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Group 15 Herbicide Resistance in Waterhemp

Article cover photo
Waterhemp resistance has not yet been found in Iowa, but farmers should handle weed management as if there is. (Photo: Joseph L. Murphy/Iowa Soybean Association)

By Scott Nelson, ISA On-Farm Network® director

University researchers in Illinois1 have discovered populations of waterhemp that are resistant to Group 15 herbicides, which include products such as Dual II Magnum®, Stalwart®, Outlook®, Zidua® and Warrant/Harness®. Waterhemp resistance to Group 15 herbicides has not been found yet in Iowa, but soybean and corn farmers need to act to prevent or slow the development of waterhemp resistance. 

One of the most important practices for effective herbicide resistance management in soybeans is to use multiple modes of action. Use of Group 15 herbicides alone is not recommended, even by the manufacturers. Farmers have a long list available of premix herbicides containing multiple modes of action. Most of these products are very effective in managing waterhemp, but some caution is warranted. For example, premixes of Group 15 herbicides that also contain ALS inhibiting Group 2 herbicides, are available in the market. Since waterhemp resistance to Group 2 herbicides is widespread in Iowa, these premixes have only Group 15 as an effective mode of action. Using these products could put unnecessary selection pressure on the Group 15. 

Many premixes with metribuzin, or Sencor, are also available. These can be very effective products, but farmers need to ensure the amount of metribuzin in the premix is enough to control weeds pre-emergence. In some cases, it may be judicial to spike additional metribuzin in the premix to achieve good pre-emergence control.

Farmers who have fields with a history of heavy waterhemp pressure are still encouraged to apply Group 15 herbicides as layered residuals. Since waterhemp emerges throughout the season, a timely application of a Group 15 in a post-emergence program will prevent germination of waterhemp until the crop is more competitive with the weed. Studies have shown the optimum time to apply a residual Group 15 is about 30 days after planting. Waiting too long to apply the residual herbicide gives waterhemp an opportunity to emerge. Group 15 herbicides are not effective on emerged weeds. 

Farmers should consider more complex premixes in corn production as well. For years, farmers have relied on Group 15 plus atrazine as the backbone for weed control in corn. Because atrazine-resistant populations of waterhemp exist in the state, these premixes are relying mostly on the Group 15 herbicides to achieve control of waterhemp. 

In Iowa, waterhemp resistance to Group 15 herbicides is neither widespread nor officially present. The University of Illinois research has proven that waterhemp can adapt to almost any herbicide. Farmers should manage fields as if waterhemp resistance already exists by using multiple effective modes of action to delay or prevent the occurrence of waterhemp in their corn and soybean fields.

Contact Scott Nelson at snelson@iasoybeans.com.

1The Bulletin: Waterhemp Resistance to Group 15 Herbicides

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