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 whole.
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 nitrogen.
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
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
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
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 residue.
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.