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Winter cover crops become weeds?

Cover crops are a great way to keep the soil rich and prevent erosion. Although, what happens when the ground frost prevents farmers from eradicating the cover crops before spring planting? In short, they become weeds.

Cover crops that are not controlled prior to planting become weeds. Whatever cover crop is present, they take moisture and nutrients from corn and soybean plants. “Weather presents a challenge this year. Last year by the first of April we’d made our first applications. We’re at least two weeks late this year,” says Kevin Bradley of the MU Division of Plant Sciences. 

After limited testing in 2012, Bradley conducted research on herbicide controls (using commonly used herbicide mixes at different stages of growth) of cover crops last year on annual ryegrass, crimson clover, hairy vetch cereal rye with vetch, and winter peas. Bradley studies the spring transition from cover crop to cash crop. Some cover crops become hard-to-control weeds, which cut cash crop yields.

Bradley came up with 93 percent control and other treatments gave only 18 percent control. That leaves enough “weeds” to compete for sunshine, fertilizer, and water. Bradley came to a major finding: controls used early work better. “It’s important to make timely application of the right herbicide,” Bradley says.

Of Bradley’s findings, widely used cover crops such as annual ryegrass, what, and crimson clover are the toughest to kill. Alternatively, cereal rye, hairy vetch, and winter peas are easy to eliminate.  The easiest were tillage radishes and oats.

Some tips to remember that Bradley found from the control ryegrass are:

  • Spray during the daylight hours when ryegrass actively grows. Preferred temperature is above 60 degrees Fahrenheit.
  • Spray at least four hours before sunset. That allows better trans-location of herbicides.
  • Avoid spraying when day or night temperatures are forecast to fall in the 30s. 
  • Adjust spray settings for more gallons per acre to get better coverage

To see the tables showing the full results, click here.

Source: University of Missouri Extension

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