Abstract
To some extent, weeds are a result of crop production, but to a
larger extent they are a consequence of management decisions. Managing
croplands according to nature's principles will reduce weed problems.
And while these principles apply to most crops, this publication
focuses on agronomic crops such as corn, soybeans, milo, and small
grains. The opportunities to address the root causes of weeds are
not always readily apparent, and often require some imagination
to recognize. Creativity is key to taking advantage of these opportunities
and devising sustainable cropping systems that prevent weed problems,
rather than using quick-fix approaches. Annual monoculture crop
production generally involves tillage that creates conditions hospitable
to many weeds. This publication discusses several alternatives to
conventional tillage systems, including allelopathy, intercropping,
crop rotations, and a weed-free cropping design. A Resources list
provides sources of further information.
Table of Contents
Introduction
FIRST, FREE YOUR BRAIN
As Iowa farmer Tom Frantzen poetically states: "Free your
brain and your behind will follow." What Tom is referring to
is discovering new paradigms. Joel Barker, author of Paradigms—The
Business of Discovering the Future (1),
defines a paradigm as a set of standards that establish the boundaries
within which we operate and the rules for success within those boundaries.
The "weed control" paradigm is reactive—it
addresses weed problems by using various tools and technologies.
"How am I gonna get rid of this velvetleaf?" and "How
do I control foxtail?" are reactive statements. The
conventional tools to "get rid of" or "control"
weeds—cultivation and herbicides—are reactive measures
for solving the problem.
Farmers would generally agree that weeds are not in the field because
of a deficiency of herbicides or cultivation. Rather, weeds are
the natural result of defying nature's preference for high species
diversity and covered ground. Nature is trying to move the system
in one direction, the farmer in another. We create weed problems
through conventional crop production methods. After we create these
problems, we spend huge sums of money and labor trying to "control"
them.
The opposite of reactive thinking is proactive thinking,
by which we seek what we want through effective design and planning.
A proactive approach to weed management asks, "Why do I have
weeds?" This publication will expose you to some proactive
principles of cropland management that can make weeds less of a
problem. It also offers some reactive strategies to deal with the
weeds that remain bothersome.
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The Successful Weed
Weeds can be divided into two broad categories—annuals and
perennials. Annual weeds are plants that produce a seed crop in
one year, then die. They are well adapted to succeed in highly unstable
and unpredictable environments brought about by frequent tillage,
drought, or other disturbance. They put much of their life cycle
into making seed for the next generation. This survival strategy
serves plants in disturbed environments well, since their environment
is likely to be disturbed again. The annual plant must make a crop
of seed as soon as possible before the next disturbance comes. Annual
plants also yield more seed than do perennial plants, which is why
humans prefer annual over perennial crops for grain production.
When we establish annual crop plants using tillage (i.e., disturbance)
we also create an environment desirable for annual weeds.
Characteristics of a highly successful annual weed include:
- sprouting requirements that can be met in many environments
- long-lived seed
- variable seed dormancy habits
- rapid vegetative growth
- high seed production
- some seed produced even in harsh conditions
- effective seed dispersal habits
Perennial weeds prosper in less-disturbed and more stable environments.
They are more common under no-till cropping systems. Their objective
is to put some energy into preserving the parent plant while producing
a modest amount of seed for future generations. After a field is
converted from conventional tillage to no-till, the weed population
generally shifts from annual to perennial weeds. Perennial weeds
possess many of the characteristics of annual weeds: competitiveness,
seed dormancy, and long-lived seed. In addition to these characteristics,
many perennial weeds possess perennating parts such as stolons,
bulbs, tubers, and rhizomes. These parts allow the parent plant
to regenerate if damaged and to produce new plants from the parent
plant without seed. Additionally, the perennating parts serve as
food storage units that also enhance survival. These stored-food
reserves allow for the rapid regrowth perennial weeds are known
for.
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The Root Cause of Weeds
When a piece of land is left fallow, it is soon covered over by
annual weeds. If the field is left undisturbed for a second year,
briars and brush start to grow. As the fallow period continues,
the weed community shifts increasingly toward perennial vegetation.
By the fifth year, the field will host large numbers of young trees
in a forest region, or perennial grasses in a prairie region. This
natural progression of different plant and animal species over time
is a cycle known as succession. This weed invasion, in
all its stages, can be viewed as nature's means of restoring stability
by protecting bare soils and increasing biodiversity.
Weeds are evidence of nature struggling to bring about ecological
succession. When we clear native vegetation and establish annual
crops, we are holding back natural plant succession, at great cost
in weed control. To better understand this process, think of succession
as a coil spring. Managing cropland as an annual monoculture compresses
the spring—leaving it straining to release its energy as a
groundcover of weeds. In contrast, a biodiverse perennial grassland
or forest is like the coil spring in its uncompressed condition—a
state of relative stability with little energy for drastic change
(Figure 1). (2) Generally
speaking, biodiversity leads to more stability for the ecosystem
as a whole.
Figure 1. Monoculture vs. Biodiversity. (2) |
Modern crop agriculture is typified by large acreages of a single
plant type, accompanied by a high percentage of bare ground—the
ideal environment for annual weeds to prosper in the first stage
of succession. Holding succession at bay in these unstable agro-ecosystems
can be managed only with big investments in cultivation, mowing,
herbicides, and fertilizers.
A high diversity of plants and animals increases the stability
of the whole agro-ecosystem. For example, an insect outbreak could
wipe out a pure stand of a crop, because the insects can easily
move from one plant to the next and breed rapidly. The insects'
favorite food is all around, and in a monocrop there are few predators
or parasites to hamper them. The same insect invasion occurring
in a mixture of many plant types would be less severe on any one
type of plant because the insects would have a harder time finding
their preferred food. The insects would be undernourished, have
more difficulty finding a mate, and have lower reproductive rates.
The end result is that the impact of an insect outbreak is much
less severe on the whole ecosystem where high biodiversity prevails.
Stability through biodiversity is one of nature's fundamental rules.
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Understanding Weed Seed Banks
and Germination
Weed seeds have a host of characteristics that assure their survival.
They are adapted to many types of disturbance and harsh climatic
conditions. Weeds have several seed-dispersal mechanisms (often
aided by humans) assuring wide distribution. Their seeds last a
long time in the soil without rotting. Additionally, seeds can go
dormant until favorable conditions come around again.
Weed seed distribution and density in agricultural soils are influenced
by cropping history and the management of adjacent landscapes, and
may be highly variable. A study of western Nebraska cropland found
140 seeds per pound of surface soil, equivalent to 200 million seeds
per acre. (3) Redroot pigweed and common lambsquarter
accounted for 86%. Growing without competition from other plants,
a single redroot pigweed plant can produce more than 100,000 seeds,
while a common lambsquarter plant can produce more than 70,000 seeds.
(4)
New weed species can enter fields by many routes. Equipment moved
from one field to the next—especially harvest equipment—spreads
weed seeds, as does hay brought from one farm to another. Crop seed
is often contaminated with weed seed, and livestock transport weed
seeds from one farm to another in their digestive tracts and in
their hair. Practical actions that can be taken to prevent the introduction
and spread of weeds include the use of clean seed (check the seed
tag for weed-seed levels), cleaning equipment before moving from
one field to the next, and composting manures that contain weed
seeds before applying them to the field.
Survival and germination of weed seeds in the soil depend on the
weed species, depth of seed burial, soil type, and tillage. Seeds
at or near the soil surface can easily be eaten by insects, rodents,
or birds. Also, they may rot or germinate. Buried seeds are more
protected from seed-eating animals and buffered from extremes of
temperature and moisture. On average, about 4% of broadleaf and
9% of grass weed seeds present in the soil germinate in a given
year. (5)
Results from seed burial experiments demonstrated that seeds of
barnyard grass and green foxtail buried at 10 inches showed germination
rates of 34 to 38% when dug up and spread on the soil surface. In
the same study, seed buried at one inch showed only one to five
percent germination. In another study, seeds were buried at different
depths for a period of three years. Seed germination was greater
with increasing depth of burial. (3) These studies
show that seeds near the surface face lots of hazards to their survival,
while those buried deeply by tillage are more protected. When those
deep-buried seed are plowed up to the surface again they have a
good chance of germinating and growing.
Table 1 shows that viable weed seeds are
widely distributed in moldboard and ridge-till systems. A higher
percentage of seed remains near the soil surface under chisel plow
and no-till. The moldboard plow and ridge-till systems are stirring
the soil more, burying lots of weed seeds, and keeping weed seed
more evenly distributed down to a six-inch depth.
Table
1. Weed seed distribution with soil depth under four tillage
systems. (6) |
Soil depth |
Moldboard plow |
Ridge-till |
Chisel plow |
No-till |
(inches) |
(percent total seed present) |
0-2 |
37 |
33 |
61 |
74 |
2-4 |
25 |
45 |
23 |
9 |
4-6 |
38 |
22 |
16 |
18 |
After a seed is shed from the parent plant, it can remain dormant
or germinate. There are several different types of dormancy. Seeds
with hard seed coats possess "innate" dormancy. Several
weed species, including pigweed, have seed coats that require mechanical
or chemical injury and high-temperature drying to break dormancy.
Another type of innate dormancy can best be described as after-ripening,
meaning the seed requires further development after it falls off
the plant before it will germinate. Several grass and mustard family
weeds require after-ripening. (7) "Induced"
dormancy results when seeds are exposed to unfavorable conditions,
such as high temperatures, after being shed from the parent plant.
"Enforced" dormancy occurs when conditions favorable to
weed germination are absent. The seeds remain dormant until favorable
conditions return. Altogether, multiple types of dormancy ensure
that some weed seeds will germinate and some will remain dormant
for later seasons.
Some weed species are dependent on light for germination; some
germinate in either light or darkness; others germinate only in
the dark. Thus, there are no hard and fast rules for managing an
overall weed population according to light sensitivity.
Manure application may stimulate weed germination and growth. Studies
have shown that poultry manure does not contain viable weed seeds,
yet weed levels often increase rapidly in pastures following poultry
manure application. Since chickens and turkeys have a gizzard capable
of grinding seeds, weed seeds are not likely to pass through their
digestive systems intact. Additionally, most poultry rations contain
few if any weed seeds. The weed germination is probably caused by
effects of ammonia on the weed-seed bank already present in the
soil. The effect varies depending on the source of the litter and
the weed species present. Manure from hoofed livestock (e.g., sheep,
cattle, and horses), on the other hand, may indeed contain weed
seed that has passed through their digestive systems. Composted
manure contains far fewer weed seeds than does raw manure because
the heat generated during the composting process kills them.
Fertilization practices can also affect weed germination. Where
fertilizer is broadcast, the entire weed community is fertilized
along with the crop. Where fertilizer is banded in the row, only
the crop gets fertilized.
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Proactive Weed Management Strategies
In the preceding sections, we saw how weeds are established and
maintained by human activities. So, how do we begin to manage an
unnatural system to our best benefit without compromising the soil
and water? We can start by putting the principles of ecology to
work on our behalf, while minimizing actions that only address symptoms.
Crops that kill weeds
Some crops are especially useful because they have the ability
to suppress other plants that attempt to grow around them. Allelopathy
refers to a plant's ability to chemically inhibit the growth of
other plants. Rye is one of the most useful allelopathic cover crops
because it is winter-hardy and can be grown almost anywhere. Rye
residue contains generous amounts of allelopathic chemicals. When
left undisturbed on the soil surface, these chemicals leach out
and prevent germination of small-seeded weeds. Weed suppression
is effective for about 30 to 60 days. (8) If
the rye is tilled into the soil, the effect is lost.
Table 2 shows the effects of several cereal
cover crops on weed production. Note that tillage alone, in the
absence of any cover crop, more than doubled the number of weeds.
Table
2. Tillage and cover crop mulch influence on weed numbers
and weed production. (9) |
Tillage |
Cover Crop |
Weeds per foot squared |
Weed weight lbs. per foot squared |
Conventional |
None |
12.0 |
.22 |
None |
None |
5.0 |
.14 |
None |
Rye |
0.9 |
.10 |
None |
Barley |
0.8 |
.09 |
None |
Wheat |
0.3 |
.07 |
A weed scientist in Michigan (9) observed that
some large-seeded food crops planted into rye mulch had high tolerance
to the allelopathic effects, while smaller-seeded crops had less
tolerance. In the study, corn, cucumber, pea, and snapbean no-till
planted under rye mulch germinated and grew as well or better than
the same crops planted no-till without mulch. Smaller-seeded crops,
including cabbage and lettuce, showed much less germination, growth,
and yield. Weeds that were reduced by rye mulch included ragweed
(by 43%), pigweed (95%), and common purslane (100%).
Dr. Doug Worsham, a North Carolina weed scientist, concluded that
leaving a small grain mulch and not tilling gives 75 to 80% early-season
reduction of broadleaf weeds. (10) Table
3 shows the results of tillage, mulch, and herbicides on weed
control in a tobacco study. (11) Just the absence
of tillage alone gave 68% grass control and 71% broadleaf control.
Table
3. Effects of mulch, tillage, and diphenamid herbicide on
weed control in tobacco at two North Carolina locations. (10) |
Treatment |
% Broadleaf control |
% Grass control |
Till, no herbicide |
8 |
47 |
Till + herbicide |
52 |
67 |
No-till, no herbicide |
68 |
71 |
No-till + herbicide |
87 |
94 |
No-till + rye, no herbicide |
79 |
80 |
No-till + rye + herbicide |
97 |
80 |
In other studies, North Carolina researchers investigated combinations
of herbicide use and cover crop plantings on weed control. (12)
Rye and subterranean clover showed the highest weed control without
herbicides (Table 4). Neither provided as
much control as herbicides, however. Tillage reduced weed control
considerably where no herbicide was used, as compared to no-tillage.
Table
4. Effect of pre-emergent herbicides, cover crop, or tillage
on corn weed control 45 days after planting. (12) |
Cover Crop |
Herbicide |
No Herbicide |
Herbicide |
No Herbicide |
% Broadleaf weed control |
% Grass weed control |
Rye |
100 |
85 |
100 |
79 |
Crimson clover |
100 |
68 |
93 |
68 |
Sub. clover |
100 |
95 |
93 |
75 |
Hairy vetch |
98 |
18 |
90 |
18 |
No-tillage |
99 |
23 |
95 |
20 |
Tillage |
99 |
0 |
96 |
0 |
By season's end the weed control resulting from cover crops alone
had decreased (Table 5). The researchers concluded
that additional weed control measures must be applied with cover
crops to assure effective weed control and profitable yields.
Table
5. Effect of cover crop or tillage on corn weed control at
harvest. (12) |
Cover Crop |
% Broadleaf weeds
control |
% Grass weeds control |
Rye |
83 |
36 |
Crimson clover |
41 |
34 |
Sub. Clover |
66 |
32 |
Hairy vetch |
10 |
25 |
No-tillage |
10 |
19 |
Tillage |
0 |
0 |
Other crops that have shown allelopathic effects include sunflowers,
sorghum, and rapeseed. Weed control ability varies among varieties
and management practices. Sweet potatoes have been shown to inhibit
the growth of yellow nutsedge, velvetleaf, and pigweed. Field trials
showed a 90% reduction of yellow nutsedge over two years following
sweet potatoes. (13)
Rapeseed, a type of mustard, has been used to control weeds in
potatoes and corn under experimental conditions. All members of
the mustard family (Brassicaceae) contain mustard oils that inhibit
plant growth and seed germination. (14) The
concentration of allelopathic mustard oils varies with species and
variety of mustard.
Researchers have begun to study ways to manage mustard's weed-suppressive
abilities in crop production. In a Pacific Northwest study, fall-planted
'Jupiter' rapeseed and sundangrass were evaluated for suppression
of weeds growing in spring-planted potatoes. In the spring, the
researchers either tilled or strip-killed the rapeseed in preparation
for potato planting. The first year of the study, rapeseed reduced
mid-season weed production 85% more than fallowing. By the end of
the season, weed production was reduced by 98% with rapeseed, but
only 50% the second year. Potato yields are shown in Table
6.
Table
6. Potato yields following weed treatments over 2 years. (14) |
Weed Treatment |
1992 (cwt/acre) |
1993 (cwt/acre) |
Rapeseed |
682 |
619 |
Fallow |
621 |
525 |
Herbicide |
658 |
680 |
In general, typical levels of cover crop residues, when left on
the soil surface, can be expected to reduce weed emergence by 75
to 90%. (15) As these residues decompose, the
weed suppression effect will decline also. Residues that are more
layered and more compressed will be more suppressive. (15)
Small-seeded weeds that have light requirements for sprouting are
most sensitive to cover crop residue. Larger-seeded annual and perennial
weeds are least sensitive to residue. Effective management strategies
include growing cover crops that produce high amounts of residue,
growing slower-decomposing cover crops, packing the mulch down with
implements that compress it, and using methods other than cover
crops to control large-seeded annual and perennial weeds.
Smother Crops and Mulch
Certain crops can be used to smother weeds. Short-duration plantings
of buckwheat and sorghum-sudangrass, for example, smother weeds
by growing faster and out-competing them. In northern states, oats
are commonly planted as a "nurse crop" for alfalfa, clover,
and legume-grass mixtures—the oats simply take the place of
weeds that would otherwise grow between the young alfalfa plants.
With enough mulch, weed numbers can be greatly reduced. Nebraska
scientists applied wheat straw in early spring to a field where
wheat had been harvested the previous August. At the higher straw
rates, weed levels were reduced by more than two thirds (see Figure
2). Wheat, like rye, is also known to possess allelopathic qualities,
which may have contributed to the weed suppression.
Figure 2. Effect of wheat straw mulch on weed levels at two locations in Nebraska. (16) |
Crop Rotations
Crop rotations limit the buildup of weed populations and prevent
major weed species shifts. Weeds tend to prosper in crops that have
requirements similar to the weeds. Fields of annual crops favor
short-lived annual weeds, whereas maintaining land in perennial
crops favors perennial weed species. Two examples would be shattercane
in continuous sorghum and downy brome in continuous winter wheat.
In a crop rotation, the timing of cultivation, mowing, fertilization,
herbicide application, and harvesting changes from year to year.
Rotation thus changes the growing conditions from year to year—a
situation to which few weed species easily adapt. Rotations that
include clean-cultivated annual crops, tightly spaced grain crops,
and mowed or grazed perennial sod crops create an unstable environment
for weeds. Additional weed control may be obtained by including
short-season weed-smothering crops such as sorghum-sudan or buckwheat.
Crop rotation has long been recognized for this ability to prevent
weeds from developing to serious levels.
In a dryland wheat study, continuous winter wheat was compared
to a rotation of winter wheat/proso millet/fallow or a winter wheat/sunflower/fallow
rotation. (17) The year before, at the start
of the study, the fields were in winter wheat and were sprayed with
Roundup™ (glyphosate) and 2,4-D. The sunflowers were treated
with Prowl™ (Pendimethalin). All other weed control was by
mechanical means, including a sweep and rodweeder as needed. During
the two-year study, weed levels were 145 plants per square yard
for the continuous wheat, 0.4 plants per square yard for the winter
wheat/proso millet/fallow system, and 0.3 for the winter wheat/sunflower/fallow
system.
Intercropping
Intercropping (growing two or more crops together) can be used
as an effective weed control strategy. Having different plant types
growing together enhances weed control by increasing shade and increasing
crop competition with weeds through tighter crop spacing. Where
one crop is relay-intercropped into another standing crop prior
to harvest, the planted crop gets off to a weed-free start, having
benefited from the standing crop's shading and competition against
weeds. Such is the case when soybeans are interplanted into standing
green wheat—the thick wheat stand competes well with weeds
while the soybeans are getting started. Planting method, planting
date, and variety must be well-planned in advance. Though soybeans
can be directly drilled into the standing green wheat, less wheat
damage occurs if the wheat is planted in skiprows. Skiprows are
created by plugging certain drop tube holes in the grain drill.
Soybeans can be planted with row units set at spacings matched to
the skiprows in the wheat. For example, if the wheat is drilled
on 7 1/2-inch rows, to create a 30-inch row spacing for soybeans,
every 4th drill hole in the wheat drill would be plugged. Tractor
tires will follow the skips, resulting in no damage to the wheat.
Studies in Missouri and Ohio showed that wheat yields were three
to six bushels per acre less when intercropped with soybeans than
when solid-drilled and grown alone. (18) Generally,
soybean yields are higher when intercropped into wheat than when
double cropped behind wheat in the central and northern Midwest,
where double cropping is risky due to a shorter growing season.
For more information on intercropping, consult the ATTRA publication
entitled Intercropping
Principles and Production Practices.
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Weed Free by Design
Thus far, we've seen that weeds are a symptom of land management
that defies nature's design. Stirring the soil with tillage creates
conditions favorable for weed germination and survival. Monocultures
of annual crops hold natural plant succession back and minimize
biodiversity, inviting weed populations to thrive. When we try to
maintain bare ground, weeds grow to cover the soil and increase
biodiversity.
If we take a proactive approach to the whole agricultural system,
rather than just looking at the parts, we can use the principles
of nature to our advantage instead of fighting them. We will never
win the war against nature, and she has much more patience than
we do. When we try to break the rules of nature, we end up breaking
ourselves against the rules.
Let's look at an agronomic system where—by design—weeds
simply are not a problem. One of the biggest shortcomings in American
agriculture is the separation of plant and animal production. Commodity
crop production of corn, milo, and soybeans is really a component
of animal production because these crops are largely fed to livestock.
It seems inefficient to grow grains separately and haul them to
animal-feeding facilities. At Shasta College in Redding, California,
Dr. Bill Burrows has developed a series of complementary crop and
animal systems. He plants a mixture of milo and cowpeas together,
with no herbicide. The milo and cowpeas are so vigorous they outcompete
any weeds present. Here nature's principle of biodiversity is obeyed
rather than fought with herbicides. Previously, when the milo was
grown separately, he had to spray for greenbugs. After he started
with the pea-milo mixture, the greenbug problem disappeared. When
the milo and peas are mature, he combines them. This produces a
milo to pea ratio of 2/3 to 1/3, which is ideal for feed.
After grain harvest he turns his animal mixture of hogs, cattle,
sheep, and chickens into the standing crop stubble, thereby adding
more diversity. All the waste grain is consumed by livestock, and
the stubble trampled into the soil, at a profit in animal gains
to the farmer. What few weeds may have grown up with the crop can
be eaten by the livestock. Under typical single-crop scenarios,
the waste grain would rot in the field and the farmer might incur
a $6/acre stalk mowing cost. In this case, following the principle
of biodiversity increased profit by lowering cost. Bill and his
team designed weeds out of the system. Other opportunities exist
to design weeds out of the farming operation. These opportunities
are limited only by human creativity—the most underutilized
tool in the toolbox.
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Reactive Measures
The reactive paradigm of weed management is typified by the word
control. This word assumes that weeds are already present,
or to be expected, and the task is to solve the problem through
intervention. Agriculture magazines are chock-full of advertisements
promising season-long control, complete control, and control of
your toughest weeds. These ads imply that the secret is in the proper
tank mix of herbicides. Examining these ads from a cause-and-effect
standpoint, we might well conclude that weeds are caused by a deficiency
of herbicides in the field.
When selecting a tool for weed management, it helps to understand
the weed's growth stages and to attack its weakest growth stage
(the seedling stage). Alternatively, management techniques that
discourage weed seed germination could be implemented. In so doing,
a farmer can identify a means of control that requires the least
amount of resources.
The various tools available for weed management fall into two categories:
those that enhance biodiversity in the field and those that reduce
it (Table 7). This is not to imply a "good
vs. bad" distinction. Rather it is meant to describe the effect
of the tool on this important characteristic of the crop/weed interaction.
In general, as plant diversity increases, weeds become less of a
problem.
Table
7. Listing of tools and their effect on biodiversity. |
Increase
Biodiversity |
Decrease Biodiversity |
Intercropping |
Monocropping |
Rotations |
Tillage |
Cover Crops |
Herbicides |
Strip cropping |
Cultivation |
Back to top
Weed Control Tools and Their
Effects
Herbicides
Since herbicide information is abundantly available from other
sources, it is not covered in detail in this guide. Herbicides can
be effective in maintaining ground cover in no-till systems by replacing
tillage operations that would otherwise create bare ground and stimulate
more weed growth. Until better weed management approaches can be
found, herbicides will continue to remain in the toolbox of annual
crop production. However, some farmers are realizing that with continued
herbicide use, the weed problems just get worse—or at best
stay about the same. Nature never gives up trying to fill the vacuum
created by a simplified bare-ground monoculture, and long-term use
of the same herbicide leads to resistant weeds, as they adapt to
the selection pressure applied to them. But compared to tillage
systems where bare ground is maintained, herbicide use may be considered
the lesser of two evils. At least where ground cover is maintained,
the soil is protected from erosion for future generations to farm.
There are many approaches to reducing costly herbicide use, such
as banding combined with between-row cultivation, reduced rates,
and using some of the other methods discussed earlier.
Least-toxic Herbicides
Corn gluten meal has been used successfully on lawns and high-value
crops as a pre-emergent herbicide. It must be applied just prior
to weed seed germination to be effective. A common rate is 40 pounds
per 1000 square feet, which suppresses many common grasses and herbaceous
weeds. (19) Two name brand weed control products
containing corn gluten meal are WeedBan™ and Corn Weed Blocker™.
Herbicidal soaps are available from Ringer Corporation and from
Mycogen. Scythe™, produced by Mycogen, is made from fatty
acids. Scythe acts fast as a broad-spectrum herbicide, and results
can often be seen in as little as five minutes. It is used as a
post-emergent, sprayed directly on the foliage.
Vinegar is an ingredient in several new herbicides on the market
today. Burnout™ and Bioganic™ are two available brands.
Both of these are post-emergent burndown herbicides. They are sprayed
onto the plant to burn off top growth—hence the concept "burndown."
As for any root-killing activity with these two herbicides, I cannot
say. The label on Burnout™ states that perennials may regenerate
after a single application and require additional treatment.
Researchers in Maryland (20) tested 5% and
10% acidity vinegar for effectiveness in weed control. They found
that older plants required a higher concentration of vinegar to
kill them. At the higher concentration, they got an 85 to 100% kill
rate. A 5% solution burned off the top growth with 100% success.
Household vinegar is about 5% acetic acid. Burnout™ is 23%
acetic acid. Bioganic™ contains 10% acetic acid plus clove
oil, thyme oil, and sodium lauryl sulfate. AllDown Green Chemistry
herbicide™ contains acetic acid, citric acid, garlic, and
yucca extract. MATRAN™ contains 67% acetic acid and 34% clove
oil. Weed Bye Bye™ contains both vinegar and lemon juice.
Vinegar is corrosive to metal sprayer parts—the higher the
acidity, the more corrosive. Plastic equipment is recommended for
applying vinegar.
Dr. Jorge Vivanco of Colorado State University horticulture department
isolated the compound "catechin," a root exudate from
spotted knapweed, Centaurea maculosa, that has strong herbicidal
properties. Knapweed uses the compound as an allelopathic method
of competing with other plants. Several companies are interested
in producing an environmentally friendly natural herbicide from
the root exudate. Since catechin is naturally occuring, new herbicides
made from it may be eligible for EPA's fast-track approval process.
(21)
Weeder Geese
Weeder geese have been used successfully both historically and
in more recent times. They are particularly useful on grass weeds
(and some others, too) in a variety of crops. Chinese or African
geese are favorite varieties for weeding purposes. Young geese are
usually placed in the fields at six to eight weeks of age. They
work well at removing weeds between plants in rows that cannot be
reached by cultivators or hoes. If there are no trees in the field,
temporary shade will be needed. Supplemental feed and water must
be provided as well. Water and feed containers can be moved to concentrate
the geese in a certain area. A 24- to 30-inch fence is adequate
to contain geese. Marauding dogs and coyotes can be a problem and
should be planned for with electric fencing or guard animals. At
the end of the season, bring geese in for fattening on grain. Carrying
geese over to the next season is not recommended, because older
geese are less active in hot weather than younger birds. Additionally,
the cost of overwintering them outweighs their worth the next season.
Geese have been used on the following crops: cotton, strawberries,
tree nurseries, corn (after lay-by), fruit orchards, tobacco, potatoes,
onions, sugar beets, brambles, other small fruits, and ornamentals.
ATTRA can provide more information on weeder geese.
Tillage
Tillage and cultivation are the most traditional means of weed
management in agriculture. Both expose bare ground, which is an
invitation for weeds to grow. Bare ground also encourages soil erosion,
speeds organic matter decomposition, disturbs soil biology, increases
water runoff, decreases water infiltration, damages soil structure,
and costs money to maintain (for fuel and machinery or for hand
labor). Some specific tillage guidelines and techniques for weed
management include the following:
Preplant tillage. Where weeds such as quackgrass
or johnsongrass exist, spring-tooth harrows and similar tools can
be effective in catching and pulling the rhizomes to the soil surface,
where they desiccate and die. Discing, by contrast, tends to cut
and distribute rhizomes and may make the stand even more dense.
Blind tillage. Blind cultivation is a pre-emergent
and early post-emergent tillage operation for weed control. It usually
employs either finger weeders, tine harrows, or rotary hoes. These
implements are run across the entire field, including directly over
the rows. The large-seeded corn, soybeans, or sunflowers survive
with minimal damage, while small-seeded weeds are easily uprooted
and killed. For corn, the first pass should be made between three
and five days after planting and a second at the spike stage. Blind
cultivation may continue until the crop is about five inches tall.
For soybeans, the first pass should be done when germinating crop
seedlings are still about ½ inch below the soil surface,
but not when the "hook" is actually emerging. The second
pass should be done three to five days after soybean emergence,
and twice later at four-day intervals. Sunflowers can be blind-tilled
up to the six-leaf stage, giving them an excellent head start on
weeds. Grain sorghum may be rotary hoed prior to the spike stage,
and again about one week after spike stage. Because the seed is
small, timing for blind-till in sorghum is very exacting. Post-emergent
blind tillage should be done in the hottest part of the day, when
crop plants are limber, to avoid excessive damage. Rotary hoes,
not harrows, should be used if the soil is crusted or too trashy.
Seeding rates should be increased 5 to 10% to compensate for losses
in blind cultivation. (22, 23)
Row crop cultivation. Cultivation is best kept
as shallow as possible to bring as few weed seeds as possible to
the soil surface. Where perennial rhizome weeds are a problem, the
shovels farthest from the crop row may be set deeper on the first
cultivation to bring rhizomes to the surface. Tines are more effective
than duck feet sweeps for this purpose. Later cultivations should
have all shovels set shallow to avoid excessive pruning of crop
roots. Earliest cultivation should avoid throwing soil toward the
crop row, as this places new weed seed into the crop row where it
may germinate before the crop canopy can shade it out. Use row shields
as appropriate. As the crop canopy develops, soil should be thrown
into the crop row to cover emerging weeds.
Interrow cultivation is best done as soon as possible
after precipitation, once the soil is
dry enough to work. This avoids compaction, breaks surface crusting,
and catches weeds as they are germinating—the most vulnerable
stage.
Generally speaking, tillage systems tend to discourage most biennial
and perennial weed species, leaving annual weeds as the primary
problem. Exceptions to this are several weeds with especially resilient
underground rhizome structures such as johnsongrass, field bindweed,
and quackgrass. Plowing of fields to bring up the rhizomes and roots
has been used to control bindweed and quackgrass.
Another interesting application of timing to weed control is night
tillage. Researchers have found that germination of some weed species
is apparently triggered by exposure to light. Tillage done in darkness
exposes far fewer seeds to light and reduces weed pressure. So far,
small-seeded broadleaf weeds (lambsquarter, ragweed, pigweed, smartweed,
mustard, and black nightshade) appear to be most readily affected.
(24)
Flame weeding
Preplant, pre-emergent, and post-emergent flame weeding has been
successful in a number of crops. The preplant application has commonly
been referred to as the "stale seedbed technique." After
seedbed tillage is completed, weed seeds, mostly in the upper two
inches of the soil, are allowed to sprout. Assuming adequate moisture
and a minimum soil temperature of 50º F (to a depth of 2 inches),
this should occur within two weeks. A fine to slightly compacted
seedbed will germinate a much larger number of weeds. The weeds
are then "seared" with a flamer, or burned down with a
broad-spectrum herbicide, preferably when the population is between
the first and fifth true-leaf stages, a time when they are most
susceptible. The crop should then be seeded as soon as possible,
and with minimal soil disturbance to avoid bringing new seed to
the surface. For the same reason, subsequent cultivations should
be shallow (less than 2 inches deep). (25)
Pre-emergent flaming may be done after seeding, and in some crops
post-emergent flaming may be done as well. Flaming is often used
as a band treatment for the crop row, and usually combined with
interrow cultivation. Early flaming may be done in corn when it
is 1.5 to 2 inches high. The growing tip is beneath the soil surface
at this stage, and the crop readily recovers from the leaf damage.
Subsequent post-emergent flamings may be done when corn reaches
6-10 inches in height, and later at lay-by. No flaming should be
done when corn is at approximately 4 inches high, as it is most
vulnerable then. The burners are offset to reduce turbulence and
to avoid concentrating too much heat on the corn. Water shields
are available on some flame weeder models. Uniform seedbed preparation
and uniform tractor speed are important elements in flaming. Hot
and dry weather appears to increase the efficacy of flaming. (26)
Searing the plant is much more successful than charring. Excessive
burning of the weeds often stimulates the roots and encourages regrowth,
in addition to using more fuel. Flaming has generally proved most
successful on young broadleaf weeds. It is reportedly less successful
on grasses, as the seedlings develop a protective sheath around
the growing tip when they are about 1 inch tall. (27)
Some concerns with the use of fire include possible crop damage,
potential dangers in fuel handling, and the cost of fuel. For more
information on flame weeding, see the ATTRA publication Flame
Weeding for Agronomic Crops.
Table 8.
Approximate Cost of Selected Weed Control Practices –
2003* |
Practice/Input |
Average Dollars per
Acre Cost |
Tillage |
Moldboard plow |
11.75 |
Chisel plowing |
10.55 |
Disking – tandem |
7.95 |
Disking offset |
9.80 |
Harrowing |
4.60 |
Soil finishing |
8.90 |
Field cultivating |
7.85 |
Cultivating – row |
6.65 |
Cultivating – ridge-till |
9.40 |
Rotary hoeing |
5.00 |
|
Spraying |
Ground – broadcast |
4.60 |
Ground – incorporated
|
8.60 |
Ground – banded |
5.05 |
Aerial |
6.15 |
Rope wick |
4.65 |
|
Other |
Chopping corn stalks |
7.05 |
Grain drill |
9.50 |
Rye seed (90 #/ac) |
10.42 |
Hairy vetch (20#/ac)** |
15.00 |
*Costs of all practices
except flaming, rye, and vetch from Iowa Farm Custom Rate
Survey averages, Iowa State University Extension publication
FM 1698, revised April 2003.
**Hairy vetch seed costs vary widely, ranging from 50¢
to $1.50. 2003 price was 75¢ per pound from Albert Lea
Seed house in Albert Lea, MN. |
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Integrated Weed Management
An integrated approach means assembling a weed management plan
that incorporates a number of tools consistent with farm goals.
Included are sanitation procedures, crop rotations, specialized
tillage schemes, cover crops, and herbicides. The best examples
of integrated approaches have been developed on-farm, by farmers
themselves. A useful book that spotlights farmers and other researchers
and the integrated weed management strategies they are using is
Controlling Weeds With Fewer Chemicals, available from
the Rodale Institute (see Additional Resources).
The next two examples are taken from this book.
Dick and Sharon Thompson of Boone, Iowa, built a herbicide-free
weed-management system around ridge-till technology for corn and
soybeans. Fields are overseeded or drilled in the fall with combinations
of hairy vetch, oats, and grain rye as a winter cover crop. The
vetch provides nitrogen, while the grasses provide weed suppression
and erosion protection. The cover crop is not tilled in before planting.
Instead, the ridge-till planter skims off enough of the ridge top
to create a clean seeding strip. Subsequent passes with the ridge-till
cultivator eliminate any cover crop in the interrow area and help
to re-shape the ridges. The Thompsons estimate savings of $45 to
$48 per acre using their methods. "Walking the Journey"
is a 20-minute video chronicling the Thompson farm, available for
$39. See Additional Resources for ordering
information.
In Windsor, North Dakota, Fred Kirschenmann has developed a diverse
rotation including cool-weather crops like oats, rye, barley, and
spring wheat, and warm-season crops like sunflower, buckwheat, and
millet. He employs selective timing to manage his principal weed
problem, pigeon grass. By planting cool-weather grains early, he
can get a competitive jump on the weed, which requires somewhat
warmer soil to germinate. The warm-season crops do best long after
pigeon grass has germinated, however. He uses shallow pre-plant
tillage to control weeds in these crops. Kirschenmann also composts
manure before spreading it. One of the many advantages of composting
is the reduction of viable weed seeds, which are killed by heat
during the curing process.
Don and Deloris Easdale of Hurdland, Missouri, reduced their annual
herbicide costs from $10,000 to less than $1,000 in three years
on their 300-plus acres of grain crops. (28)
They use hairy vetch, winter rye, or Austrian winter peas in combination
with their ridge-till system. They flail chop hairy vetch or winter
peas ahead of the ridge-till planter and plant directly into the
remaining cover crop residue. This practice eliminates using a burndown
herbicide. The legumes replace much of the nitrogen needed for the
corn or milo crop. Some liquid starter and liquid nitrogen is placed
below the seed at planting. They more than recover the seed costs
of their cover crops in savings on fertilizer and herbicide.
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Other ATTRA Publications
of Interest
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References
- Barker, Joel A. 1993. Paradigms—The
Business of Discovering the Future. Harper Business. New York.
240 p.
- Savory, Allan. 1988. Holistic Resource
Management. Island Press. Washington, DC. 564 p.
- Wilson, Robert G. 1988. Biology
of weed seeds in the soil. p. 25-40. In: Miguel Altieri and Matt
Liebman (eds.). Weed Management in Agroecosystems: Ecological
Approaches. CRC Press, Inc. Boca Raton, FL. 354 p.
- Stevens, O.A. 1954. Weed Seed Facts.
North Dakota Agriculture College Extension Circular. A-218. 4
p.
- Lehnert, Dick. 1996. Breaking the
weed-seed bank. Soybean Digest. Mid-March. p. 52.
- Clements, David R., Diane L. Benoit,
Stephen D. Murphy, and Clarence J. Swanton. 1996. Tillage effects
on weed seed return and seedbank composition. Weed Science. Volume
44. p. 314-322.
- Schlesselman, John T., Gary L.
Ritenour, and Mahlon Hile. 1989. Cultural and physical control
methods. p. 45-62. In: Principles of Weed Control in California.
Second edition. California Weed Conference. Thompson Publications.
Fresno, CA.
- Daar, Sheila. 1986. Update: Suppressing
weeds with allelopathic mulches. The IPM Practitioner. April.
p. 1-4.
- Putnam, Alan R., Joseph DeFrank,
and Jane P. Barnes. 1983. Exploitation of allelopathy for weed
control in annual and perennial cropping systems. Journal of Chemical
Ecology. Volume 9, Number 8. p. 1001-1010.
- Worsham, A.D. 1991. Allelopathic
cover crops to reduce herbicide input. Proceedings of the Southern
Weed Science Society. 44th Annual. Volume 44. p. 58-69.
- Schilling, D.G., A.D. Worsham,
and D.A. Danehower. 1986. Influence of mulch, tillage, and diphenamid
on weed control, yield, and quality in no-till flue-cured tobacco.
Weed Science. Volume 34. p. 738-744.
- Yenish, J.P., and A.D. Worsham.
1993. Replacing herbicides with herbage: potential use for cover
crops in no-tillage. p. 37-42. In: P.K. Bollich, (ed.) Proceedings
of the Southern Conservation Tillage Conference for Sustainable
Agriculture. Monroe, LA. June 15-17.
- Anon. 1993. Sweet potato plants
vs. weeds. HortIdeas. January. p. 8.
- Boydston, Rick, and Ann Hang.
1995. Rapeseed green manure crop suppresses weeds in potato. Weed
Technology. Vol. 9. p. 669-675.
- University of Connecticut IPM
Program. No date. Contribution of cover crop mulches to weed management.
www.hort.uconn.edu/ipm/
- Crutchfield, Donald A., Gail
A. Wicks, and Orvin C. Burnside. 1985. Effect of winter wheat
straw mulch level on weed control. Weed Science. Volume 34. p.
110-114.
- Lyon, Drew J., and David D. Baltensperger.
1995. Cropping systems control winter annual grass weeds in winter
wheat. Journal of Production Agriculture. Vol. 8, No. 4. p. 535-539.
- Helsel, Z.R. and T. Reinbott.
Circa 1989. Intercropping: planting soybeans into standing green
wheat. University of Missouri Agronomy Department. University
of Missouri, Columbia, MO. 3 p.
- Quarles, William. 1999. Non-toxic
weed control in the lawn and garden. Common Sense Pest Control
Quarterly. Summer. p. 4-14.
- Anon. 2002. Vinegar wipes out
thistles organically. Stockman Grass Farmer. July. p. 1.
- Malone, Marty. 2003. Uses for
Knapweed? The Ag Perspective. Park County Extension Service. www.parkcounty.org/Extension/News/Weed_news/weed_news.html
- Anon. 1991. Non-chemical weed
control for row crops. Sustainable Farming News. September. p.
1-8.
- Jordan, C. Wayne. 1981. Sunflower
Production In Mississippi. Cooperative Extension Service. Mississippi
State University. Mississippi State, MS. 2 p.
- Becker, Hank. 1996. Nightmare
in tilling fields—a horror for weeds. Farmers' Digest. March.
p. 20-24.
- Pieri, Paul B. No date. Flame
Weeding. Maurolou Farm. Little Compton, RI. 6 p.
- Cramer, Craig. 1990. Turbocharge
your cultivator. New Farm. March-April.
p. 27-30, 35.
- Drlik, Tanya. 1994. Non-toxic
weed control. The IPM Practitioner. October. p. 20.
- Easdale, Deloris. 1996. Controlling
weeds and maintaining soil fertility with cover crops. National
Conservation Tillage Digest. February. Vol. 3, No. 2. p. 28-30.
Back to top
Additional Resources
Shirley, C., and New Farm staff. 1993. What Really Happens When
You Cut Chemicals. 156 p.
This book contains a series of farmers' experiences with
adopting new strategies for higher profits and lower input costs,
while enhancing the environment.
Available for $14.95 from:
Rodale Institute
611 Siegfriedale Road
Kutztown, PA 19530
800-832-6285
610-683-6009
ribooks@fast.net
Cramer, Craig, and the New Farm Staff (eds.). 1991. Controlling
Weeds with Fewer Chemicals. Rodale Institute, Kutztown, PA. 138
p.
Available for $14.95 from Rodale Institute (see address above).
Bowman, Greg (ed.). 1997. Steel in the Field. Sustainable Agriculture
Network Handbook # 2. 128 p.
This book is a farmer's guide to weed management tools using
cultivation equipment. Available for $18.00 + $3.95 shipping and handling from the Rodale Institute listed above or:
Sustainable
Agriculture Publications
210 Hills Building
University of Vermont
Burlington, VT 05405-0082
802-656-0484
sanpubs@uvm.edu
Walking the Journey: Sustainable Agriculture that Works. 1992.
A 20-minute video of Dick and Sharon Thompson's ridge-till
farming in Iowa. Available for $25 from:
Extension Communications
Attention: Lisa Scarborough
3614 ASB, Room 1712
Iowa State University
Ames, IA 50011
515-294-4972
Principles of Sustainable Weed Management for Croplands
By Preston Sullivan
NCAT Agriculture Specialist
Cole Loeffler, HTML Production
IP 039
Slot 152
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