While the USDA estimates only about 10% of U.S. cropland is being managed with cover crops for various soil health and weed suppression reasons, recent research across the Corn Belt shows over-wintering cereal crop covers can significantly reduce weed biomass.
While on-farm and research results vary widely, an early 2023 symposium of the Weed Science Society of America revealed several core trends tying cover crop use to weed suppression. Get Rid of Weeds (GROW), a collaboration of integrated weed control among land-grant universities, reports the following takeaways from the symposium:
1. Biomass is Key
The more biomass a cover crop produces before termination, the greater the likelihood of successful weed suppression.
LIVING MULCH. Cotton planted between a living mulch of white clover showed fewer Palmer amaranth weeds for two seasons after the university trial began. UGA Crop and Soil Sciences Weed Science
“A thick mat of residue can suppress weeds’ germination, emergence and reproduction, as well as aid in decay of weed seeds beneath it,” says Erin Haramoto, University of Kentucky weed scientist.
Haramoto is quick to note that not all regions of the country can consistently produce the biomass figures that seem fairly consistent in the Mid-Atlantic area with its warm springs and ample rainfall.
Using Cover Crops for Weed Control
The USDA’s Natural Resources Conservation Service offers these generalized recommendations for using cover crops to combat weeds:
- Species Selection — Blending cover crop species is often more effective than planting a monocrop. Some species grow quickly and die during the winter, while others take longer to establish, then live on into the spring. A mix provides continuous living vegetation for increased weed suppression.
- How & When — Seeding cover crops as soon as possible before or after cash crop harvest reduces the fallow period when weeds can emerge. No-till drilling offers a uniform, controlled distribution of cover crop seed across the field. Broadcast seeding is also effective but may require higher seeding rates to provide an even, complete blanket of growth.
- Terminate for Weed Control — Cover crop residue can remain in the field long after termination, providing a blanket to shade weeds and interfere with germination. Regardless of method, terminate as late as possible to get maximum biomass production in the spring.
“Good luck growing a towering stand of cereal rye by soybean planting time with only 2 inches of rain between January and June, a scenario faced several times by many Great Plains growers over the past 20 years,” Haramoto says. “To grow the amounts of small grain biomass needed for 75% weed suppression before soybean planting, the cover would have to be terminated as late as early June or later in some areas.”
2. Consider Your Region
John Holman, cropping systems agronomist for Kansas State University, says cover cropping every year poses obvious risks, particularly in the semi-arid western Great Plains.
He says in dry years, cover crops potentially can drain soil moisture needed for the following cash crop. In years with average or above-average precipitation, however, his research indicates covers can provide many benefits for Plains growers, such as wind and water erosion control, livestock forage, and weed suppression.
BATTLING WEEDS. A crimson clover cover crop roller-crimped and chemically terminated showed some reduction in Palmer amaranth emergence. UGA Crop and Soil Sciences Weed Science
To help strip-tillers decide whether to plant covers or fallow their fields because of dry conditions, Holman and his research team developed the Flex-Fallow cover crop concept.
“With Flex-Fallow, we recommend growers only plant cover crops in place of a fallow rotation if a Paul Brown Moisture Probe finds at least a foot of soil moisture and the NOAA Climate Prediction Center is forecasting average to favorable moisture for the season,” he says. “We try to take advantage of moisture in good years and crop less extensively in the dry years.”
Living Mulch Fights Palmer Amaranth
For several years, researchers at the University of Georgia have been experimenting with “living mulch” cover crops, usually legumes, to improve cash crop performance and reduce commercial fertilizer use. In 2020, the university’s weed science group began trials using similar tactics to fight Palmer amaranth in cotton production. While not conclusive, the results show promise.
Under the leadership of Crop and Soil Sciences assistant professor Nicholas Basinger, graduate research assistant Hannah Lindell has been actively involved in work examining the impact on Palmer amaranth populations in Georgia cotton, as well as the effects of cereal rye and crimson clover compared with no-till practices over the past four seasons.
The study, conducted on 36-inch row spacing, was set up with cotton planted into 20-cm vegetation free strips between a Durana white clover living mulch in the middles, no-till bare ground, roller-crimped and terminated crimson clover, and roller-crimped and terminated cereal rye.
“We did not use post-crop emergence or residual herbicides in years 1 and 2, primarily because the cover crop effect on Palmer amaranth was our focal point,” Lindell says.
In the end, cereal rye and living mulch cover crops greatly reduced the total number of Palmer amaranth plants in years 1 and 2, but not the number of seeds or biomass produced per Palmer amaranth plant, she explains.
“Living mulch created an environment that caused individual female Palmer amaranth plants to produce 38-60% more biomass and between 9,570 and 19,800 more seeds than if they were exposed to cereal rye, crimson clover, or bare ground (no cover crop) conditions,” she says.
Since 2020, Lindell and her associates saw shifts in cover crop and weed material occurring based on cover crop type.
“In year 1, cover crop biomass was abundant, but in years 2 and 3, cereal rye, crimson clover and living mulch began to decline,” she says. “One theory is that during year 1, the land had no prior cotton production, so cover crops could be planted within the optimal sowing window resulting in considerable biomass. But with cotton harvest usually occurring in late October to early November, we missed the sowing window in years 2 and 3.”
In the end, Lindell found cereal rye and living mulch cover crops greatly reduced the total number of Palmer amaranth plants the first 2 years. Lindell says with a combination of a cover crop barrier and a broadcast herbicide application, Palmer amaranth populations were nearly completely controlled.
Farther east, John Wallace, Penn State weed scientist and research leader for several GROW projects, says no-tillers in areas with dependable spring rains are showing interest in planting green, in which cash crops are planted into growing cover crops to increase time for covers to produce sufficient biomass for weed suppression.
Haramoto says in such conditions, farmers run the risk of the cover crop becoming a weed itself if weather delays spring termination with herbicides or crimpers. Also be aware of pest pressure from slugs and voles, which benefit from increased residue in the field.
3. Covers Won’t Control All Weeds
While obviously useful to suppress weeds, cover crops cannot control all weeds all season long, Haramoto cautions.
“We need a lot of biomass from cover crops to get 80-90% weed control,” she says, citing recent studies of roller-crimped cereal rye. “Maxing out at those levels of weed control is likely not sufficient for most growers, who will need additional tools.”
COVERS NEEDED. No-till cotton planted on bare ground with no cover crops showed obvious presence of Palmer amaranth as seedlings emerged. UGA Crop and Soil Sciences Weed Science
USDA Agricultural Research Service weed ecologist Steven Mirsky says covers will never be the total solution for weed control.
“If you’re trying to truly reduce the seed bank, you’ll need a holistic effort using tools like the new targeted harvest weed seed control systems and sprayer/weeder robotics,” he says.
4. Do Your Weed Control Homework
Haramoto says despite the risks, the benefits of cover crops for weed suppression, soil health and less erosion are becoming increasingly well known. The two remaining questions are determining if cover crops can truly reduce herbicide use and if they slow herbicide resistance.
WEED CONTROL. Burned down and roller-crimped cereal rye showed strong control of Palmer amaranth emergence. UGA Crop and Soil Sciences Weed Science
She says studies show cover crop residue allows fewer weeds to emerge and forces them to be smaller, leaving them more vulnerable to timely herbicide applications. But overall, Haramoto says more research is needed to determine cover crops’ long-term benefits of weed suppression across a wide geographic area.
