's Crop News at www.extension.umn.edu/go/1063.