Weeds Could Develop Resistance to Glyphosate

February 14, 1997

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; http://www.msstate.edu/Entomology/EntHome.html. PANNA.

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