Strategic Farming: Field Notes Addressed Cover Crops and Weed Management

Jun 02, 2023

By Phyllis Bongard

With the increasing cases of herbicide resistance, a diversified management strategy is needed to control problematic weeds in row crops. Using cover crops to manage waterhemp is one strategy that researchers at both the University of Minnesota and North Dakota State University are evaluating. UMN Extension Weed Scientist Debalin Sarangi and special guest Joe Ikley, NDSU Extension weed specialist, joined moderator Liz Stahl, Extension crops educator, for the May 31st Field Notes session to discuss the value of cover crops for weed management in conventional cropping systems.

Herbicide resistance is not going away

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. For more information, see the recent Crop News article, Multiple herbicide-resistant waterhemp is spreading rapidly in Minnesota.

Diversification is the key

With herbicide options becoming limited and the very long time it takes to bring a herbicide with new sites of action to the market, 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 using cover crops as a weed management tool? While they provide many benefits – including soil protection – cover crop adoption significantly lags in Minnesota. The biggest challenge is our unique environmental conditions, according to Sarangi. Minnesota and North Dakota have narrow planting windows for both the cover crops in the fall and row crops in the spring.

Despite the challenges, the weed scientists see value in assessing cereal rye as a cover crop for weed management. Establishing a cover crop could be an easier practice to adopt as herbicide options continue to diminish.

Cover crops as a weed management tool

Cereal rye termination and preemergence herbicides

For the past two years, the specialists from NDSU have evaluated waterhemp control using different cereal rye termination times with and without a preemergence herbicide. The termination times included 1) no rye, 2) rye terminated 14 days before planting, and 3) rye terminated day of planting or day after (planting green). A postemergence herbicide was applied when waterhemp reached 4 inches.

When both cereal rye cover crop and a preemergence (PRE) herbicide were included, they complemented each other and waterhemp control improved. In contrast, when there was no cereal rye or no PRE was applied, waterhemp control diminished by the end of the season.

As their research expands, Ikley will also be looking at how delayed termination might impact competition for soil moisture and resulting yield losses.

How much of the PRE is intercepted by the cover crop?

When cover crops are used, herbicide interception is a concern. A national study has determined that while up to roughly half of the PREs can be intercepted, most does make it to the soil surface. There was always a net benefit to using a PRE within a cover crop compared to either no PRE or not cover crop.

Minnesota cover crop recipes

To find the best cereal rye weed management recipes, researchers started looking at termination timings, seeding rates and seeding dates in southern Minnesota in 2021. Continuous corn, corn-soybean, and corn silage systems are also incorporated into the recipe research.

The fall of 2021 was a good establishment year for the cereal rye. The amount of biomass produced by the cover crop impacted weed management in 2022. For example, when crops were planted on May 13, the cereal rye produced about 1000 pounds of biomass per acre and provided some weed control benefits in Lamberton and Rochester, MN. Biomass increased to 4000 lb/A 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.

How much biomass is enough?

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% 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..

Terminating the cereal rye cover crop

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 most effective. Cereal rye was controlled within 10 days, while some herbicides took several weeks for cereal rye termination.

One option for organic producers is to use a roller-crimper, although it may not result in 100% control. Control is also stage-specific, so if the cereal rye is cut at the tillering stage for forage or grazing, control may only be 10 to 20%, while waiting for the heading stage may result in about 50% control. Including light tillage may be needed to achieve 100% control of the cover crop.

Final thoughts from the specialists

Sarangi stresses to ‘start clean and stay clean’ this season. If producers missed applying PREs this spring because of the rain and other unfavorable weather conditions, 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, Ikley reminds everyone that certain PREs cannot be applied once soybeans are emerging, due to injury risks. Crops are emerging quickly now in the warm weather, so check both the fields and the herbicide labels.

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