“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
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
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
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
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 yield competitively.
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
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 time-frames 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
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.