Many of Iowa's popular winter hardy cover crops like cereal rye, wheat and clover, need to be terminated at least two weeks prior to planting corn and soybeans to minimize yield loss risk.
As beneficial as cover crops are, killing them is critical, says Barb Stewart, state agronomist with USDA's Natural Resources Conservation Service (NRCS). "Killing cover crops as late as feasible will maximize plant growth and residual nutrient accumulation," she said.
Stewart says, however, to be sure and allow at least two weeks for cover crops to decompose, release nutrients, and recharge soil moisture.
Soybeans grow through a terminated cereal rye cover crop.
If spring is exceptionally dry, or if the long-range forecast predicts dry conditions, Stewart recommends terminating cover crops when they are six to eight inches tall.
She says small grain cover crops can reduce corn yields similar to the effects of continuous corn. "Plant chemicals called allelochemicals released into the soil can inhibit corn growth," says Stewart. "This is why terminating the small grain cover crops at least two weeks prior to corn planting is necessary."
Some cover crop species, such as oats, spring wheat, and annual ryegrass, typically do not survive Iowa winters. Brassica cover crops, such as radishes or turnips, should also winter kill in Iowa. However, Stewart says these plants produce hard seeds that can stay viable in the soil for years. "Scout each year in fields where you have grown brassicas," she said. "Hard seeds may germinate the next spring. Watch these areas for seed germination, and if there is significant germination from hard seed you may need to terminate them."
Producers can terminate cover crops with herbicides that are compatible with the spring-planted crop. Harvesting, crimping, or mowing can be used only if the cover crop is mature.
To maximize soil health benefits from cover crops, Stewart recommends terminating them without tilling the soil. "Cover crops improve overall soil health by adding living roots to the soil during more months of the year and increasing organic matter in the soil," she says. "Tillage destroys all of those natural benefits."
Farmers should not plant corn or soybeans into a living cover crop, or risk losing out on crop insurance benefits.
Iowa farmers planted more acres of cover crops in 2012 about 100,000 than ever before. By comparison, only about 5,000 Iowa acres were planted to cover crops in 2009. Cover crops are helpful in limiting nitrogen leaching, reducing soil erosion, suppressing weeds, improving soil health, feeding livestock, among other benefits.
For more information about cover crop termination, visit your local NRCS office.