Cover crops have multiple benefits for sustainable farming
systems. They can be used to manage nutrient cycling, add
organic matter, reduce leaching, prevent soil erosion, break
up pest cycles, attract beneficial insects and promote biodiversity.
They can also be used to manage weeds. Here at The Rodale
Institute, we use a number of winter annual cover crops in
our rotation, including hairy vetch, cereal rye, wheat and
barley. All are planted in late summer or early fall, and
all can suppress the germination and growth of weeds through
competition and shading.
In no-till systems, cover crops can be used to create a vegetative
mulch for weed suppression after the primary crop has been
planted. Unlike conventional no-till systems, which use herbicides
to kill standing cover crops and/or weeds, our organic no-till
system uses a roller mounted to the front of the tractor to
kill the cover crop mechanically. (Although herbicides are
generally regarded as benign compared to insecticides, they
remain the largest agricultural pesticide class, accounting
for 60 percent of the total amount of pesticide used in 2001.
Intensive herbicide use has been associated with a variety
of human health risks, negative impacts on wildlife and the
development of herbicide-resistant weeds.)
Soybeans are the most weed-sensitive crop we grow and the
most difficult to keep weed-free. Data from our Farming Systems
Trial show a strong negative linear relationship between soybean
yields and weed biomass: for every kilogram (2.20 lb) of weeds,
soybean yields drop by one-third to two-fifths kg (0.73 lb
to 0.88 lb).
These factors have made soybeans a priority candidate for
our organic no-till trials. Unlike our traditional-till, cultivated
organic soybeans, where most of the weeds are found in the
rows, in the no-till system the majority of the weeds are
found in between the rows. If and when they break through
the mat, they grow fast and big, creating the appearance of
a very weedy field. Our measurements have shown that overall
weed biomass in our no-till soybeans tends to be somewhat
higher than in our traditional-till soybeans, but it seems
likely that competition from the weeds is reduced because
of their location.
What makes a good mulch
The two key factors related to the weed suppressive effect
of rolled down cover crops are aboveground biomass and carbon
to nitrogen ratio. We measure biomass by placing a half-meter-square
quadrat in the field and then cutting, drying and weighing
all the plant material within the square. From this we can
calculate pounds of dry matter per acre in the field as a
The carbon to nitrogen ratio (or C:N ratio) is the proportion
of the two chemical elements carbon and nitrogen in the cover
crop plant tissue. Carbon is stored in carbohydrates such
as simple sugars (monosaccharides), complex sugars (polysaccharides),
starch, cellulose, hemicellulose and lignin, as well as in
plant oils, fats and waxes. Nitrogen is stored in amino acids,
the protein compounds in the plants. Protein is about 16 percent
The microbes that decompose crop residues use carbon as an
energy source and nitrogen to build tissue. If residues have
a C:N ratio higher than 20:1, the microbes will need to gather
N from the surrounding environment to do their work. Generally
speaking, the higher the C:N ratio of the cover crop residue,
the more slowly decomposition will occur and the longer the
residue will serve as a weed-suppressing mat.
For our no-till soybeans we've trialed three small grains
as cover crops: barley, wheat and rye. Our goal was to document
not just how much biomass each cover crop produced, but how
it responded to rolling and how well it suppressed weeds.
We believed that rye would provide the most biomass, for instance,
but we weren't sure the roller could handle all that growth
and knock it down effectively.
Our goal was to document not just how
much biomass each cover crop produced, but how it responded
to rolling and how well it suppressed weeds.
As many organic farmers know, rye has great advantages as
a cover crop. It's the most winter hardy of the small grains,
with the lowest germination temperature and thus the greatest
flexibility within rotations. Rye is also allelopathic, producing
chemicals that inhibit germination of other seeds. When the
standing rye is knocked down with the roller, these chemicals
slowly leach out of the dying plant tissue and help prevent
germination of small seeded weeds. This effect has been shown
to last 30-60 days depending on environmental conditions.
Cover crop biomass amounts will vary from year to year, of
course, depending on rainfall and temperatures. Generally
speaking, however, rye grows taller and, as a result, produces
more biomass than wheat or barley, a total of about 6,000-8,000
lbs/ac dry weight. It also has the highest C:N ratio (about
40:1), making it the most resistant to decomposition. Barley
produces the least biomass (4,000 to 5,000 lbs/ac) and has
the lowest C:N ratio (about 20:1). Wheat is in between, producing
5,000-6,000 lbs/ac of biomass with a C:N ratio of about 35:1.
One potential disadvantage of rye is that it can dry out
a field, making it unsuitable as cover crop in drier regions,
especially before soybeans, which require more water for germination
than do other grain crops. As the saying goes, “If it’s
too dry, don’t try."
Rolling small grain covers
To kill standing small grain cover crops mechanically you
have to wait at least until the plants are in the "boot"
stage, with the head fully developed but still inside the
rolled leaf sheath. For us the boot stage arrives in mid to
late May, with heading and flowering in late May to early
June. As a rule, the longer you wait past the boot stage,
the more complete kill you get, provided you have enough weight
to crimp the stems. We normally wait until the head has emerged
and is flowering.
So far, we've been rolling our barley, wheat and rye all at
the same time, in late May or early June. In the future we
plan to adjust the time of rolling according to each crops'
maturity. This could give us an earlier planting of beans
in the barley, for instance, maximizing the effectiveness
of the cover and giving us an earlier jump on the weeds.
Another consideration when rolling down small grain covers
is the direction of rolling. We seed our small grains with
a drill, so there are 7½-inch spaces between the rows.
If the plants are rolled in the same direction as they were
sown, gaps can appear along the inter-row spaces. Rolling
perpendicularly or diagonally to the direction of drilling
can avoid this, but may not be practical depending on the
size and layout of the field.
Aiming for better seed-to-soil contact
The biggest challenge we're having now with our organic no-till
system is not with killing the cover crop, however, but with
getting a good stand planted through the knocked-down residue.
Rye offers the best weed suppression, but the heavy mass of
vegetation causes the planter's depth wheels to ride up and
makes it difficult to get the seed placed well into the furrow.
Soybeans in particular require good seed-to-soil contact
for germination. In conventional no-till it's recommended
to increase seeding rates by 15 percent to compensate for
loss in the residue, and it may be that a higher increase
is needed for organic no-till. Our current planter set-up--a
Monosem vacuum pickup mounted to a four-row Kinze planting
unit--has a maximum seeding rate of 150,000-155,000 seeds/ac,
so we're somewhat limited in how we address this.
The above graph shows how as cover crop biomass increases,
soybean plant populations decrease. With a planting rate of
150,000 seeds/ac and a labeled germination rate of 88 percent
(these were Iowa 3006 beans), ideally we would have had 132,000
plants/ac. Planting into the barley and wheat covers, with
an average fresh-weight biomass of 26,370 lbs/ac, we obtained
soybean populations of 121,417 plants/ac. But planting into
the rye cover, with up to 37,041 lbs/ac biomass, soybean populations
fell to 82,667 plants/ac. Soybeans are relatively “plastic,”
so smaller plant populations have some ability to grow larger
and produce similar yields to denser populations--but there
are limits to that plasticity.
The biggest challenge we're having now
is getting a good stand planted through the knocked-down
Because of these challenges, yields in our organic no-till
soybeans have been lower than our organic traditional-till
beans—around 25 bu/ac in 2004 (compared to our typical
40 bu/ac). It's unclear whether the limiting factor in the
no-till system at this point is seed placement and resulting
crop plant populations, weed pressure, moisture or even fertility.
In future seasons we'll continue to try to improve our planter
setup, experiment with the timing of rolling and planting
and begin to examine other questions like the role of cover
crops in nutrient availability.