January 9, 2004: Want to learn how to manage
weeds naturally on your diversified farm? Here’s what
the experts at the 2003 National Biodynamic Conference in
Ames, Iowa, had to say about effective methods to help you
“just say no” to herbicides.
Combination of strategies helps control
weeds
Weeds are often the biggest problem that farmers face with
biodynamic and organic farming systems. Unfortunately, heavy
reliance on cultivation can be harmful to soil and crops,
and hand weeding is labor intensive. So what’s the answer?
“To manage weeds, you have to understand the life
cycle of a weed species,” said Matt Liebman, an agronomist
at Iowa State University. “Focusing on soil-crop-weed
interactions will aid the development of ecologically sound
farming systems.”
Crop rotations play a big role in weed management, he said.
“Some crops are much more effective in suppressing weeds,
so you need to tune in to where weed seeds are being added
in a rotation. In a rotation that includes corn, soybeans,
triticale and alfalfa, soybeans are the weak link when it
comes to weed control.” For vegetable producers, crops
such as carrots and onions are often less competitive against
weeds. These differences and characteristics illustrate the
point that the starting phase in your crop rotation can be
just as important as the crops you include in the rotation,
Liebman said.
“In the first example, if you start the rotation with
soybeans, you’ll have more weed pressure later on. But
if you start the same rotation with triticale, you’ll
have less weed pressure.”
Because weeds can’t get up and move, their fate depends
on the soil conditions surrounding them, he said. “Research
has compared soil amended with composted manure and green
manure versus a rotation with herbicides. Where the soil has
been built up with organic matter, weeds are less of a problem.”
The amended soils also show less variance in weed pressure
compared to unamended soils, Liebman said. “Not only
do you get better weed control in fields with healthier soils,
but you get more consistent control, too.”
Green manures can provide another weed control benefit,
he said. “Researchers have studied the effects of synthetic
nitrogen and clover green manure on bean crops and weed performance.
The results showed that the biomass of weeds was a lot less
with green manure than synthetic nitrogen. This means thousands
fewer weed seeds were produced per meter.”
Understanding seed predation
Fewer weed seeds can also be achieved by encouraging seed
predators like ground beetles, field crickets, field mice
and birds, Liebman said. “Research has shown that these
predators are taking very large quantities of seeds from fields
at certain times of the year.”
Averaged over several sampling dates during the growing
season, 18 percent and 22 percent of velvetleaf and giant
foxtail seed, respectively, were removed per day by predators,
according to the research cited by Liebman. “This can
strongly regulate weed populations over the course of 20 years,”
he said.
Perennials such as alfalfa and red clover tend to have higher
levels of seed predation than wide-row crops like corn, he
said. “Habitat is the key to increasing the number of
seed predators. Rodents remove about six times as much weed
seed as insects, and they like the canopies that form in alfalfa
and small grains.”
Allelopathic weed suppression
In weed seed tests involving red clover extract, the clover’s
toxicity effects led to fewer weed seeds germinating, Liebman
said, and of the ones that did germinate, their radicles were
much smaller. Researchers have also learned that larger seeds
have a relatively strong susceptibility to the allelopathy
from red clover, he said. This means you can get good weed
suppression for three to four weeks in large-seeded crops
that follow red clover, but, Leibman warned, “there
can be damage to small-seeded crops, unless you wait at least
three weeks before planting.”
Other plants offer similar results, he said. “Sorghum
is very potent. If you incorporate it green, it will virtually
obliterate weeds in your fields. Cereal rye also works, but
be careful with this, because it can sequester a lot of nitrogen.”
Little things mean a lot
Another important weed strategy, Liebman said, is to delay
fertilizing your fields until your crops are ready to take
up the nutrients. “Weeds will thrive on a big dose of
fertilizer,” he said. “That’s why a slow
release of nitrogen, for example, is much more effective than
a big pulse of fertilizer.”
The underlying message, Liebman said, is that ecologically-based
agriculture uses a lot of little things that add up to success
or failure with weed management. “While these tools
are individually weak, they are powerful together and can
provide weed control that’s as good as what you get
from synthetic herbicides.” 
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