PANUPS: Roundup Resistance

                         P A N U P S
                   Pesticide Action Network 
                        North America
                       Updates Service

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, 
Winter 1996. 
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 22513mew@msu.edu; 

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