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OVERVIEW OF NONLEGUME
COVER CROPS
Commonly used nonlegume cover crops include:
 Annual
cereals (rye, wheat, barley, oats)
Annual
or perennial forage grasses such as ryegrass
Warm-season
grasses like sorghum-sudangrass
Brassicas
and mustards
Nonlegume cover crops are most useful for:
Scavenging
nutrients—especially N—left over from a previous crop
Reducing
or preventing erosion
Producing
large amounts of residue and adding organic matter to the soil
Suppressing
weeds
Annual cereal grain crops have been used successfully in many different
climates and cropping systems. Winter annuals usually are seeded
in late summer or fall, establish and produce good root and topgrowth
biomass before going dormant during the winter, then green up and
produce significant biomass before maturing. Rye, wheat, and hardy
triticale all follow this pattern, with some relatively small differences
that will be addressed in the section for each cover crop.
There is growing interest in the use of brassica and mustard cover
crops due to their “biofumigation” characteristics.
They release biotoxic chemicals as they break down, and have been
found to reduce disease, weed and nematode pressure in the subsequent
crop. Brassicas and mustards provide most of the benefits of other
nonlegume cover crops, while some (forage radish, for example) are
thought to alleviate soil compaction. See the chapter, Brassicas
and Mustards, for more information.
Perennial and warm-season forage grasses also can serve well as
cover crops. Forage grasses, like sod crops, are excellent for nutrient
scavenging, erosion control, biomass production and weed control.
Perennials used as cover crops are usually grown for about one year.
Summer-annual (warm-season) grasses may fill a niche for biomass
production and weed or erosion control if the ground would otherwise
be left fallow (between vegetable crops, for example). Buckwheat,
while not a grass, is also a warm-season plant used in the same
ways as summer-annual grasses.
Nonlegume cover crops are higher in carbon than legume cover crops.
Because of their high carbon content, grasses break down more slowly
than legumes, resulting in longer-lasting residue. As grasses mature,
the carbon-to-nitrogen ratio (C:N) increases. This has two tangible
results: The higher carbon residue is harder for soil microbes to
break down, so the process takes longer, and the nutrients contained
in the cover crop residue usually are less available to the next
crop.
So although grass cover crops take up leftover N from the previous
crop, as they mature the N is less likely to be released for use
by a crop grown immediately after the grass cover crop. As an example
of this, think of how long it takes for straw to decompose in the
field. Over time, the residue does break down and nutrients are
released. In general, this slower decomposition and the higher carbon
content of grasses can lead to increased soil organic matter, compared
to legumes.
The carbon content and breakdown rate of brassicas is usually intermediate
to grasses and legumes, depending on maturity when terminated. Brassicas
and mustards can take up as much N as grass cover crops, but may
release that N more readily to the subsequent crop.
Nonlegume cover crops can produce a lot of residue, which contributes
to their ability to prevent erosion and suppress weeds while they
are growing or when left on the soil surface as a mulch.
Although grasses and other nonlegumes contain some nitrogen in
their plant tissues, they generally are not significant sources
of N for your cropping system. They do, however, keep excess soil
N from leaching, and prevent the loss of soil organic matter through
erosion.
Management of nonlegumes in your cropping system may involve balancing
the amount of residue produced with the possibility of tying up
N for more than one season. Mixtures of grass and legume cover crops
can alleviate the N-immobilization effect, can produce as much or
more dry matter as a pure grass stand and may provide better erosion
control due to the differences in growth habit. Suggestions for
cover crop mixtures are found in the individual cover crop sections.
In addition to grasses, another summer non-legume is buckwheat,
which is described in detail in its own section.
Buckwheat is usually classed as a non-grass coarse grain. While
it is managed like a quick-growing grain, it has a succulent stem,
large leaves and white blossoms.
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Ryegrass
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