Horseweed is undoubtedly one of the most troublesome weeds in the world. Herbicide-resistant populations are now found in 18 countries, and many are resistant to multiple herbicide sites of action. Two recent university studies – one published by the journal Weed Science and the other by the journal Weed Technology – explore the role cover crops might play in controlling horseweed and reducing the need for herbicides.

Cover Crops Alone Are Not Enough

One study explored the impact of cover crop planting date and seeding rate on the need for herbicide treatments typically applied in the fall and spring in no-till soybean crops.

Researchers compared cover crops planted in late September to those planted in late October. Some were seeded at 50 kg ha^-1, while others were seeded at double that rate. The reduction in horseweed density was similar, regardless of planting time or seeding density. In all instances, horseweed was best controlled when a comprehensive herbicide treatment program was added.

“Planting Green” Reduces Horseweed Density

A second team explored whether the time the cover crop is terminated makes a difference in horseweed control.

Test plots of cereal rye and of winter wheat were planted in soybean fields at two seeding densities. Some were terminated a week before soybean planting with a treatment of glyphosate. Others were left in place until one week after soybean crops were in the ground – a technique called “planting green.”

Planting green resulted in a cover crop biomass that was up to 272 percent greater than on plots terminated before soybeans were planted – reducing the biomass of glyphosate-resistant horseweed by 46 to 93 percent compared to fields with no cover crop. Early termination plots provided a less consistent suppression. Planting green also resulted in soybean yields that were as much as 108 percent greater than those on early termination plots.  

As with the first study, researchers found cover crops alone were unable to suppress glyphosate-resistant horseweed through to harvest time. Instead, they are best used as part of an integrated control strategy that includes herbicides.

Want to know more? Articles detailing both studies are now available online, and free to access for a month:

“The effects of integrating a cereal rye cover crop with herbicides on glyphosate resistant horseweed (Conyza canadensis) in no-till soybean”

“Effects of fall-planted cereal cover crop termination time on glyphosate-resistant horseweed suppression”