Most vegetable legume growers have yet to adopt cover crops due to the potential for reduced germination and yield in thick residue.
But a new study from the University of Illinois and the U.S. Department of Agriculture’s Agricultural Research Service shows early-terminated rye could be a promising part of an integrated weed management program for some vegetable legumes, including edamame.
“In general, the more cover crop biomass you have, the better the weed suppression. We found a sweet spot in an earlier experiment with edamame when we terminated rye at tillering and then planted into the stubble. The rye provided measurable weed suppression without harming the crop,” said Marty Williams, USDA-ARS ecologist and affiliate professor in the UI Department of Crop Sciences. “We wanted to follow that up with yield experiments for edamame and decided to broaden to some mainstream vegetables legumes including snap bean and lima bean.”
Williams and his research team looked at weed density and biomass in bare soil and in plots planted with a rye cover crop terminated at tillering, about a month before vegetable crops were planted. For three growing seasons, the researchers either sprayed weeds with one of the few registered herbicides, hand-pulled weeds in addition to spraying or left weeds alone to grow. Then they gathered information on weed suppression, crop establishment and yield as well as soil moisture and nitrogen.
In edamame and snap bean, early-terminated rye reduced weed biomass by 53% and 73%, respectively, comped with bare soil. And, consistent with his earlier studies, Williams found no reductions in edamame establishment or yield.
“We found the early-terminated rye system worked well in edamame. The rye suppressed weed biomass without impacting the crop,” Williams said. “For lima bean, the system failed. For snap bean, there is room for improvement.”
He suspects the poor performance in lima and snap bean is related to the crops’ weak ability to fix nitrogen in soils depleted of the nutrient by the cover crop. In turn, poor crop growth and canopy development favored the weeds.
Although early-terminated rye worked well in edamame, the cover crop didn’t eliminate the need for other weed control measures.
“It reduced the weed load, but did not eliminate it,” Williams said. “Early-terminated rye roughly halved the weeds compared to bare soil. The weed load was further reduced by using an herbicide. Hand-weeding, while generally used as a last-resort tactic, was used in this system to entirely eliminate interference with the crop and weed seedbank additions. That combination is the most effective approach.”