Cover Crops for Use in North Carolina Field Crop Production
Written by Molly Hamilton, Crop Science Extension Assistant, NCSU
Adapted, with permission, from Managing Cover Crops Profitably. Sustainable Agriculture Network (www.sare.org), Handbook 3. 1998
Benefits and Limitations of Cover Crops
Cover Crops for North Carolina
Web Resources for Cover Crop information
Benefits and Limitations of Cover Crops:
Benefits: There are economical and ecological benefits of using cover crops in organic production agriculture. Nitrogen-fixing legume covers can provide a significant source of nitrogen for subsequent crops, reducing fertilizer costs. Many cover crops effectively suppress weeds through physical means (out-competing weeds) and allelopathy. Cover crops can improve soil tilth, increase soil organic matter, 
improve soil structure (increasing the rate of water infiltration and reducing compaction), and improve nutrient cycling. Enhancing soil health by the use of cover crops can improve crop yields. Cover crops can also be used to prevent erosion, conserve soil moisture, and protect water quality (by slowing erosion and runoff). Cover crops may also be used to enhance insect pest management on farms. Cover crops can do this by providing habitat for beneficial organisms that are the natural enemies of crop insect pests. By lengthening a rotation through the use of cover crops, crop pests (disease, nematode, insects and weeds) may become less problematic.
Limitations: While nitrogen-fixing legumes can add significant amounts of nitrogen to the soil, that nitrogen can be lost to the atmosphere almost as easily as conventional fertilizers if not managed effectively. Shallow (and un-aggressive) incorporation of the green manure can help prevent N losses. Planting a grass/legume mix can also slow down the release of N so it is not lost as quickly when incorporated. Another challenge of using a cover crop is killing the cover at the desired time. Some cover crops are very easy to kill mechanically, others are more difficult. No conventional herbicides are allowed in certified organic fields to kill a cover crop. Some cover crops need to be killed at a certain time in their life-cycle to ensure that they do not set seed and therefore become a weed problem for future years. Some cover crops can hinder the cash crop if the cash crop is planted too soon after killing the cover. Covers with allelopathic properties can hinder germination or establishment of the cash crop, and a large amount of dry matter left by some cover crops can tie up nitrogen needed by the cash crop. Cover crop residues can also lead to cooler soils in the spring.
Cover crops should be selected to fill a need of your production system and fit well (in time and place) in your crop rotation. Below are steps you can take to help you find cover crops that will work well for your farm.
Identify the cover crop use:
Decide what you want the cover crop's function to be. What is needed from the cover crop (provide nitrogen, suppress weeds, expand rotation, add organic matter etc.)?
Where/how will the cover crop fit?:
You want to identify times in the year where you will need or have the opportunity to utilize a cover crop. Planning is important. Identify times of the year and crop rotation where the cover crop will fit, thrive, and fulfill its function. It may also be important to consider times of heavy labor or equipment demand when planning where and when the cover crop will fit.
Some common cover crop niches: A winter fallow field is an ideal time to have a cover crop. It is possible to seed a cover after harvesting a summer crop, or to overseed a shade-tolerant cover crop before the cash crop is harvested (e.g. cover crop can be overseeded before soybeans are harvested). Shade-tolerant cover crops include white clover, annual ryegrass, rye, hairy vetch, crimson clover, red clover, and sweetclover. To ensure adequate sunlight for the cover crop, overseed before full canopy closer of the primary crop or a few weeks before the cash crop starts to die. A summer fallow field can also be considered for cover crop use. Sometimes there are fields with a month or more summer or warm-season fallow (after early corn harvest, etc.), and a quick growing summer annual cover crop can provide erosion control, weed suppression, organic matter and possibly some nitrogen during that time. Buckwheat, millet, sorghum-Sudangrass, or cowpeas can be overseeded into a spring crop or planted after a small grain or corn crop is harvested. A full year fallow field can also be beneficial if a cover crop is planted to rebuild fertility or organic matter over a longer period. This cover crop is low-maintenance and can provide year-round erosion and weed control. There is an opportunity to harvest this cover crop for forage also.
Cover crop management:
It is important to determine how the cover crop will need to be managed in the time slot available for it. Think about things such as: how to seed the cover; weather conditions; soil temperature and moisture; cover crop canopy architecture; cover crop vigor (can it stand weather extremes and field traffic); cover crop regrowth; how to kill and plant into it; does cover winter-kill; time and energy constraints; and equipment and labor needs. These are important factors when thinking about which cover crop to use. How easily will it fit into your rotation? How easy is it to manage? Is it economical?
Select the cover crop:
Choose a cover crop that will work well with your goals and niche. Or, select one or two species of cover crop may come close to fitting with the goals and niche and can be used (individually or together). Mixed cover crop stands can be very beneficial, especially grass/legume mixes.
Cover Crops for North Carolina
Legumes:
Winter species:
hairy vetch
Austrian winter pea
Cahaba white vetch
crimson clover
subterranean clover
red clover
berseem clover
sweetclover
white clover
Summer species:
soybeans
velvetbean
cowpeas
Non-legumes:
Winter species:
ryegrass
rye
wheat
barley
triticale
oats
Summer species:
sorghum hybrids (sorghum-Sudangrass)
buckwheat
German millet
pearl millet
Japanese millet
LEGUMES:
Hairy vetch (Vicia villosa)
- winter annual in NC
- N-fixing legume—can fix 90-200 lb/A of total N
- good soil builder and weed fighter
- does well planted in combination with grass crops
Austrian winter pea (Pisum sativum subsp. arvense) also called field pea
- winter annual in eastern half of NC (unreliable growth in western half)
- N-fixing legume—can fix 90-150 lb/A of total N
- can be a weed suppressor
Cahaba White Vetch (Vica sativa)
- winter annual in eastern half of NC (unreliable growth in western NC)
- does not produce as much biomass as hairy vetch or accumulate as much N
- tolerant of wide range of soil conditions
Crimson clover (Trifolium incarnatum)
- winter annual in NC
- N-fixing legume—can fix 70-130 lb/A of total N
- can be good weed suppressor
- could be overseeded into soybeans for a winter cover and N-supplier for corn
Subterranean clover (Trifolim subterranean, T. yanninicum, T. brachycalcycinum)
- reseeding cool season annual (fall seeded) in piedmont and coastal plain of NC; cool season annual (spring seeded) in mountains of NC
- N-fixing legume—can fix 75-200 lb/A of total N
- can be good forage crop
Red Clover (Trifolium pratense)
- winter annual
- N-fixing legume—can fix 70-150 lb/A of total N
- can be a good forage source for livestock
Berseem clover (Trifolium alexandrinum)
- winter annual in NC
- N-fixing legume—can fix 75-220 lb/A of total N
- can also be used for weed control and grazing
Sweetclovers: Yellow sweetclover (Melilotus officinalis) and white sweetclover (M. alba)
- biennial (fall sown) in NC piedmont and coastal plain; biennial (spring sown) in NC mountains
- N-fixing legume—can fix 90-170 lb/A of total N
- soil builder, weed suppressor, good forage
White clover (Trifolium repens)
- perennial
- N-fixing legume—can fix 80-200 lb/A of total N
- erosion protection, good forage
Soybeans (Glycine max)
- summer annual
- late maturing varieties give highest biomass yields and fix most nitrogen
- economic choice for summer legume crop
- competes well with weeds
Velvetbean (Mucuna deeringiana)
- warm-season annual; viney habit
- performs well in sandy and infertile soils
- produces high amounts of biomass and decomposes readily to provide N for subsequent crops
Cowpeas (Vigna unguiculata)
- summer annual in NC piedmont and coastal plain; poor growth in NC mountains
- N-fixing legume—can fix 100-150 lb/A of total N
- can be a weed suppressor and forage crop
NON-LEGUMES:
Annual ryegrass or Italian ryegrass (Lolium multiflorum)
- winter annual (cool season annual grass)
- prevents erosion, improves soil structure and drainage, adds organic matter
- weed suppressor (but can become a weed, itself)
- nutrient scavenger
Rye (Secale cereale)
- cool season annual (cereal grain)
- prevents erosion, nutrient scavenger, adds organic matter
- weed suppressor
Wheat (Triticum aestivum)
- winter annual (cereal grain)
- can be good weed suppressor
- scavenge excess nutrients, adds organic matter, prevents erosion
Barley (Hordeum vulgare)
- winter or cool-season annual (cereal grain)
- can be good weed suppressor
- provides erosion control; helps with nutrient cycling
Oats (Avena sativa)
- cool season annual (cereal grain)
- can be good weed suppressor
- scavenges excess nutrients, adds organic matter
Sorghum-sudangrass hybrids (Sorghum bicolor X S. bicolor var. sudanese)
- summer annual
- weed and nematode suppressor
- prevents erosion, builds soil organic matter
- very fast growing
Buckwheat (Fagopyrum esculentum)
- summer or cool-season annual (broadleaf grain)
- good weed suppressor
- quick soil cover
German (foxtail) millet (Setaria italica)
- annual warm season grass; 2-4 feet tall
- matures quickly; low water requirement (however, not drought-hardy)
- needs fine, firm seedbed for adequate germination
Pearl millet (Pennisetum glaucum)
- summer annual bunchgrass; 4-10 feet tall
- well adapted to sandy and/or infertile soils
- not as easily killed by mechanical means as German or Japanese millet
- good weed suppressor
Japanese millet (Enchinochloa frumentacea)
- summer annual grass; 2-4 feet tall
- grows rapidly (mature seed in as little as 45 days)
- can be late-season green forage
Web Resources for Cover Crop Information
Managing Cover Crops Profitably -- Sustainable Agriculture Network, USDA, 1998 -- pdf file
NCSU Soilfacts: Winter Annual Cover Crops -- pdf file
Growing Small Farms: Cover Crops for Sustainable Production (from Chatham County, NC)
Summer Cover Crops (NCSU Horticulture Information Leaflet)
Overview of cover crops and green manures -- ATTRA publication
Sustainable Farming Connection: cover crop page
Back to Crop Production Home Page
