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Cover Crop Use and Management
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Hairy vetch is a nitrogen-providing
powerhouse, a cover crop that “fixes” significant
atmospheric nitrogen for the subsequent crop.
– Photo by Rob Myers |
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Planting cover crops between cash crops provides a great opportunity for farmers
and ranchers to diversify. In addition to improving soil quality, cover crops
slow runoff, crowd out weeds, prevent nitrogen leaching, provide habitat for
wildlife and beneficial organisms, and can even be grazed or harvested.
“Preventing erosion is perhaps the most obvious soil benefit of cover
crops,” said Marianne Sarrantonio, a professor of sustainable crop production
at the University of Maine, “but providing organic matter is a more long-term
and equally important goal.” For more information about cover crops, see
Managing Cover Crops Profitably, 2nd Edition, a detailed reference manual from
the Sustainable Agriculture Network. (See “Resources")
Most farmers plant annual grasses or legumes as covers, sowing winter annuals
in late summer or early fall to provide cover until the following spring. The
grasses scavenge nitrogen and prevent its loss while the legumes add nitrogen
to the system; mixing grasses and legumes optimizes rapid soil cover and overall
soil improvement.
Although any fast-growing herbaceous plant has potential as a cover crop,
seed cost or availability can be limiting. Other challenges include selecting
a cover crop that will survive the winter, finding a method of low-cost seeding
and developing an efficient way to kill or control the cover before planting
the primary economic crop. Some tips for successful cover cropping include:
 Choose
covers that are easy to plant and establish and also easy –
and affordable – to kill and control.
Satisfactory
varieties should provide reliable and persistent ground cover
and have no negative impact on the following economic crop –
such as using up too much soil moisture or harboring pests.
When
selecting a legume, make sure it has good ability to provide nitrogen.
To boost
biomass accumulation or improve winter survival, some producers
seed a cover crop before the previous cash crop has finished growing.
You can overseed a cover crop with a planter early in the growth
cycle of a select few crops – such as clover into wheat
– and you can seed some other covers during the last cultivation.
For
appropriate crops in your region, consult Managing Cover Crops
Profitably, 2nd Edition (see “Resources").
Because killing or controlling cover crops is a key management consideration,
many farmers favor spring oats for their ability to die reliably during cold
winters. However, the longer a cover crop survives into the spring, the longer
it curbs erosion or traps moisture, so other farmers select a cover that must
be killed with herbicide or tilled in before they plant their summer cash crop.
In Lancaster County, Pa., Steve Groff rolls down his jungle-like cover-crop
mixtures of hairy vetch, rye and crimson clover with a Buffalo rolling stalk
chopper before transplanting tomatoes into those 25 acres.
Some farmers prefer to suppress, rather than kill, the cover crop while the
cash crop is getting established, then let the ground cover grow gradually as
an understory crop. White Dutch clover, for example, provides erosion control,
weed suppression, supplemental nitrogen and possible habitat for beneficial
insects. This can only be done successfully with ample moisture. Don’t
jeopardize a cash crop by allowing the intercrop to compete for available soil
moisture!
Rotational Benefits
Whether farmers rotate their crops primarily for economic or environmental
reasons, agro-ecosystems clearly benefit from a diversity of crops. Rotational
crops curb erosion, improve soil structure, conserve soil moisture and help
break up insect, disease and weed cycles. They also contribute soil nutrients:
small-seeded legumes like alfalfa or sweet clover are an economically competitive
nitrogen source with commercial fertilizer.
In the long-term Wisconsin Integrated Cropping Systems Trial, launched in
1989, soybeans benefited from the so-called “positive rotation effect”
when small grains were added to a traditional corn-soybean rotation. While the
corn seemed to need only soybeans to maximize its performance, reduced disease
pressure in the longer rotation bumped up soybean yields. Altogether, the expanded
rotation returned $43 more per acre, on average, than the original rotation
– and its income was more stable. The research was funded in part by a
SARE grant. The randomized, replicated studies were conducted on two research
farms – in Dane and Walworth counties – representing different climates
and soil types.
In southeastern Minnesota, grower Andy Hart, using a SARE grant, provided
$7-an-acre incentives to neighbors who wanted to try cover crops of oats, barley
or winter rye after harvesting their peas and sweet corn. With most of southeastern
Minnesota’s 35,000 acres of peas and sweet corn left open to wind and
water erosion for up to 10 months each year, Hart was determined to find –
and encourage planting of – a better rotation.
Not only did the participating neighbors continue using cover crops, but other
farmers in the area began adopting the practice as well. “They see the
advantage of cover crops,” said Hart. So does Lakeside Foods, a local
canning company that is encouraging their use among growers.
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Rotations that maintain
crop residue on the soil surface reduce erosion, conserve water
and build soil organic matter. Using a kit, Wendell Jones, a
district conservationist with USDA’s Natural Resources
Conservation Service, measures residue on an Iowa City, Iowa,
farm.
– Photo by USDA-NRCS |
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Soil Benefits
Diverse rotations improve soil and, thus, crop yields. Rotations
that keep crops or their residues in the ground longer provide more
protection from wind and water erosion. Fields planted in small
grains or hay are less vulnerable to erosion than those planted
in row crops.
You can enhance the beneficial effects if you also reduce tillage. Surface
residue moderates soil temperature and conserves soil moisture. Residue also
builds more soil organic matter, which improves water-holding capacity and boosts
populations of beneficial soil microorganisms. On the other hand, excessive
tillage promotes erosion, dries soils and destroys soil structure as well as
the food sources on which soil organisms depend.
In an Ohio State University study, a three-year reduced-tillage rotation of
corn-soybean-wheat-hairy vetch compared favorably with the typical two-year
corn-soybean system. Both were superior to the corn monoculture system, resulting
in less soil erosion, less nitrate pollution and a better distribution of labor.
The use of hairy vetch contributed at least 25 pounds of nitrogen per acre to
the subsequent corn crop. The benefits of the three-year system can be attributed
to a combination of factors, including greater diversity of crops, soil-building
cover crops and small grains, and reduced tillage.
Farmers can use reduced-tillage practices in many rotational strategies. Excessive
tillage promotes erosion, dries soils and destroys soil structure as well as
the food sources and micro-niches on which beneficial soil organisms depend.
Reduced tillage slows the turnover of nutrients, encourages diverse communities
of beneficial insects and leaves temperature- and moisture-moderating quantities
of crop residues on surfaces.
In Springerton, Ill., Ralph Upton received a SARE grant to test cover crops
in his corn-soybean-wheat rotation. He wanted to learn whether cover crops like
buckwheat, hairy vetch, rye, rye grass and sunnhemp would penetrate his compacted
soil. Results have been promising. The rye grass, into which he plants corn
or soybeans, put down roots 52 inches deep. The roots of his cereal rye, into
which he plants soybeans, extend 48 inches below the soil surface. Upton also
has measured his hairy vetch roots at 43 inches.
Comparing his fields to a neighbor’s, he found that a probe went 10 inches
into his ground compared to just 3 inches next door. The benefits extend to
his crops, too.
“We’re getting great root systems,” he said. “We had
corn roots down 54 inches deep where we had the cover crop and our soybeans
will go down 42-43 inches.”
With crops able to pull moisture from subsoils, Upton has seen yield increases
of 3 to 6 bushels per acre, and he expects more benefits to come. “I think
the benefits of this project will be very good in the long run,” he said.
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