Cereal rye as a cover crop can reduce waterhemp

COLUMBIA, Mo. – Cereal rye as a cover crop may reduce waterhemp populations without yield loss in soybean, according to a three-year study at the University of Missouri.

MU Extension weed specialist Mandy Bish and a team of researchers studied how planting soybean into living cereal rye — “planting green” — and then terminating the cereal rye affected biomass accumulation, soybean stand and yield, and early-season waterhemp emergence. The Missouri Soybean Merchandising Council funded the experiment.

The team seeded cereal rye at 30, 50, 70, 90 and 110 pounds per acre in late October and early November. At soybean planting, cereal rye ranged 40-43 inches. Bish found that higher seeding rates of cereal rye did not increase biomass. Regardless of seeding rates, biomass generated was about 14,500 pounds per acre.

Bish says accumulated biomass did not affect soybean stand compared to soybean planted into no cover. However, soybean planted into cereal rye grew taller than soybean planted into no-cover plots. Soybean yield was about the same, regardless of seeding rate.

Biomass from cereal rye cover crop reduced waterhemp emergence for four weeks in 2018 experiments and six weeks in 2020 compared to plots without a cover crop. But this was not true in 2019, when waterhemp pressure at the study site was extremely high.

Bish says residual herbicides were not applied in this study, but the group continues to research integration of cereal rye cover crop with residual herbicides. In a USDA-funded study, MU’s results show that early-season weed emergence was suppressed by about 97% when cereal rye was terminated before stem elongation was complete (about two weeks before soybean planting) and a residual herbicide was applied at soybean planting.

“We are currently evaluating combinations of soybean planting dates, cereal rye termination timings and residual herbicide applications in a collaborative project funded by the United Soybean Board,” Bish says.

“Cereal rye, nor any cover crop, should be viewed as a silver bullet for weed control,” Bish says. Understanding the soil seed bank is extremely important when making decisions about integrating cover crops and chemicals for weed control, she adds.

“In studies where waterhemp seed in the soil seed bank was extremely high (corresponding to roughly 500 or more plants per square meter), we have not seen cereal rye consistently suppress waterhemp from emerging,” Bish says. “However, when waterhemp densities are around 100 to 200 plants per square meter, we can see the effects of cereal rye on waterhemp suppression.”