Manage Weeds Using Cover Crops

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Restriction of synthetic herbicide use in organic agricultural systems increases the complexity of weed management, leading farmers to cite weeds as one of the greatest barriers to successful organic production. To meet this challenge, most organic growers employ a wide range of tools in an integrated weed management system.

My 2011 survey of Midwest organic farmers found that the top four most commonly adopted weed management practices were crop rotation (86%)   between-row cultivation (78%), primary tillage (76%), and cover cropping (66%).

A vigorous cover crop can prevent weeds from germinating and growing. Photo credit: James DeDecker.

A vigorous cover crop can prevent weeds from germinating and growing. Photo credit: James DeDecker.

Define Cover Crops
Cover crops are crops grown between harvest and planting of commodity or feed crops, usually not for harvest, but for the production of biomass and the various agroecological benefits this additional biomass can provide.

While cover crops are most often thought of as a means of preventing erosion and improving soil health, they can also be applied as an effective weed management tool. Cover crops can suppress weeds in four primary ways:

  1. By diversifying and filling gaps in a crop rotation;
  2. By competing with weeds for light, nutrients and moisture;
  3. By releasing chemicals that inhibit the germination and growth of weeds (allelopathy); and
  4. By attracting beneficial organisms that feed on weeds and weed seed.

The following will expound on how each of these suppressive mechanisms functions and can be applied.

Diversifying And Filling Gaps
Diverse crop rotations suppress weeds by forcing them to grow in different crops with varying life cycles, growth habits, spatial arrangements, and management requirements. Rotating dissimilar crop species also allows the application of a wide range of control measures and exposes weeds to more natural mortality factors.

For example, adding a year or two of a perennial legume crop like clover between annual vegetable crops can reduce the germination of annual weeds normally triggered by soil disturbance and also permits regular mowing that can suppress problem perennial weeds like Canada thistle.

It’s also important to remember that many weeds are pioneer species that tend to establish quickly and thrive in disturbed habitats. While mechanical control methods like cultivation can knock back weeds for a period of time, disturbing the soil essentially resets the succes sional clock, bringing new weed seed to the soil surface and initiating more weed germination and growth.

While it may be possible to till enough to keep weeds down, tillage degrades soil health and equipment is costly to operate. As an alternative, cover crops can be grown in short windows before and after cash crops to fill gaps in a rotation, minimizing exposure of bare soil and limiting the ability of most weeds to become established.

Research at Michigan State University’s Kellogg Biological Station found that oilseed radish grown following snap beans reduced weed biomass 98.5% compared to plots left bare after beans were harvested.

Choosing The Best Option
Many cover crop options are available, and each farming system is different. Successful use of cover crops for weed management depends on strategic implementation.

Before planting a cover crop, consider:

  • Is a particular species/variety likely to fit your objective of weed suppression via a particular mechanism?
  • Is the seed readily available at an acceptable cost?
  • Is the cover crop suited to your field conditions, soil, cropping system, traffic patterns, etc?
  • Is there a high likelihood of good establishment given your equipment and labor availability, timing, etc.?
  • Do you have a viable strategy for suppression and residue management?
  • Is the cover crop likely to cause any additional problems due to weediness, insects, etc.?

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