Weeds could develop resistance to Roundup
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 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, there are fewer constraints to weeds evolving resistance to glyphosate than previously thought.
The review, written by Dr Jonathan Gressel in the journal Resistant Pest Management, pointed out that sub-populations of several crop species already naturally exhibit varying degrees of resistance to glyphosate. If naturally occurring variation leads to resistance in crops, then it is likely that variation will also lead to resistance in weeds.
The review also challenged the argument that, 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 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 paraquat is less environmentally persistent than glyphosate.
The author called for development of resistance management plans for glyphosate to be made a priority. He stated that the lack of such plans contributed to the appearance in 1996 of a glyphosate-resistant population of annual rye-grass in Australia in a field that had been sprayed with glyphosate 10 times in the previous 15 years.
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 to 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 US for the first time in 1997. [From Pesticide Action Network North America Updates Service.]