Cover Crops
Cover
Crop Publications Columbia
Basin Cover Crops Choosing
a Cover Crop Costs
and Problems Benefits
of Cover Crops
How to Choose a Cover Crop
(This section has been adapted with permission from Northwest Cover Crop Handbook by Marianne Sarrantonio, Rodale Institute, 1994.)
Cover crops provide many benefits, but they're not do-it-all "wonder crops". To find a suitable cover crop or mix of covers:
- Identify your problem or use
- Identify the niche (the best time and place for a cover crop in your system)
- Describe the niche
- Describe the ideal cover crop
- Select the best available species
- Test a few options
This makes selection of cover crops a little easier by focusing on some proven ones. Thousands of species and varieties exist, however. The steps that follow can help you find crops that will work best with a minimum of risk and expense.
1. Identify Your Problem or Use
Review Benefits
of Cover Crops to decide what you want most from a cover crop. This
simplifies your search.
Some common goals for covers are to:
- Provide nitrogen
- Add organic matter
- Improve soil structure
- Reduce soil erosion
- Provide weed control
- Manage nutrients
- Furnish moisture-conserving mulch
Having one or two secondary goals can narrow the hunt when comparable covers could satisfy a primary role. You might want habitat for beneficial organisms, better traction during harvest, faster drainage or another benefit.
2. Identify the Niche
Sometimes it's obvious where and when to use a cover crop. You might want some nitrogen before a corn crop, or a perennial ground cover in a vineyard or orchard to reduce erosion or improve weed control. For some goals, such as building soil, it may be hard to decide where and when to schedule cover crops.
Look at your rotation first. Make a timeline of 1-24 monthly increments across a piece of paper. For each field, pencil in current or probable rotations, showing when you typically seed crops and when you harvest them. If possible, sketch in a rough graph showing average daily temperature during the timeline, and another for average rainfall. Add other key information, such as frost-free periods and times of heavy labor or equipment demand.
Look for open periods in each field, open spaces on your farm or opportunities in your seasonal work schedule. Also consider ways to extend or overlap cropping windows.
Here are examples of common niches in some systems, and some tips:
Winter fallow niche.
In many regions, seed winter covers at least six weeks before a hard frost. Winter cereals are an exception and can be planted a little later. If ground cover needs are minimal, plant rye until the frost period for successful overwintering, although N recycling will be limited.
You might seed a cover right after harvesting a summer crop, when the weather is still mild. In cooler climates, consider extending the window by overseeding (some call this underseeding) a shade-tolerant cover before cash crop harvest. Annual ryegrass, hairy vetch, crimson clover, red clover and sweetclover tolerate some shading.
If overseeding, irrigate
immediately if possible, or seed just before a soaking rain. Species with
small, round seeds, such as clovers, don't need a lot of moisture to germinate
and can work their way through tiny gaps in residue.
If you want to harvest a cereal grain cover crop, interseeding a legume
might increase disease risks due to lower air circulation or insect pest
risk, so plan accordingly. Changing seeding rate or the rotation sequence
may lessen this risk.
To ensure adequate sunlight for the cover crop, overseed before full canopy closure of the primary crop (at last cultivation of field corn, for example) or a few weeks before the cash crop starts to die).
Expect excessive field traffic around harvest time? Choose tough, low growing covers such as grasses or clovers. Limit traffic or delay a field operation to allow for cover crop establishment.
Summer fallow niche.
Many vegetable rotations present cover crop opportunities - and challenges. When double cropping, you might have fields with a three-to-eight-week summer fallow. Quick-growing summer annuals provide erosion control, weed management, organic matter and perhaps some N.
Consider overseeding into a spring crop with buckwheat, millet or sorghum-Sudangrass, or a warm-season legume such as cowpeas. Or till out strips in the cover crop for planting a fall vegetable crop and control the remaining cover between the crop rows with mowing, partial cultivation or herbicide spraying.
Small grain rotation niche.
Companion seed a winter annual cover crop with a spring grain, or frost seed a cover into winter grains. Soil freezing and thawing pulls seed into the soil and helps germination. Another option if soil moisture isn't a limiting factor in your region: broadcast a cover before the grain enters boot stage (when seedheads start elongating) later in spring.
Full-year improved fallow niche.
To rebuild fertility or organic matter over a longer period, perennials or biennials - or mixtures - require the least maintenance. Spring-seeded yellow blossom sweetclover flowers in summer of Year 2, has a deep taproot and gives plenty of aboveground biomass. Also consider perennial forages recommended for your area.
Creating new niches.
Have you honed a rotation that seems to have a few open time slots? Plant a cover in strips alternating with your annual vegetable, herb or field crop. Switch the strips the next year. Mow the strips periodically and blow the top growth onto adjoining cash crops as mulch. In a bed system, rotate out every third or fourth bed for a soil-building cover crop. Another option: Band a cover or some insect-attracting shrubs around fields or along hedgerows to suppress weeds or provide beneficial habitat where you can't grow cash crops.
Or Build a Rotation Around cover Crops.
It's hard to decide in advance every field's crops, planting dates, fieldwork or management specifics. One alternative is to find out which cover crops provide the best results on your farm, then build a rotation around those covers.
With this "reverse" strategy, you plan covers according to their optimum field timing, and then determine the best windows for cash crops. A cover crop's strengths help you decide which cash crops would benefit the most.
3. Describe the Niche
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