Cover Crop As A Natural Grown Weed Killer

There is something unique about rye that researchers at USDA’s Agricultural Research Service (ARS) have been trying to figure out. Used as a cover crop, rye is inexpensive, easy to grow, winter-hardy, and suppresses weed growth without herbicides.

Other cereal cover crops function similarly to rye, as they also produce high levels of biomass and have the potential for weed suppression, according to John Teasdale, research leader at the Sustainable Agricultural Systems Laboratory in Beltsville, MD. He says weed suppression is almost a “secondary ecological service,” compared to rye’s other benefits.

“Rye grows at cooler temperatures than other cover crops and tolerates colder winter temperatures than most cover crops,” Teasdale says. “It develops an extensive root system, which makes it adapted for preventing erosion and capturing excess nitrogen in soil — nitrogen that remains after a summer crop and could become an environmental contaminant during winter and spring. It can contribute to carbon sequestration, since it produces abundant biomass in spring. Thus, it is often the cover crop of choice for overwinter protection of soil and water and building organic matter in soil.”

Residual Benefits

So what is it that made Teasdale want to investigate rye and its effects on suppressing weeds further? Teasdale, along with ARS chemist Cliff Rice, hypothesized that something in rye affects soil chemistry.

“Much research has been done with benzoxazinones in the past and they have been promoted as a reason for weed suppression for a couple of decades,” Teasdale says. “But little research has been done on their activity in soil, so we have pursued that to see if these chemicals can account for weed suppression in soil.”

Cover crops can suppress weeds through several mechanisms, Teasdale explains, and an aggressively growing cover crop like rye will smother weeds that try to grow or establish at the same time as the cover crop. After the cover crop is killed in preparation for the subsequent cash crop, the residue can be left on the soil surface in no-till or incorporated into the soil in plow-till.

“Residue on the surface can suppress summer weeds by physically smothering weeds, by preventing environmental signals that stimulate weed seeds to germinate, and by releasing toxic chemicals,” says Teasdale. “Residue plowed into the soil would inhibit weeds primarily by releasing chemicals during decomposition.”

Research to date has shown that that surface residue is likely to suppress weeds more through physical than chemical mechanisms, Teasdale says.

“Chemical effects can account for some weed suppression but the effect only lasts for approximately a week when fresh tissue is decomposing rapidly,” he says. “Our current research shows that benzoxazinones can be isolated from soils, but they are present in small quantities for only a week or two and probably are not responsible for weed suppression. Other as yet unidentified compounds are probably more likely responsible for weed suppression.”

While the practical benefits of getting temporary weed suppression are well-documented and known by many growers, it cannot be relied upon for full-season weed control, according to Teasdale. As a cover crop that is commonly used throughout the eastern half of the U.S., rye’s soil and water protection benefits are well-known.

“The state of Maryland has a program that subsidizes farmers for growing rye to protect the Chesapeake Bay watershed, so additional research is needed to understand mechanisms for what is happening and provide more specific guidance for management of rye crops,” Teasdale says.