Wallaces Farmer

Cropping Systems: Enough cover crop biomass and ground cover can thwart weeds from emerging.

Meaghan Anderson

February 25, 2020

5 Min Read
cover crop residue on the soil surface
BEANS EMERGING: Large amounts of cover crop residue on the soil surface can help farmers control tough weeds. Meaghan Anderson

While cover crops are still planted on relatively few acres in Iowa compared to total acreage of corn and soybeans, I regularly get questions about managing herbicides in fields with an overwintered cover crop. As farmers become more comfortable with cover crops, they have begun to look for additional benefits from growing the cover crop. One opportunity is to use cover crops to help manage weeds. 

The most important aspect of suppressing weeds with a cover crop is to have sufficient cover crop biomass and groundcover present when weeds germinate and emerge. The cover crop residue must be persistent enough to cover the ground through much of the early summer. Most weed suppression is provided by the mulch on the soil surface. Research has shown that more than 4,000 pounds of dry biomass per acre is necessary to provide significant, consistent weed suppression. A uniform cover crop stand is needed to prevent weed escapes through gaps or missed areas.  

Suppress weeds with cover crop 

Studies show that cereal rye planted in September in central Iowa produced nearly 4,000 pounds of biomass per acre by the first week of May, while cereal rye planted in October had less than 500 pounds per acre of biomass by early May. 

While the size and timing of cereal rye development varies from year to year based on growing conditions, the 4,000 pounds of dry biomass likely translates to at least 24-inch-tall rye at time of termination. More consistent weed control is achieved with even stands and higher accumulation of biomass (bigger cereal rye). Since soybeans are typically planted after corn, there is a better potential for summer annual weed suppression in soybeans than corn due to increased spring growth of the cover crop prior to termination. 

Cover crop species such as legumes and brassicas do not consistently produce enough biomass or persist long enough after termination to suppress summer annual weeds. Thus, the focus is on using cereal rye for weed suppression, as it has the best potential to help manage weeds, especially in Iowa’s weather conditions. 

Increase herbicide effectiveness 

It’s important to understand what you can do to increase the efficacy of residual herbicides when sprayed on cereal rye. Large cover crops may affect the performance of residual herbicides by reducing the amount of herbicide that reaches the soil.  

Recent research at the University of Missouri found that interception of a residual herbicide by the cover crop greatly reduced the amount of herbicide reaching the soil. Furthermore, the research showed that the herbicide was not “released” from the cover crop residue either with rain or as the residue degraded. This suggests that residual herbicides applied to large cover crops may not be an effective use of that herbicide.  

We have used the rough rule of thumb that most of the residual herbicide likely reaches the soil when applied to cover crops smaller than 12 inches tall, so continuing to include that residual herbicide in the cover crop termination pass with the sprayer is probably the best method of application. When the overwintered cover crop gets larger, there may be an opportunity to delay residual herbicide application timing, thus extending activity later into the season and possibly reducing herbicide costs. 

Reducing residual herbicide cost 

There are opportunities to reduce herbicide expense by using cereal rye as a cover crop. A cereal rye cover crop has the potential to control early-season weeds, therefore acting as a short-lived residual herbicide. In this system, the cover crop might act as the first pass of a layered residual herbicide program (see chart ).  

chart demonstrating a herbicide program with cover crop acting as a short-lived “residual herbicide” at or shortly after crop planting, followed by a postemergence herbicide application with a residual herbicide at an early postemergence timing.

HOW IT WORKS: This chart demonstrates a herbicide program with a cover crop acting as a short-lived “residual herbicide” at or shortly after crop planting, followed by a postemergence herbicide application with a residual herbicide at an early postemergence. (Source: Bob Hartzler)

This requires several factors to fall in place, most importantly having a uniform stand of large cereal rye. Here are the steps outlining how this might work: 

  • Prioritize planting the cereal rye in a way to ensure an even stand. 

  • Scout the field before termination to check for stand gaps or thin areas where weeds are likely to establish within a few weeks after termination of the cover crop. 

  • Terminate the cereal rye with an appropriate burndown herbicide at or shortly after soybean planting, making sure to allow the cereal rye to grow to near or at flowering to have enough biomass present to suppress weeds. 

  • Scout the field to determine the appropriate timing for the postemergence herbicide pass with a residual herbicide included, as this will likely need to occur earlier than if a residual was included in the burndown pass. 

  • Spray the field with an appropriate postemergence herbicide and include a full rate of a residual herbicide effective against target weeds.  

Using cover crops as a weed control tool requires more effort than simply seeding the cover crop the previous growing season. Termination date must be based on cover crop size rather than when fields are fit for planting. Rigorous scouting is required to ensure the cover crop is uniform throughout the field and to determine the optimum time for postemergence applications.

We constantly talk about how important it is to integrate alternative weed management strategies into our crop production system to take pressure off herbicides, and a cereal rye cover crop is one strategy that can fit that need if managed right. 

Anderson is the ISU Extension field agronomist for central Iowa. Contact her at [email protected]



About the Author(s)

Meaghan Anderson

Meaghan Anderson is the Iowa State University Extension field agronomist for east-central Iowa. He areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and Integrated Pest Management.

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