Part of what we are
seeing is that crop-weed interactions can be very different
between these two cropping systems. Weed competition appears
to be much less in the organic plots than in the conventional
“Damn, that’s a lot of weeds!”
is a common response when conventional farmers visit The Rodale
Institute® for the first time. Before our farm manager, Jeff
Moyer, throws an organic tomato our way, let us assure you that
he and the rest of the farm crew do an excellent job with weed management.
The point is that many conventional farmers are simply not used
to seeing any weeds in their fields at all.
Most organic farmers, on the other hand, are very familiar with
their non-crop vegetation. The next comment we usually get from
our visitors is, “how do you get such good yields with all
those weeds?” Our answer is that, based on our experience
and experimentation, weeds in organic crops just don’t compete
as strongly as they do with conventional crops.
This season at The Rodale Institute we are studying the effects
of weeds on corn and soybeans in our Farming Systems Trial, a long-term
comparison of organic and conventional farming systems. Part of
what we are seeing is that crop-weed interactions can be very different
between these two cropping systems.
Weed competition appears to be much less in the organic plots than
in the conventional plots. Although the organic corn was planted
two weeks after the conventional corn, the organic corn is greener,
taller, and shows no obvious nutrient deficiency. The conventional
corn shows reduced growth, yellowing foliage, and necrosis of the
lower canopy, indicating nitrogen deficiency. This pattern emerged
following a very dry May and June.
The weed management dilemma
Weeds are a serious problem for farmers because they can reduce
crop yields, decrease crop quality, foul equipment, and poison livestock.
Costly chemical herbicides, potentially dangerous to both farmers
and the environment, are normally used to combat weeds in conventional
agricultural systems. Organic farmers generally rely on labor-intensive
mechanical methods to remove weeds from their fields.
Farmers also fight weeds for aesthetic or cultural reasons. Many
people regard weed control as an index of farming skill, regardless
of crop yield or farm profitability. This attitude can lead to excessive
weed control—weed control beyond what makes sense economically—which
in turn can lead to increased water pollution and herbicide resistance
development. Finding a balance between yield loss and excessive
management is a challenge faced by farmers every year.
To find that balance we must start with an understanding of how
the presence of weeds affects crop yield. Weeds compete with crop
plants in three major ways. First, weeds may simply shade out crop
plants, resulting in poor growth from decreased photosynthesis.
Second, weeds may use up water that would otherwise be available
to crop plants. This type of competition is especially important
in arid regions and drought years. The third form of competition
occurs when weeds use up nutrients that would otherwise be available
for crop plants.
Although the organic corn was planted two weeks
after the conventional corn, the organic corn is greener, taller,
and shows no obvious nutrient deficiency.
The conventional corn shows reduced growth,
yellowing foliage, and necrosis of the lower canopy, indicating
Crop-weed interference has been studied extensively over the past
50 years. Most of this research has examined the effect of a single
weed species on specific crop plants under controlled conventional
management systems, resulting in "typical yield reduction"
indices for various weeds in different crops. Information about
yield reduction potential, crop quality loss, and harvesting difficulties
is used in combination with data on the costs of weed removal to
produce "economic weed threshold" levels.
The economic weed threshold is defined as the weed density at which
a control practice is economically justified. This approach acknowledges
that at low weed density levels, crop plants are not negatively
affected and therefore removal of additional weeds is more expensive
than it's worth. Most economic weed threshold values are expressed
as number of weeds per area. On a practical level, however, few
farmers--conventional or organic--calculate weed thresholds when
deciding how to manage weeds on their farms.
Possible explanations for an unexpected result
Okay, back to the tale of two competitions. At The Rodale Institute
we occasionally have a field or two where weeds develop prolifically.
This is usually a result of poor weather conditions that prevent
normal mechanical weed management (rotary hoeings and cultivations).
Interestingly, however, in most of these cases our corn crops still
According to the Penn State agronomy guide, these corn crops should
suffer massive yield losses. The 2005 agronomy guide, for example,
indicates that you can expect a 10 percent corn crop yield reduction
when you have 150 pigweed plants per 100 feet of row. Additional
10 percent losses should be expected with 150 lambsquarter plants,
400 giant foxtail plants, or 50 velvetleaf plants. If you add these
plant populations and losses up, than you should expect to have
a 40 percent yield loss from these 750 weeds per 100 ft row. After
many trials examining weed interference with crops, I can assure
you that we do not see a 40 percent loss of yield when we have weed
populations like that.
You should expect to have a 40 percent yield
loss from these 750 weeds per 100 ft row . . . However, in most
of these cases our corn crops still yield competitively.
Since most weed-induced yield reduction information and economic
weed thresholds have been determined under conventional management
practices, the question arises, do organic systems have different
economic weed thresholds than conventional systems?
One fundamental difference between conventional and organic farm
management that could be affecting crop-weed interactions relates
to the understanding and treatment of soil resources. For many conventional
farmers, soil is just a substrate to store synthetic fertilizer
and hold crop plants in place. Most organic farmers, by contrast,
view the soil as a biotic entity that needs to be fed and cared
for like any other living organism.
Soil fertility and health are essential to understanding crop-weed
competition. In organic production systems, plowed-down cover crops,
manures and composts provide nutrients over a longer period of time
than do the synthetic fertilizers used in conventional corn and
soybean production systems. Research has shown that weeds normally
take up synthetic fertilizer faster and in larger quantities than
crop plants. Nutrients from organic matter sources that are available
for a longer period of time can potentially provide a buffer against
nutrient competition. Weeds do not have the opportunity to deplete
organic nutritional sources within critical timeframes as they do
with use of synthetic fertilizers.
Soils on organic farms also typically have increased organic matter,
which allows more rainwater to penetrate and be retained than in
soil with lower OM levels. Because organically managed soils hold
more water and nutrients, it is likely that crop-weed competition
will be reduced.
Another reason why weeds may reduce crop yields in conventional
systems more than in organic systems is because organic farmers
tend to plant later than conventional farmers. Conventional farmers
can use seeds treated with fungicides, allowing them to plant into
cool, moist soils in early spring without having the seed rot before
it germinates. By the time organic corn is planted, the soil is
warmer and the plants grow more quickly, making it more competitive
against weeds than slow-growing conventional corn. The organic crop
is likely to reach canopy earlier, reducing competition for sunlight.
A closer look
These are more than just hypotheses. After 25 seasons comparing
organic and conventional corn and soybeans, we have found that there
is no significant difference between organic and conventional crop
yields. This occurs despite the significantly increased weed biomass
typical of the organic systems compared to conventional systems.
Long-term data from the Farming Systems Trial shows that excessive
weed infestations do indeed cause reductions in corn yields, but
only when the dry weight of the weeds exceeds 4,000 pounds per acre.
After 25 seasons . . . we have found that
there is no significant difference between organic and conventional
crop yields. This occurs despite the significantly increased
weed biomass typical of the organic systems.
We are currently conducting more studies to investigate weed thresholds
in organic and conventional corn and soybean fields. Within the
Farming Systems Trial, we have set up an experiment with four treatments—weed-free,
additional weeds added, normal weed management, and no weed management—in
order to evaluate crop yield and quality under different levels
of weed infestation. This study will be used to resolve this issue
of differences in weed-crop competition, but will also be useful
to determine effectiveness of normal weed management practices and
to see if differences exist in the types of weeds present in organic
and conventional fields.
Our weed management research is part of a larger collaboration
between The Rodale Institute, Dr. John Teasdale from the USDA-ARS
Sustainable Agricultural Systems Laboratory and Dr. Dave Mortensen
from the Penn State Weed Ecology Laboratory (click
here for more on the collaboration). Additional trials are seeking
to evaluate the relative weed tolerance of different corn and soybean
varieties. These trials are now in their third season and we hope
to soon have the ability to make recommendations on commercial varieties
that are notably weed tolerant. So stay tuned.