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7 Ways Cover Crops Help Fight Weeds

Follow these steps to bolster the weed-fighting strength of cover crops.

Cover crops are no magic bullet for controlling weeds, but they can go a long way in helping to keep weeds in check.

 “By planting cover crops, you can reduce the density of weed populations as well as the size of the weeds,” says Kansas State University (KSU) agronomist Anita Dille. “While trying to compete with the cover crop, the weed seedlings get less moisture, nutrients, and light. Because the weeds stay relatively small, you can get better control with herbicides.”

A case in point comes from a KSU trial evaluating the effectiveness of a cover crop in combatting Palmer amaranth. A cover crop of spring oats and peas was no-till planted in early March. The cover crop was terminated in mid-May, and soybeans were planted soon after.

The Palmer amaranth plants emerging in the cover crop treatment were smaller in stature as well as fewer in number than the Palmer amaranth emerging in the treatment without a cover crop. “Living cover crops or a layer of cover residue will reduce sunlight reaching the soil surface,” says Dille. “This will serve to smother and out-compete weeds for light, water, and nutrients.”

Some cover crop species further fight weeds through the process of allelopathy. This occurs when the decaying residue of certain plants releases chemicals that inhibit the germination of weed seeds.

7 Weed-fighting Strategies

Overall, there are seven key practices that can help cover crops fight weeds most effectively.

1. Determine the time of emergence of target weeds.

“Identify the weed species you want to control,” says Dille. “It’s important to get a cover crop well established before the target weed begins to emerge.

“Here in Kansas, summer annual weeds, like kochia, will come up very early in the spring – first of March in the western part of the state,” she points out. “Horseweed, a winter annual, begins to emerge in mid-September.”

2. Plan for robust growth of the cover crop.

To help the cover crop effectively compete with weeds, seed it well in advance of the time the problem weeds begin to emerge.

“Timeliness of planting the cover crop is critical to getting enough growth on the cover crops in order for them to be effective,” says DeAnn Presley, KSU soil scientist. “Cover crops should be planted six weeks before a hard freeze in order to produce enough biomass to outcompete weeds. Cover crops need a minimum of 6 inches of growth by the time weeds emerge.”

3. Choose cover crop species that produce a significant amount of biomass.

“Cereals are good choices for producing a lot of biomass,” says Dille. “These include cereal crops like triticale, oats, barley, wheat, and cereal rye.

“Because cereals tend to grow upright in narrow rows, you can increase the effectiveness of the canopy by planting a mix of species including brassicas, like daikon radish or turnip,” she says. “Including legumes like clover will add nitrogen as well as help thicken the density of the cover crop canopy.” 

4. Choose cover crop species that meet your goals.

For starters, choose species whose growth habits most closely match the needed weed-control time.

“Oats won’t overwinter, but they do a nice job of controlling weeds if you plant early enough in the spring to outcompete spring-emerging weeds,” says Presley. “In Kansas, barley and cereal rye will overwinter and provide weed control in spring. In one of our on-farm experiments, the farmer chose winter wheat as a cover crop, and it did a great job competing with both fall- and spring-emerging weeds.”

Cover crops planted in midsummer like sorghum-sudan or the millets can tolerate late-summer heat and will minimize fall-emerging weeds. Barley and oats seeded in September will also suppress weeds emerging later in the fall.

End use of the cover crop, such as for grazing, also figures into the choice of cover crop species.

5. Plan a strategy for allelopathy to work.

When intending to take advantage of allelopathy as a weed-fighting mechanism, consider that allelopathy occurs most effectively as plant material is decomposing.

“Allelopathy occurs after allelopathic plants such as rye or the brassicas are killed and their plant material is breaking down,” says Dille. The chemicals being released from the decomposing plant material act like preemergent herbicides, reducing the likelihood that weed seeds will germinate.

Giving this process time to work after termination of the cover crop and before the planting of the subsequent crop requires planning. “It’s a short window of opportunity,” says Dille. “Plan to terminate the allelopathic cover crop two to three weeks ahead of planting the main crop.”

6. Consider planting conditions and seeding rates.

“When flying cover crop seed on over soybeans or corn, we’ve seen that the population of the resulting cover crop tends to be uneven,” says Presley.

Aerially applying the seed at a higher rate helps to even out the cover crop stand.

Apply seed at a higher rate, too, when seeding late. “When planting late, increasing the seeding rate helps to compensate for the fact that some of the seeds aren’t going to come up,” she says.

Drilling cover crop seed right after harvesting the previous crop tends to produce a uniform stand. “Drilling results in a thick stand but one that is not as tall as a stand resulting from aerial seeding,” says Presley.

7. Monitor the effectiveness of the cover crop.

“In our experience, cover crops won’t eliminate the need for herbicides,” she says. “The weeds that do grow are smaller and fewer in number, making them easier to control with herbicide applications.”

By monitoring weed populations, you can evaluate the effectiveness of the cover crops as control agents under your growing conditions. To count weeds and compare their populations from year to year, lay a 36-inch square of plastic tubing on the ground and count the weeds inside, Presley says.

“Cover crops have some good potential for helping to fight weeds, but scouting is going to be as essential as ever,” she adds.

Online Tool for Choosing Cover Crops 

The Midwest Cover Crop Council offers an online decision-making tool, mccc.msu.edu/selector-tool/, to help in the selection of cover crop species best suited to specific regions and goals.

The interactive site allows you to key in your location, crop rotation, and objectives. In response, the site will list cover crop options best matching your needs.

“If your rotation is corn and soybeans, and you pick weed control as your goal, the site will show a list of cover crops that can fit that rotation, and it will show their ranking for weed control,” says Kansas State University soil scientist DeAnn Presley.

“For each suggested species of cover crop, the decision-making tool will provide [management details] such as the recommended planting time, planting depth, and seeding rate,” she says.

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