By
Dr. Mohammed Sa’id Berigari, Senior Soil &Environ. Scientist, USA,
30/9/2012.
Abstract: Weeds compete with
crops for soil and air space, nutrients, water; and sunlight. Using a single
herbicide alone repeatedly and intensively for a long time to control weeds will
increase the selection pressure. That
would ultimately lead to evolution of some weed biotypes and spread in growth
to the extent that crops can’t compete with the weeds. A good example is when Culpepper, S. in 2004
identified the first sample of Palmer amaranth or pigweed that had evolved
resistance to Roundup (generic name glyphosate). The weed was so out of control
that Georgia farmers were mowing down huge cotton acres. Today about 98% of Culpepper‘s farmers have
glyphosate-resistant pigweed in their farms and it has spread to 76 counties in
Georgia, USA. This problem is not unique
to the farmers of Georgia. The
agriculture communities worldwide face this problem. Therefore, scientists conducted field
experiments in many states in USA and arrived at alternative solutions to
control weeds. The focus now is on
fighting weeds through diverse managements which include crop rotations,
minimum tillage, and cover cropping.
However, experts now consider weed resistance to herbicides a
sociological dilemma more than a physiological problem.
Herbicides
are used to kill weeds that compete with crops for nutrients, water, soil and
air space, and sunlight for their growth, thus, reduce their yields. And also,
they are used to minimize tillage, therefore, save the cost of fuel and extra
labor. John E.Franz, organic chemist
worked at Monsanto Co., Missouri, USA synthesized glyphosate [N-(phosphonomethyl)
glycine] in 1970 as a very effective herbicide.
When genetically developed cotton cultivars survived
sprays with glyphosate herbicide in 1997, many US farmers began using it with
Roundup Ready varieties to control weeds especially the cotton growers of the
State of Georgia, USA. Such a practice
was simple, safe, and did not harm crops whenever applied.
However, in
2004 an extension agronomist at the University of Georgia, Stanley Culpepper,
identified the first samples of a Palmer amaranth or pigweed that had developed
resistance to Roundup (generic name glyphosate). By the end of 2007 the weed was out of
control to the extent that the Georgia farmers began cutting down thousands of
their cotton acres. Currently, about 98%
of the Culpepper’s farmers face glyphosate-resistant pigweed in their fields,
and it has spread to 76 counties of Georgia.
Amaranthus palmeri (pigweed)
is a fast growing C4 dioecious plant, having male and female plants
leading to a high degree of genetic diversity, thus, high capacity for
reproduction and developing resistance to herbicides. On the average a female plant of pigweed
bears 40,000 seeds and few have produced 500,000 seeds per plant.
Herbicide resistant
weeds created problems not only for Georgia farmers and for other US farmers
but the agriculture communities worldwide are facing this problem. However, nothing
is really new in certain ways. Weeds have developed resistance to herbicides
since the first triazine herbicides were used about half a century ago. What is new, however, is too much reliance
now on glyphosate to control weeds.
Consequently, this led to remarkable popularity of glyphosate-resistance
crops. Currently about 95% of the USA soybean crops and 70% of USA cotton and
corn are Roundup Ready.
The outcome has been wide spread weed resistance to
herbicides. Glyphosate tolerance
recently found in at least 20 weed species that reportedly infest millions of
crop acres. Some weeds, such as water
hemp (Amaranthus tuberculatus) in the Midwest, USA and Lolium spp. in Australia
have evolved resistance to three or more herbicide mechanisms of action.
There is very little we can do to resolve that
problem. The primary solution seed
companies are seeking is the engineering of new cotton, soybean, and corn
cultivars that can tolerate additional herbicides, such as glufosinate
(DL-phosphinothricin); 2,4-D( dichlorophenoxyacetic acid); and dicamba( 3,6-dichloro-2-methoxybenzoic
acid)- including “stacked trait” varieties that resist more than one. While
these tools may help, scientists, also , want to see wider options of
integrated weed management that relies less on technology and more on diverse
weed fighting strategy, including crop rotations, minimum tillage, and cover
cropping.
According to
Dave Mortensen, weed scientist at Penn State University, diversity management,
although more difficult, is the only sustainable method forward especially with
farmers’ who favor strict, no tillage farming.
He said”We are putting more and more pressure on herbicides to do all
the killing and that is just not going to get the job done down the road”.
Diversification Will Reduce Selection
Pressure
When an herbicide,
like glyphosate, is applied repeatedly year after year over very large areas
that could increase selection pressure on weeds to evolve resistance just like
how bacteria develop resistance when antibiotics are overprescribed. The
probability that any one plant will become tolerant to an herbicide is very low
but is not zero. However, in the case of
glyphosate repeated use on millions of acres did ultimately select for few
individual plants among billions to survive it.
And as glyphosate continued to exist in the environment, those
glyphosate-tolerant biotypes - such as pigweed- began to reproduce in much
greater numbers than their susceptible ones.
This
phenomenon, however, is not in any way unique to glyphosate or even to
herbicides. Any weed-killing method will
eventually select for plants that can resist it. The idea is to prevent or delay the evolution
of resistance by reducing the selection pressure that any one method
exerts. This is where diverse management
plays a major role, and according to Bill Curran, weed scientist at Penn State
University, it has three components:
herbicides, tillage, and crop rotation.
If a farmer does not diversify those three, he will face problems, but
what is the right mix? It is a
challenging question to answer.
For that
reason prominent weed scientists Curran, Mortensen, and others have been
conducting research to reduce herbicide use, thus, the selection of resistant
weeds. Apparently they borrowed ideas
from crop growers who combat weeds without any chemicals: organic farming producers. Curran, about 10 years ago, began working
with growers who wanted to reduce their dependence on tillage for weed
control. Soon he realized the research
might have implications for conventional growers as well. Many have been tilling less but spraying more
herbicides as a result.
One
alternative method to both deep tillage and heavy herbicide use that Curran’s group
focused on is winter cover crops, for example, cereal rye in soybean and hairy
vetch in corn. Curran, Mortensen, and
coworkers adopted a system in which they grow a rye cover crop under organic
and conventional conditions and then rolled/crimped it down to form
weed-suppressive mulch. When soybean was
sown into the mulch, the researchers suppressed emerging weeds in the organic
plots with a high-residue cultivator specifically designed for reduced tillage
systems. In the conventional plots, the
researchers sprayed a post-emergence herbicide.
Therefore,
herbicides are not eliminated completely under diversified practice, in
reality, chemicals are very useful when soils are too wet or dry for mechanical
weed control, Curran notes. However, herbicide
use can be decreased significantly in these systems because the mulch does the
weed-fighting work. In effect, you are
using mulch as a preemergence herbicide according to Curran and you are hoping
to get four to six weeks of weed control.
In another
investigation Curran and coworkers found that a combination of banded residual
herbicide and high-residue cultivation reduced the need for residual
application by 67% without significantly changing corn yield. However, this does not occur when weed
densities are very high. Then it will be
necessary to use heavy dose of an herbicide.
These
findings demonstrate the value of sharing ideas between organic and traditional
systems. Curran thinks that both sides could benefit
from each other when exchanging ideas take place more often.
Impacts of many small hammers
Matt Ryan,
now assistant professor at Cornell University.
As a graduate student working with Mortensen, Curran, and others he not
only embraced cover crops and other organic methods to reduce herbicide use, but
also, his efforts took him to the opposite direction of studying herbicide
chemistry. Ryan stated that there is a rich body of literature describing the
interactions between herbicides including methods of determining whether two or
more chemicals applied together interfere with one another’s activity, fail to
interact or combine to produce a stronger weed killing effect. That led scientists to the idea: could they, also, find interactions between
non-chemical weed control practices?
The idea is
based on the “many little hammers” impacts of ecological weed management, Ryan
explains. Certain weed-control
strategies are weak when used alone, for instance, just increasing the seeding
rate to make a crop more competitive against weeds “does not work”. However, when these tactics are used
together, they can be more effective than would be expected from each method to
control weed alone. That is really what the concept of” many little hammers”
means.
Ryan designed
an experiment with two gradients: five levels of rye cover crop biomass and
five different soy bean seeding rates.
The result showed that the two treatments together gave greater effect
on weed control than either one individually.
Moreover, the findings offer insight into the reasons behind the
improved control, which to Ryan is the most important point of all. The action of the rye mulch was related to
seed size. The mulch delayed emergence
of small-seeded annual weeds long enough for the larger-seeded soybean to
germinate and develop a competitive, weed-suppressing canopy. That mechanistic
outlook is important for expanding the use of these methods and improving
management in general, according to Ryan.
He said “once you know why a practice is effective, you can apply that
information to other situations and tweak the practice so that you maximize its
effectiveness”.
What these
mechanisms often come down to, adds Culpepper, is weed biology”. Glyphosate
kills regardless of biology or used to anyway.
But now that glyphosate-resistant pigweed emerged, one has to understand
the biology of this and other weed resistant weeds to herbicides if the
objective is to combat them effectively.
Pigweed is
very competitive against cotton to the extent that Georgia farmers can’t allow
it to exceed one or two inches in height or” they are mowing their crop down”
Culpepper said. Therefore, now they use
2-3 residual herbicides at planting, followed by another three during the
growing season to prevent the weed from sprouting. Pigweed seeds, also, can’t emerge from a soil
at depth greater than 2-3-inches that is why many farmers have returned to deep
moldboard plowing as another way to control pigweed emergence. Even then enough emerges that farmers
hand-weed the plant to keep it from reproduction. Culpepper said that 92% of his growers last
year hand-weeded 54% of the entire cotton crop.
That reflects the scope of the problem.
Culpepper
adds that while this combination of tactics is working now, however, it is also
expensive and economically not sustainable as cotton prices have fallen. That is why he, too, for the past six years
has been conducting experiments with an integrated approach that includes cover
crops and herbicides. In four of the
on-going farm trials in Georgia, he and his coworkers have found that a robust
crop of rye can reduce pigweed emergence by 70 to 90%. Like the Penn State weed scientists Culpepper
then controls the remaining pigweed with herbicide, and the system still
achieves important goals. It remarkably
reduces the need for chemicals, thus, cutting the costs. It encourages the farmers to adopt
conservation tillage. Most importantly,
cover crop will extend the life of valuable herbicide chemistry such as
glufosinate (Liberty) as Culpepper asserted” we have reduced the selection
pressure. We are spraying fewer plants”.
With only 0.5%
of Georgia cotton farmers planting rye cover crops now, Culpepper hopes that $1.1million
grant will help convince more growers to try his program, “because it is
difficult, it is challenging,” he said.
“You grow the rye, you roll the rye, and then you strip-till cotton
right into the rye.”
Will Farmers Adopt Diverse Management?
This is the
question facing everyone: How to
convince farmers to embrace more complex weed management practices when they
have been used to and dependent on the strength of herbicides like the
glyphosate and the simple technologies such as Roundup Ready. The work is easier to embrace in areas like
Georgia because farmers there essentially have no choice. However, in Iowa and Pennsylvania the goal of
integrated practice is primarily to prevent future problems rather than deal
with the current ones, thus, making it harder to sell integrated strategy. Owen said “Truthfully, until growers get
[herbicide-resistance] in their fields, the likelihood of them actually doing
something about it is unfortunately not very high”.
According to
Owen one thing could help weed scientists is more research into the economics
of diversified approaches. Farmers are
more concerned that these practices will cost more time and money, while
offering little benefits other than preventing a problem they may not actually
face. However, Owen and coworkers found,
in a number of on-farm studies in six states for five years, an integrated
system involving crop rotations and diverse mix of herbicides actually saved
money, or at least didn’t cost farmers anything more than their standard
practices.
Even so, change
is difficult, that is why Owen thinks agronomists must collaborate with
sociologists and psychologists to overcome human barriers. Scientific facts are established “Now what do
the social sciences say about getting growers to accept?” Owen is also, working with a group from the
Weed Science Society of America to develop a set of best practices-with a clear
and simple language - that will hopefully bring more farmers to adopt such
practices.
Mortensen
suggests handling this issue from a totally different angle. Emphasis should be focused on the benefits of
diverse management. Cover crops, for
example, don’t just suppress weeds; they also reduce insect pressure, enhance
soil tilth and soil organic matter, and stop nutrient and herbicide runoff.
Yet, they still are mainly deployed to fix one environmental problem at a time;
for instance, concerns over the steady decline of Chesapeake Bay recently led
Maryland State to adopt a program that pays farmers to plant winter cover crops
to curb nutrient runoff.
Consider
taking the same program and listing all the benefits that cover crops provide
as well as eliminating the negative effects that would help growers to adopt
diversified approach. And as long as
research continues to focus on the benefits of the diverse system, there is
hope that more farmers will embrace such a system.
Conclusion
As a long
term management strategy, experts agree, that if farmers do not use nonchemical
methods for weed control such as crop rotations, cover crops, and tillage. Then
eventually, few individual weed biotypes will emerge resistant to 2, 4-D,
dicamba, glufosinate, glyphosate, and to other new and old herbicides selected
for their successful weed killing capacities.
References
The author
of the above article relied significantly on various parts of the one presented
in reference (2).
1.
Bomgardner, M.M. 2012. War on weeds. CSA 90(21): 20-22.
2.
Fisher, M. 2012. Many Little Hammers: Fighting Weed Resistance with Diversified
Management. Crops, Soils, Agronomy-CSA News
Sep. Issue: 4-10.
3.
Gunsolus, J.L. 2008. Herbicide resistant weeds.
