Discover the Cover: Managing Cover Crops to Suppress Weeds and Save Money on Herbicides

Posted by Elizabeth Creech, NRCS in Conservation Farming
Jul 24, 2018

If you’re farming in the United States, you’ve likely heard two major suggestions for building healthier soils across your operation: reduce tillage and plant cover crops.

Reduced tillage is often the easier sell. On average, farmers who switch from continuous conventional till to continuous no-till save more than four gallons of diesel fuel per acre each year. In a very direct way, fuel saved is money saved.

The potential economic benefits of cover crops aren’t so immediately obvious, but they’re just as important to consider. Cover crops can and often do lead to annual economic benefits for farmers. How? Weed suppression is one part of the answer.

Cover crops suppress weeds.

Conventional cropland fields are left bare each year after cash crops are harvested. Planting cover crops reduces or eliminates this fallow period, providing living roots and aboveground biomass over winter and into the spring to directly compete with weeds.

After termination, cover crop residue can block sunlight from weed seeds, providing increased suppression into the cash crop growing season. Some cover crop species are even allelopathic; they produce chemicals that reduce weed seed germination.

Zeb Winslow – a corn, soybean, and cotton producer in eastern North Carolina – has reduced his herbicide use by roughly 50% by planting cover crops. “Cover crops provide us with moisture retention and weed control,” says Winslow. “I only sprayed my soybeans one time after planting last year, and we’ve gone from spraying our cotton four or five times to spraying it twice with some spot treatments.”

Zeb Winslow is a fifth-generation farmer in Scotland Neck, North Carolina.

Zeb Winslow is a fifth-generation farmer in Scotland Neck, North Carolina. Winslow uses cover crops to reduce input costs and build the overall health and resiliency of his soil.  Photo Credit: Zeb Winslow

If you’re interested in using cover crops to suppress weeds and reduce herbicide use on your operation, here are three management decisions to consider.

1. Species selection matters.

Farmers interested in weed suppression should generally select cover crop species that establish quickly and produce large amounts of biomass. The goal here is direct competition: cover crops need to use the resources that would otherwise go towards weed germination.

Seeding a blend of cover crop species is often more effective than seeding a monocrop. Some species grow quickly and die during the winter, while others take longer to establish then live into the spring. A mix of species that collectively provides continuous living vegetation is often recommended – though not always critical – for increased weed suppression.

Winslow uses a diverse mix of cover crop species including rye, triticale, oats, crimson clover, rapeseed, and wooly pod vetch.  Photo Credit: Zeb Winslow

Winslow uses a diverse mix of cover crop species including rye, triticale, oats, crimson clover, rapeseed, and wooly pod vetch.  Photo Credit: Zeb Winslow

2. When seeding cover crops, consider the “how” and “when.”

A large part of successful weed suppression lies in rapid and complete cover crop establishment. It’s generally best to seed cover crops as soon as possible after harvest – or even before – to reduce or eliminate the fallow period when weeds could establish.

No-till drilling offers a uniform, controlled distribution of cover crop seed across the field.

Broadcast seeding is another option, where seed are mechanically distributed across the field’s surface. While some farmers till after broadcasting to improve seed-to-soil contact, others report successful broadcast seeding on no-till operations.

Broadcast seeding may require a higher seeding rate than no-till drilling to provide an even and complete blanket of cover crop growth, an important factor in successful weed suppression.

3. Terminate your cover with weed management in mind.

Cover crop residue can remain in the field long after termination, providing a physical blanket of ground cover that shades out weed seeds and hinders their germination. Rolling certain high biomass species is one way to effectively create this cover, though it’s by no means the only option.

Left: Cover crops are rolled while soybean seeds are no-till drilled. Right: Soybeans emerging from cover crop residue three weeks after planting.  Photo Credits: Jason Johnson, NRCS

Left: Cover crops are rolled while soybean seeds are no-till drilled. Right: Soybeans emerging from cover crop residue three weeks after planting.  Photo Credits: Jason Johnson, NRCS

Cover crops can be terminated chemically – with herbicides – or mechanically – using techniques such as mowing, plowing, undercutting, or rolling.

No matter the method, terminating as late as possible is generally advised for continued weed suppression. “Aboveground biomass can double between early April and early May in our area,” says Winslow. “Biomass grown during this period produces the residue that gives us a shot at season-long weed control.”

We’re here to help.

Like any cash crop, cover crops should be selected and managed with your specific operation in mind.

Visit your local service center for one-on-one support with selecting cover crop species and managing them through termination.

Visit our soil health page to learn about additional benefits of cover crops including improved water infiltration, reduced erosion, increased soil organic matter, and improved nutrient cycling.

Category/Topic: Conservation Farming