COLUMBIA, Mo. – Winter cover crops growing on crop ground must be eradicated before spring planting, says a University of Missouri Extension weed scientist.

“Earlier the better,” says Kevin Bradley of the MU Division of Plant Sciences.

Cover crops not controlled prior to planting become weeds. They take moisture and nutrients from corn and soybean plants.

“Weather presents a challenge this year,” Bradley says. “Last year by the first of April we’d made out first applications. We’re at least two weeks late this year.

“We can’t give a date on when to spray. Weather changes everything.  And the winter was a lot rougher on cover crops this year.”

Bradley stepped up his research on herbicide controls of cover crops last year. He tried different commonly used herbicide mixes at different stages of growth.

Cover crops have drawn increasing attention as a way to cut winter erosion and improve soil health long-term.

Bradley conducted tests in 2013, after limited testing in 2012, on the cover crops of annual ryegrass, crimson clover, hairy vetch, cereal rye with vetch, and winter peas.

Bradley studies spring transition from cover crop to cash crop. Some cover crops become hard-to-control weeds, which cut cash crop yields.

While 93 percent control might sound good, that leaves enough “weeds” to compete for sunshine, fertilizer and water. Some treatments gave only 18 percent control.

A major finding: Controls used early work better. “It’s important to make timely application of the right herbicide,” Bradley says. Small weeds are easier to control than mature weeds.

Some widely used cover crops are difficult to control.

So far, Bradley’s tests show annual ryegrass, wheat and crimson clover are toughest to kill.

On the other hand, cereal rye, hairy vetch and winter peas are easy.

Easiest of all were tillage radishes and oats. “Both winterkilled in central Missouri. They required no controls and were removed from later tests.”

There is a lot of confusion between annual ryegrass and cereal rye. Bradley says. “Annual or Italian ryegrass (Lolium multiflorum) is much different than cereal rye. I have no problems with cereal rye.”

 Cereal rye is fairly easy to kill in the spring and has shown good results from the standpoint of weed suppression. But cereal rye is not a ryegrass species.

Since annual ryegrass became so troublesome and persistent, Bradley ran more intensive herbicide tests on it.

Best control was achieved when annual ryegrass was shorter. At just under 6 inches tall, 91 percent reduction was obtained. However, that dropped to 77 percent at 14 inches. At 36 inches, it was 58 percent.

Larger plant growth takes more herbicide.

From the ryegrass test, Bradley developed control tips:

-Adjust spray settings for more gallons per acre to get better coverage.

-Spray during daylight hours when ryegrass actively grows. Preferred temperature is above 60 degrees F.

-Spray at least four hours before sunset. That allows better translocation of herbicides.

-Avoid spraying when day or night temperatures are forecast to fall into the 30s.

Most herbicide mixes tested were based on Roundup PowerMax at 32 ounces per acre. Other herbicides were added to the tank mixes. “Looking at results, it’s clear this rate should be higher,” Bradley says.

For 2014, Bradley will study several new herbicide mixes that were not available last year. “We’ll have a lot to talk about at our field days and extension meetings.”

Tables showing full results are included in the most recent MU Integrated Pest & Crop Management newsletter at

Bradley ran his tests at the MU Bradford Research Center, Columbia, a part of the MU College of Agriculture, Food and Natural Resources.

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