The adoption of cover crops in corn-soybean production systems can help growers improve cropping system resiliency to extreme weather events, improve soil conservation and health, and slow the evolution of weed herbicide resistance. To get the most out of cover crops prior to corn and soybean establishment, growers should have a sound termination plan in mind to maximize cover crop benefits and successful establishment of the following cash crop.
Cover Crop Termination with Herbicides
The cover crop species, weeds present under cover crop residue, and following cash crop should all be considered when selecting the proper herbicide to terminate a cover crop. This will ensure the cover crop does not later become a weed problem, all weeds present are terminated, and the cash crop isn’t negatively impacted by the cover crop and/or herbicide program of choice.
Research conducted in Wisconsin indicates that glyphosate is an effective herbicide for termination of annual grass cover crops, including cereal rye and wheat. A 2019 study comparing termination of a winter wheat cover crop found glyphosate (Roundup) to be more effective than glufosinate (Liberty) (Dewerff et al., 2020; Figure 1). See also: Termination of Cereal Rye and Annual Ryegrass Using Glyphosate.
For termination of broadleaf cover crops there are differences regarding most effective herbicide(s) depending upon cover crop species. For information on effective chemical termination of broadleaf cover crops see: “University of Missouri Final Results from a Multi-state Study on Cover Crop Termination with Herbicides” or “UW NPM Cover Crop Termination”.
Other considerations for herbicide selection:
If glyphosate is used for cover crop termination, it should be sprayed when day temperatures are above 55° F and night temperatures are above 40° F. To further improve effective cereal rye control, glyphosate should be applied before the boot stage.
If glyphosate-resistant weeds are present in your cover crop at termination (e.g., marestail, giant ragweed) adding herbicides such as 2,4-D and/or saflufenacil (e.g., Sharpen) to a glyphosate tank-mix can assist in improving control. This has also been shown to aid in termination of broadleaf cover crops (check product labels for planting restrictions).
If using contact herbicides such as paraquat (e.g., Gramoxone) and glufosinate (e.g., Liberty, Cheetah, Scout, etc.), practices that improve spray coverage will improve efficacy (e.g., medium spray droplet size and higher application carrier volumes [≥20 GPA]).
Applicators and handlers of paraquat (e.g., Gramoxone) are now required by the U.S. EPA to complete training every 3 years.
Benefits associated with using cover crops are correlated to the amount of biomass produced. Allowing cover crops to accumulate as much biomass as possible prior to termination will increase extent and longevity of benefits received, including reduced soil erosion and weed control. Ideally a grower would want to terminate their cover crops at a time when they can obtain the maximum amount of biomass without reducing resources for their cash crop (i.e., water, nutrients, sunlight). In general, the later a cover crop is terminated, the longer cover crop biomass will persist on the soil surface, suppressing weeds and protecting the soil surface (Figure 2).
Potential causes of cash crop yield reduction from cover crops include early season nitrogen tie-up limitations in corn, water stress during drought situations when cover crops can reduce available soil water for an early growing crop, competition for light, and/or poorly adjusted and maintained planting equipment.
Wet springs, such as in 2019, can make timely termination of cover crops difficult. A study conducted by the Wisconsin Cropping Systems Weed Science program (part of Kolby Grint’s MS graduate research) in 2019 at Arlington and Lancaster Agricultural Research Stations, Wisconsin found that:
In soybean, termination of a cereal rye cover crop 2 weeks after planting had no impact on yield compared to tillage, no-till, and earlier terminations of a cereal rye cover crop (Figure 3).
In corn, yield was consistently reduced when cereal rye was terminated after planting (Figure 4). It is suspected early season available nitrogen was a limiting factor for corn.
A Regional Soybean Yield Rye Cover Crop Study conducted in 2019 by the University of Nebraska-Lincoln, University of Minnesota, and University of Wisconsin-Madison also found that cover crop termination shortly after planting had no effect on soybean yield (Figure 5). These findings indicate that soybean is less likely to experience a yield reduction from cover crop termination after planting compared to corn when adequate soil moisture is available (which was the case at all these research sites in 2019).
UW-Madison researchers and many others across the Midwest typically recommend terminating cover crops before crop planting when possible (see: Cover Crops Do’s and Don’t’s). Delaying the termination and planting of fields with cover crops until non-cover cropped fields have been planted can be one way to maximize cover crop biomass accumulation, while minimizing potential for cash crop yield reduction.
Other considerations for timing of cover crop termination:
What are the forecasted weather conditions? Consider the following question: Is there time to delay termination a my cover crop while still allowing timely establishment of a cash crop?
When using herbicides for cover crop termination, efficacy can be reduced after cover crops reach reproductive stages. Termination prior to reproductive stages is recommended for best chemical control.
Producers are encouraged to visit with their insurance provider when deciding the time for cover crop termination.
If cover crops will be harvested for forage, herbicides should be sprayed after biomass harvest and removal from the field to control the re-growth.
Considerations for Herbicides following Cover Crop Termination
PRE-emergence Herbicide Application
A grower may decide to spray their PRE-emergence herbicides before, during or after cover crop termination; see Iowa State University Managing Residual Herbicides with Cover Crops. Cover crop residue can alter the fate of PRE-emergence herbicides. When an abundance of cover crop biomass is present at application there is potential for spray interception and subsequent reduced soil concentration of PRE-emergence herbicides. This is most prevalent when precipitation doesn’t occur soon after herbicide application. Residual herbicide incapable of reaching the soil will not have the desired impact on controlling germinating weed species.
If concerned about a PRE-emergence herbicide being ineffective when high amounts of cover crop residue are present, growers may consider including soil residual herbicides with a POST-emergence application (after some of the cover crop residue has decomposed).
POST-emergence Herbicide Application
Cover crops alone will likely not provide season-long weed control. POST-emergence herbicides are necessary to maintain control of in-season weeds. As always, scouting is very important for determining when to make a POST-emergence herbicide application. Cover crops typically aren’t as effective for suppression of large seeded weed species (e.g., velvetleaf, giant ragweed). Including a residual herbicide with POST-emergence applications can provide extended in season weed control. This can be especially effective when a PRE herbicide wasn’t used or when cover crop residue quickly degrades.
If planning to plant cover crops again in the fall, herbicides with soil residual activity should be carefully selected so they don’t negatively impact establishment of the cover crop species intended to use. For additional information on herbicide rotational restrictions for cover crops see: UW NPM Herbicide Rotational Restrictions for Cover and Forage Cropping Systems.
Timing is the most important factor for termination of a cover crop when using a roller-crimper. For grass cover crops, termination should occur when flowering has begun (Figure 6). Full anthesis of cereal rye in WI typically occurs around Memorial Day. For legume cover crops, termination should occur when the plants begin producing pods.
If roller-crimping occurs too early the cover crop will not be successfully terminated, if done too late there is risk the seed produced by the cover crop will be viable and add to the weed seedbank. Using a roller-crimper creates a dense mat of residue parallel to the soil surface which has been shown to be one of the best ways to suppress weeds with cover crop residue.
For additional information on roller-crimpers, watch the video: Advances Using the Roller-crimper for Organic No-till in Wisconsin.
Nebraska and Wisconsin Planting Green Cover Crop Survey
Nebraska and Wisconsin farmers were surveyed in early 2020 regarding their successes and challenges from planting green. The target audience was those who already adopt cover crops. A quick look at the responses show that more than half (53%) of growers indicated that they used this practice for planting both corn and soybean. Most farmers reported that they observed none or minor impacts of planting “green” on crop emergence, yields, allelopathic reactions, pests (disease, insects, weeds), and/or fertility. Once we summarize all the survey responses we will share more details about what has worked well and what has been challenging for farmers planting green.
If you planted green in 2019 or previous growing seasons, please take the time to fill out the quick Planting Green into Cover Crops SURVEY from our colleagues at the University of Nebraska-Lincoln. Your participation assists our programs by directing our research to best meet the growers we serve. Thank you!
- Kolby Grint (UW-Madison Weed Science Graduate Student)
- Dan Smith (UW NPM Southwest Wisconsin Regional Specialist)
- Nicholas Arneson (UW-Madison Weed Science Outreach Specialist)
- Ryan DeWerff (UW-Madison Weed Science Research Specialist)
- Shawn Conley (UW-Madison Extension Soybean and Small Grain Specialist)
- Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)
The Utility of a Planter Mounted Pulse Width Modulation Spray System. Ryan P. DeWerff, Nicholas J. Arneson, and Rodrigo Werle. University of Wisconsin-Madison, Madison, WI (2020 WSSA Poster 159).
Impact of Cover Crop Termination Timing on Arthropod Abundance, Defoliation, and Soybean Yield. Justin McMechan, Thomas Hunt, and Robert Wright. University of Nebraska-Lincoln, Lincoln, NE (2019 Soybean Management Field Days Research Update).