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Cover Crop Benefits Include Weed Control

By Dorothy Noble




This Flying Media drone utilizes an electronically stabilized, high-definition GoPro video camera. At this demonstration at Steve Groff's Cover Crop Solutions Field Day near Lancaster, Pa., last October, the drone hovers over a cornfield to show growers how to quickly and efficiently monitor cover crops. It emits various colors and blinking sequences to signal battery status, distance from home, and mode-manual, ATTI or GPS. This drone is about 30 inches in diameter overall and weighs about 2.75 pounds. Flying Media is in Lancaster, Pa.
Photo by Bob Ferguson unless otherwise noted.

Cover crops provide many benefits for production agriculture. They can provide nitrogen, contribute to soil health, add organic matter, reduce erosion, aid pest management, improve air quality, attract pollinators, provide forage, add moisture-conserving mulch and suppress weeds.

Cover crops help control weeds in two ways. First, they compete with weeds for available water, light and nutrients. Second, many have allelopathic effects, producing biochemicals that influence weed emergence, growth or persistence.

Barley absorbs soil moisture early to overtake weeds. Brassica cover crops outcompete weeds with their rapid growth and full canopy in the fall. In the spring, brassica residues can inhibit small-seeded annual weeds such as pigweed and hairy nightshade. Besides attracting beneficial insects and pollinators, buckwheat chokes out weeds by shading and competition. Fast growth enhances crimson clover's prowess in crowding out weeds.

In the mid-Atlantic, forage radish planted in late August dies in the first hard frost. Its decomposing residues suppress winter annual weeds until April, allowing a weed-free seedbed for no-till crops. Although hairy vetch residue has a weak allelopathic effect, its shading restricts the light that early weeds demand. Also, the vigorous spring growth of fall-seeded hairy vetch inhibits weeds.

The allelopathic compounds in oat roots and their residue can hinder weed growth for weeks. However, subsequent crops can also be slowed. Delay planting the cash crop for a few weeks after killing the oats to minimize the effect.

Rye suppresses weeds by shading and allelopathy. In fact, the allelopathic effects can be prolonged by keeping the killed residue on the field. Sorghum-sudangrass smothers many annual weeds, including crabgrass, smooth pigweed and purslane. Moreover, its seedlings, shoots, leaves and roots secrete allelopathic compounds injurious to weeds.

In the past 20 years, considerable research has revealed a multitude of advantages for cover crops. Yet their diversity, regional and seasonal differences in performance, and sometimes conflicting grower goals can make selecting cover crops a challenge. A cover crop mixture can be highly desirable, but complicates selection.

At the 2014 Mid-Atlantic Fruit and Vegetable Convention, Tianna DuPont advised, "Match the cover crop to the farm." DuPont, a Penn State sustainable agriculture educator, said that mixing species can enhance the benefits of cover crops. Furthermore, she pointed out that many benefits increase with greater biomass. Selecting complementary species will increase biomass.

She illustrated complementary growth periods by using oats, sorghum-sudangrass, fava beans, sunn hemp and soybeans that are winter-killed. Cereal rye, annual ryegrass, triticale, red clover and canola are winter-hardy crops. To mix species by growth form, she suggested mixing tall species such as sunn hemp with low-density and vining species, and not planting either type too densely. For a complementary nutrient acquisition strategy, she used Austrian winter pea, forage radish and oats; another was oats and crimson clover. She noted that a diverse mixture can adapt to different fertility levels. Annual ryegrass and crimson clover was a favorite mix because the ryegrass holds soil aggregates well. It offers sufficient biomass without excessive height for easier plowing, and the low seeding rate along with its relatively inexpensive cost was a good value.

In DuPont's on-farm experiments, the specific needs of the individual farmers were a significant factor. For one, the planting date was crucial because both early spring planting and harvesting the cash crop in September were essential. One farmer lacked equipment. Another had constraints due to planting dates and seed costs.

When seeding cover crop mixes at the proper depth, DuPont advised separating the seeds by size and into different drill boxes, but that method requires the proper equipment. Making more than one pass will also work, but she cautioned against simply mixing the seeds and shooting for the middle.



To illustrate the importance of timing, Tianna DuPont compared the cover crop on the left, which was established on August 23, with the one on the right, established on September 24. The photo was taken on November 11 at Juniperdale Farms near Nazareth, Pa.
Photo by Tianna DuPont.

Finding the most desirable seeding rate for specific farm mixes may require some experimenting. She suggested starting with a small acreage and adjusting as needed. To determine rates in a grass/legume mix, keep the legumes at a monoculture rate and reduce the grass seeding rate by half to a quarter of the monoculture rate. For highly competitive species, use 2 to 3 pounds per acre for forage radish, 3 to 4 pounds per acre for canola, 15 to 20 pounds per acre for sorghum-sudangrass, and 20 to 40 pounds per acre for oats. When species share the same growth period, growth form and nutrient acquisition strategy, divide the seeding rate by the number of species in the group.

For DuPont's research, the cover crops were planted in August 2012 at three farms in eastern Pennsylvania. Each farmer planted one cover crop monoculture, a custom three-species mix and a four-species mix. The research evaluated more than weed control.

The preliminary findings showed that biomass and nitrogen uptake were dominated by nonlegumes such as cereal rye and canola; the monocultures and mixtures containing only legume species tended to be weedy; soil nitrogen uptake by nonlegume cover crops and weeds reduced nitrate leaching into the subsoil; the highest amount of nitrate leaching was observed under a frost-seeded red clover monoculture where weed growth was suppressed by mowing; and soil nitrate concentrations were not different between cover crop treatments at each site.

The research indicated that the cover crops with the most biomass had the most nitrogen. Red clover was used in the monoculture system, three species of clover made up another system, and Austrian winter pea, canola, red clover and rye made up the third system. The three-species clover system showed the most fall and spring weeds, even outstripping the cover crop measure of both the biomass and nitrogen accumulation. The red clover system produced fall and spring weeds, but the cover crop itself had appreciably larger accumulations of biomass and nitrogen than the three-clover mix. In the third system, both fall and spring weeds were negligible, and it excelled in both the dry matter and nitrogen measures as well.

Even with plenty of benefits, cover crop production must be managed, and challenges remain. In her presentation at the Mid-Atlantic Fruit and Vegetable Convention on reduced tillage for vegetable production, Dr. Anu Rangarajan, director of Cornell University's Small Farms Program, discussed several cover crop basics in organic no-till systems. She recommended killing the cover crop early and flail mowing if it's large. She added that rolling can restrict planting and interfere with cultivation. She also noted that employing row cleaners on planters will help manage residue. "Residue is a real problem in organic no-till production," she said, adding that excess biomass and root mass created in moist areas must be managed.



Cover Crop Solutions' David Weaver, of Robesonia, Pa., shows the size of an August 20 planting of Tillage Radish in late October. The feeder roots of the long taproot, which can reach close to 3 feet, are broken when simply pulled from the soil. Planted on Steve Groff's Lancaster County farm, this TillageMax Indy blend includes Tillage Radish, RootMax and CCS Crimson Clover. The rate was 15 pounds per acre.
Photo by Bob Ferguson.

The systems that excel in the Northeast may be unworkable in another region. Successful cover crop and weed control strategies depend on regional as well as seasonal conditions. Growers should consult their local extension offices for recommendations and compare experiences with their neighbors. "Managing Cover Crops Profitably," a publication from the Sustainable Agriculture Research & Education program, includes detailed descriptions of various cover crop species. It is available at http://bit.ly/1k8Xkft.

Dr. Juan Carlos Diaz-Perez, professor of vegetable production and physiology at the University of Georgia, said, "Herbicides must be used in no-till organic vegetable production systems here." He added that cover crops are helpful, but need to be supplemented by additional methods. He studied organic broccoli to determine the effects of integrating mulching, cover crops and tillage on weed control and yield, and found that weed control was higher in conventional tillage than in no-till. The cover crops used were pearl millet, cowpea and soybean.

The residue from the cover crops provided partial weed control during the first weeks after transplanting the broccoli. However, as the season progressed and the cover crop residues gradually broke down, the ability of the residue to control weeds was reduced. Pearl millet provided the highest control, although it was not statistically significant. The research noted that the level of weed control was the most important factor determining broccoli yield. Furthermore, the research revealed that under no-till, the residue from the cover crop that preceded broccoli provided only partial weed control.

The state of California has developed guidelines on cover crops in vegetable production. The University of California Vegetable Research & Information Center website (http://vric.ucdavis.edu) has links to detailed data and information on how to obtain "Cover Cropping for Vegetable Production," UC ANR publication 3517.

The cover crops used in California's warm season typically include sudangrass and cowpeas. Rye with various legumes is often used for cool-season crops. Dr. Milton McGiffen, vegetable crops specialist at the University of California, Riverside, said, "Cover cropping now is undertaken in areas in which water is not an issue."

In Midwest vegetable systems, rye and legumes, including red clover, hairy vetch and sweet clovers, are common in vegetable production. In the Pacific Northwest, brassicas, Austrian winter pea, sudangrass and hairy vetch provide cover crops for vegetables. In all areas, cover crop selection depends on climate (including rainfall patterns), the needs of the crop and the grower's objectives.

The author is a writer/researcher specializing in agriculture.