PANUPS: Roundup Resistance
P A N U P S
Pesticide Action Network
February 14, 1997
Weeds Could Develop Resistance to Glyphosate
The evolution of pesticide resistance plays a key role in
keeping farmers on the so-called "pesticide treadmill" -- a
cycle in which farmers feel compelled to move on to other,
sometimes more toxic, chemicals as older pesticides lose
their effectiveness. Yet a widespread assumption prevails
that weeds are unlikely to develop resistance to the
herbicide Roundup (glyphosate), leading many experts to
dismiss resistance management strategies as unnecessary.
According to a recent review of available data, however,
fewer constraints to weeds evolving resistance to glyphosate
exist than previously thought.
The review, written by Dr. Jonathan Gressel in the journal
"Resistant Pest Management," challenged the reasoning
underlying several common arguments that weeds will not
develop resistance to glyphosate. According to Gressel, some
researchers have suggested that the process of genetically
engineering crops to be glyphosate-resistant has been so
complicated that it could not be repeated by weeds in nature.
In fact, sub-populations of several crop species already
naturally exhibit varying degrees of resistance to
glyphosate. The review pointed out that if naturally
occurring variation leads to resistance in crops, then it is
likely that variation will also lead to resistance in weeds.
According to Gressel, some of the crops that have been found
to exhibit glyphosate-resistance (in sub-populations) include
maize, rice, carrot, barley, chicory and peanut.
The review also challenged the argument that glyphosate's low
persistence precludes the development of resistance -- that
is, because glyphosate does not remain in the environment for
very long, there is not enough selection pressure over time
for weeds to develop resistance. The review pointed out that
the effect of chemical persistence on weeds is relative to
the weeds' life cycle. A short-lasting herbicide has, in
effect, season-long persistence for a weed species that
germinates all at one time and is standing when the herbicide
is used. In addition, there are many cases of paraquat
resistance, and, according to the review, paraquat is less
environmentally persistent than glyphosate.
Several mechanisms have been identified that could lead to
glyphosate resistance. Glyphosate kills plants by targeting
an enzyme called EPSP-synthase. Theoretically, genetic
variation in weed species could produce plants capable of
over-expressing this enzyme -- i.e. producing more of the
enzyme than glyphosate destroys -- thus making the weed
somewhat resistant. Plants could also develop a form of EPSP-
synthase enzyme that is not susceptible to glyphosate. In
addition, weeds could develop an ability to break down the
herbicide before it does damage.
The author called for development of resistant management
plans for glyphosate to be made a priority. He stated that
the lack of such plans contributed to the appearance of a
glyphosate-resistant population of annual ryegrass in
Australia. In 1996, a Australian farmer found Roundup-
resistant ryegrass weeds in a field that had been sprayed
with glyphosate 10 times in the previous 15 years. According
to the U.S. Environmental Protection Agency, there are at
least 270 weed species resistant to herbicides worldwide.
Strong sales of Roundup pushed Monsanto's agricultural
product sales to almost US$3 billion in 1996. End of year
sales increased 22.8% over 1995 due largely to increased
sales of Roundup outside the US, notably in Europe and Latin
America. This growth is also due in part from sales of
Roundup for Monsanto's Roundup Ready soybeans (soybeans
genetically engineered to be glyphosate-resistant) which were
introduced commercially in 1996. Roundup Ready cotton will be
available in the U.S. for the first time in 1997, and it is
estimated that it will be planted on up to 600,000 acres.
Sources: Resistant Pest Management, Volume 8, Number 2,
Benbrook, Charles. Pest Management at the Crossroads, 1996.
Agrow, January 31, 1997.
Contacts: Jane Rissler, Union of Concerned Scientists, 1616 P
Street, NW, Suite 310, Washington, DC 20036.
Resistant Pest Management, Pesticide Research Center,
Michigan State University, East Lansing, MI 48824; phone
(517) 355-1768; fax (517) 353-5598; email email@example.com;
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