Quest for Best Cover Crop
For more than 6 years, ARS agronomist Seth M. Dabney has been searching for
the ideal winter cover cropone that makes no-till farming more economical
and productive in southern areas.
The problem with most cover crops is that their seed is expensive, and
they're often difficult to kill with one spraying of herbicide. In late spring,
when they make their most growth and are about to set seed, farmers are getting
ready to plant.
And trying to plant a primary crop, like cotton, while cover crops are still
dying often subjects the new seedlings to diseases, says Dabney, who works at
the ARS National Sedimentation Laboratory in Oxford, Mississippi.
Dabney's goal is to protect fragile topsoils in areas where crop residue
from cotton and soybeans gets easily decomposed, leaving the soil vulnerable to
erosion by heavy rains. During the cool period, which is more than half the
year, the South gets half its annual rainfall and a good portion of its solar
"We're trying to increase the amount of organic residues returned in
the cool period and, if possible, add a legume that fixes nitrogen and recycles
nutrients in soil," he says.
Dabney thinks the ideal cover crop is one that reseeds itself before it
needs to be killed. Such a crop doesn't have to be planted each year and yet
builds more organic matter than native weeds like chickweed and annual
Last year, Dabney received funding from USDA's Sustainable Agricultural
Research and Education Program for a 3-year cooperative research project on
cover crops. Its purpose: to integrate cover crops into southern cropping
systems by reducing seeding and weed control costs.
To this end, he and ARS research technician Calvin Vick at the Oxford lab
have been evaluating several soil-building legumes for winter hardiness and
their ability to reseed earlier than traditional cover crops like crimson
clover. This work is being done on small plots managed by cooperators at 10
locations across the South.
"Early growth and winter hardiness are generally negatively correlated,
so finding a legume with both features is difficult," Dabney says.
"We're looking for hard-seededness to the extent that, if a seed crop is
left to mature, it will come back for more than 1 year.
"Sometimes, in the Midsouth, a cold snap will kill these legumes.
"A burclover we collected locally in 1991 has the best combination we
have found so far for some upland sites," says Dabney. "It's called
southern spotted burclover and belongs to the genus Medicago that also
includes alfalfa, buttonclover, black medic, and other leguminous cover crops.
"This burclover, Medicago arabica, has an interesting history in
the Mississippi Delta, where it was one of the most widely grown cover crops in
the 1950's, rivaling hairy vetch."
He adds, "Burclovers are generally hard-seeded but have little cold
tolerance. This one has better winter hardiness than any of the commercially
available annual medics we have evaluated. The fact that it now grows wild here
shows it can persist."
Another way to cut cover crop costs is to use them to save on weed control.
According to Dabney, one system that shows promise is to quickly kill a rye
cover crop by mowing, then plant no-till cotton soon after, using tined-wheel
row cleaners to sweep aside dense residues.
The heavy mulch from the mowed cover crops suppresses weeds, thus reducing
herbicide needs. It also helps control erosion, increases organic matter, and
insulates the soil to keep cotton seedlings thriving when the temperature turns
"With this system, weed control in no-till cotton requires no more
herbicide than is used in conventional tillage systems," he says.
This type of alternative no-till management is being tested on farms in
Tiptonville, Tennessee; Denwood, Arkansas; and Yazoo City, Mississippi.
Finding the Right Combo
Since 1983, Dabney and agronomist Wayne Reeves at the National Soil Dynamics
Laboratory in Auburn, Alabama, have been working together on winter cover and
main crop combinations.
One that looks promising is crimson clover, Trifollum incarnatum L.,
as a winter cover for tropical corn. Tropical corn varieties are bred and
adapted to tropical and semitropical climatic zones, but they can be grown in
Four years of field tests at two locations in Alabama showed that the
tropical corn allowed for later planting in spring, which enabled the clover to
naturally reseed every year in a no-till system.
"With the reseeded clover, corn yields with just 45 pounds per acre of
nitrogen fertilizer equaled those achieved with 180 pounds of fertilizer
without the clover," Reeves says.
After growing clover for five seasons and 36 to 44 months without tillage.
Reeves studied the effect of the winter clover and maize combination on the
physical and chemical properties of two soilsHartsells sandy loam and
Norfolk loamy sand.
"Results showed that such a system can also improve the soil chemical
properties in a relatively short time. The clover increased soil carbon and
nitrogen on the Hartsells soil and potassium to the 2-½-inch depth on both
Calcium and magnesium concentrations were increased in the surface inch of
soil under clover, compared with winter fallow.
Several soil properties like water-stable soil aggregates, hydraulic
conductivity, and bulk density were also improved.
Since 1987, in tests near Auburn, Reeves has been studying another cover
cropwhite lupin, Lupinus albus L.for its potential as a
dual-use cover/cash crop in sustainable cropping systems. White lupin produces
seeds with protein similar to soybeans, so they can also be grown as a grain
crop. Lupin varieties tested included Tifwhite-78 and Tifblue-78 (L.
angustifolius L.); both were developed in Georgia by ARS.
"In more than 12 tests over 6 years, white lupin produced 30 bushels of
grain per acre. The grain-sorghum crop that followed required no additional
nitrogen and produced top yields," Reeves says.
His tests showed that white lupin will produce up to 360 pounds of nitrogen
per acre if it is grown to maturity. "It produces 100 to 120 pounds per
acre if growth is halted in mid-April to early May. Studies in Minnesota and
Australia show that when harvested for grain, lupin plant residue provides
about 60 pounds of nitrogen per acre," he adds.
Reeves cautions that lupin is susceptible to several fungal diseases and
should only be grown in well-drained soils. By Hank Becker, ARS.
M. Dabney is at the USDA-ARS
Sedimentation Laboratory, Oxford, MS 38655; phone (662) 232-2975, fax (662)
Soil Dynamics Laboratory, Auburn, AL 36832-5806; phone (334) 844-4741, fax
"Quest for Best Cover Crop" was published in
the August 1995
issue of Agricultural Research magazine.