Diversified strategy needed to control weeds

With herbicide resistance increasing, a diversified management strategy is needed to control problematic weeds. Using cover crops to manage waterhemp is one strategy that researchers at both the University of Minnesota and North Dakota State University are evaluating.
For several years, Midwestern states have been struggling with herbicide-resistant weeds. A recent Minnesota survey found that waterhemp resistant to multiple herbicides is prevalent in the state, with some populations showing resistance to four, five or even six sites of action.  North Dakota also struggles with multiple herbicide-resistant weeds, including both kochia and waterhemp, according to Dr. Joe Ikley, North Dakota State University (NDSU) Extension weed specialist.
With herbicide options becoming more limited, the key to managing problematic weeds is to integrate other tactics into corn and soybean production. Expanding the crop rotation, diversifying herbicide sites of action, and even mechanically destroying weed seeds are viable strategies.
What about cover crops as a weed management tool? While they provide many benefits – including soil protection – cover crop adoption lags in Minnesota. The biggest challenge is our unique environmental conditions, according to Dr. Debalin Sarangi, University of Minnesota Extension weed scientist. Planting windows for both the fall cover crops and spring cash crops are narrow.
Despite the challenges, Sarangi and Ikley see value in assessing cereal rye as a cover crop for weed management.
For the past two years, NDSU has evaluated waterhemp control using different cereal rye termination times with and without a pre-emergence (PRE) herbicide. When both a cereal rye cover crop and a PRE were included, they complemented each other and waterhemp was controlled. In contrast, when there was no cereal rye or no PRE applied, waterhemp control decreased.
When cover crops are used, herbicide interception is a concern. A national study has determined that while up to roughly half of the PRE can be intercepted, most does make it to the soil surface, and there was always a net benefit to using a PRE within a cover crop compared to either no PRE, or no cover crop.
In southern Minnesota, researchers started looking at termination timings, seeding rates and seeding dates in 2021 to find the best cereal rye recipes. An early May planting date produced a modest amount of cereal rye biomass and some weed control benefits. Biomass increased four-fold and weed control dramatically increased with a late May planting date. However, potential yield penalties for the cash crop should be balanced against cover crop benefits when planting is delayed.
NDSU has seen about 4,000 pounds biomass per acre the past couple of years when termination was delayed until early June, which translated to roughly 50 percent waterhemp control. Other states recommend twice as much biomass for complete weed control. Consequently, cereal rye should not be treated as a standalone weed management treatment or as a replacement for herbicides.
Sarangi also looked at different termination options for the cereal rye. Chemical options are the fastest and most economical way to terminate a cover crop. Of the herbicides, glyphosate (Roundup, etc.) was the most efficient option.  The cereal rye was controlled within 10 days and the field was ready to plant.
“Start clean and stay clean,” Sarangi stresses. If PRE applications were missed this spring, he recommends coming back with a postemergence application as soon as possible and before the weeds reach three inches tall. For waterhemp control, tank mixing a POST herbicide with a residual will help. However, “certain PREs cannot be applied once soybeans are emerging, due to injury risks,” reminded Ikley. “Crops are emerging quickly now in the warm weather, so check both the fields and the herbicide labels.”
Thanks to the Minnesota Corn Growers Research and Promotion Council and the Minnesota Soybean Research and Promotion Council for their generous support of this program!
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