Cover crops: How you can use covers as a weed management tool

There are many benefits to using cover crops on the farm including improved soil structure, reduced erosion, better moisture management, and nutrient cycling among others.

We’re very close to spring planting season which is a perfect time to get a head start on weed management planning. As herbicide-resistant weeds increase, it is important that we look into additional methods in weed management. As of 2023, The International Herbicide-Resistant Weed Database shows there are 518 unique cases of herbicide-resistant weeds including over 14 glyphosate-resistant species in the United States. Cover crops can work as a weed management tool in most systems, including non-GMO and organic, when managed correctly.

Below are some ways that cover crops can work as a tool for weed management in conventional systems:

How do cover crops suppress weeds?

  • Competition   

Weed germination, like all other seeds, requires the right conditions with moisture, sunlight, and temperature being the main factors. Cover crops prevent germination of weeds by competing for moisture and shading out seedlings.

  • Physical Barrier

A cover crop has live biomass and residue that acts like a mulch which further prevents germination and stunts the growth of weed seedlings. By using cover crops to control seedling size, our herbicides can prove to be more effective.

  • Allelopathy

Some cover crop species also use allelopathy, a process in which chemical compounds exuded by the plant directly affects surrounding seeds or plants. Allelopathy can come from decaying plant residue; it can also come directly from plant roots and leach through leaves.

In instances where there are negative impacts, allelopathy can prevent germination and stunt growth. Studies show that it is difficult to directly correlate allelopathy effectiveness. Some cover crop species are known to already have allelopathic compounds including sunflowers, Sorghum, Buckwheat, and Cereal Rye.

There are concerns among producers that allelopathy from these cover crops may impact the growth of a cash crop however, there is not strong evidence for this. We usually see more impacts from poor nitrogen management than we do allelopathy for fields early in their transitions to regenerative agriculture. 

What are different weed life cycles like?

It is important to understand what weeds you’ll be dealing with on your farm and the life cycle of those weeds. As we discussed above, weed germination requires the right conditions but a weeds life cycle can also determine germination timing.

Summer annuals

Summer annuals are weeds that germinate in the spring and summer as the soil warms and moisture conditions are right. These weeds will reproduce and die in one season. Examples of summer annuals include fall panicum, lambsquarter, mallow, pigweed, amaranths, ragweed, spurge, cocklebur, and waterhemp. Most of these can flush throughout summer with rainfall, but a good cash crop canopy can reduce those flushes.

Winter annuals

Winter annuals are weeds that germinate in late summer to early fall. These weeds will grow, flower, reproduce, and, usually, die in hot summer weather. Some examples of winter annuals include marestail, henbit, mustards, nettles, and chickweed.

Biennials

Biennials are weeds that will germinate and form a rosette the first year of growth. In their second year, biennials bolt, flower, reproduce, and then die. Biennial examples include mullein, chicory, and ragwort.

Perennials

Perennials are weeds that live for more than two years. The most common perennials in row crops include nutsedge, bindweed, dock, Johnsongrass, foxtail, and thistles. 

When we know the life cycles of weeds, we can understand which weeds are most affected by cover crops. Understanding weed physiology can help guide what decisions might need to be made around cover crop species selection, planting, and termination. 

What cover crops are most effective for weed control?

A cover crop with good biomass and uniform stand throughout the field is going to have the most effective influence on weed control. Effective control starts with cover crop seeding, including the seeding date, seeding method, and cover crop seed selection. It is imperative to plan according to the weeds you are looking to control. 

Some of the best covers used to suppress weeds include oats, cereal rye, and buckwheat. All three of these germinate and grow quickly and can suppress weed germination and growth. For longer season growth and coverage, you would want to choose cereal rye, millet, sorghum, and sudangrass. With these choices, you would see not only more biomass, but you would also see a longer, thicker mulching effect before and after planting. This provides longer-lasting weed control.

For most Midwest cropping situations, a winter annual cover crop, such as cereal rye or winter wheat, planted after harvest will establish through the fall and winter and suppress weeds as we move into spring planting. This is, of course, dependent on seeding rate and quality of germination.

Seeding an early spring cover crop such as oats can have effective weed suppression, but planting may be delayed in order to achieve adequate mulch.

For growers raising wheat, or in instances of prevented planting, there are numerous summer annual cover crops that can be implemented to suppress weeds and improve soil quality. Millet, sorghum, and sudan grass all offer rapid summertime growth to shade soil, suppress weeds, and set growers up for their best chance of eliminating a herbicide pass.

Note: This  is not a rescue plan for fields with heavy weed pressure, but it can be used along with herbicides and/or additional weed management. Timely planting of covers and termination just before or after planting will provide your fields with the best weed control. For more information on cover crop application methods follow this link. Farming methods to consider when implementing regenerative ag practices (cargillag.com)

Are there economic benefits to using cover crops in weed management?

Getting a good stand of cover crops can replace a fall or spring burndown. University of Maryland claims that covers can buy an extra 4-16 weeks of weed control after termination. Iowa state suggests that a heavy cover crop residue can control weed emergence for 2-3 weeks after burndown, and to follow up with a delayed application of preemergent herbicide. This practice allows for even longer protection against later weed flushes before canopy closure. 

It can be difficult to see the financial economics of using cover crops in the first year, because of the up front costs that it takes to implement covers. However, the benefits aren’t limited to just reducing herbicide use and field passes. Your return on investment will increase year after year. Research, read articles, talk to your fellow farmers, and gather information to fully understand how cover crops can help your farm with weed control.

 

Below are several articles for review.

https://weedscience.org/Home.aspx

Selector Tools - MCCC (midwestcovercrops.org)

Using cover crops for weed control: Consider all aspects (k-state.edu)

Impact of cover crops on weed management | Integrated Crop Management (iastate.edu)

Offing Cover Crops for Weed Suppression: Featuring the Roller Crimper and Other Mechanical Contraptions | University of Maryland Extension (umd.edu)

Cover Crops for Weed Management in Oklahoma | Oklahoma State University (okstate.edu)

2016-2017-Cover-Crop-Survey-Report.pdf (sare.org)

Resources - Soil Health Institute

Janelle Leach

Janelle Leach is a Conservation Agronomist for our Cargill RegenConnect™ program. Janelle brings a wealth of experience with her as she has written Variable Rate Fertility prescriptions for farmers across six states for nearly five years and was a Product Agronomist before that as well. She obtained her Associate in Science, and Associate in Applied Science degrees in Agriculture as well as a Bachelor of Science in Sustainable Agronomic Systems. Janelle is excited to educate growers on using conservation practices to build soil health for the long term.

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