Farmers Weekly  
23 November 2001


Glyphosate resistance is showing a worldwide rise

New Products, new advice and new problems were debated in detail at last
week's International "Weeds Conference" staged by the British Crop
Protection Council in Brighton.  Over the following four pages Charles
Abel, Andrew Blake and Andrew Swallow report the highlights

Resistance to glyphosate (Roundup) is emerging all around the world,
potentially jeopardising the 2.5 billion dollar market for genetically
modified herbicide tolerant crops.

The latest discovery is glyphosate resistant ryegrass in South African
vineyards where growers have used Roundup for 23 years.

"Monsanto is very sensitive because half the soya and maize is in GM
herbicide tolerant varieties, creating a market worth $2.5b,"  said
Andrew Cairns, of Natal University, Pietermaitzburg.

Resistance has also been found in ryegrass in New South Wales and
Western Australia, a grass-weed in Malaysia, a broad-leaved weed in Delaware and
ryegrass in California.  In one US case, the repeat use of Roundup in GM
herbicide tolerant maize has been blamed for the development of
resistant weeds.

"I agree with the technology, it's a very good idea,"  said Prof Cairns.

"But they need to have the stewardship in place to prevent this
becoming a bigger issue."

Resistance problems are predicted to increase but a simple change in
herbicide use should offer a solution, said Michael Owen of Iowa State
University.

"I don't see that the shifts on establishment of resistant populations
is going to be a big issue."  Using an alternative mode of action should
solve the problem, although the number of new modes of action becoming
available is decreasing, he said.

Over-use of glyphosate by growers is to blame for the rapid development
of resistance in common waterhemp and horseweed.  One or two applications
would often be enough to give economic control, but growers frequently use
three or even four applications to keep fields cosmetically clean, said Mr
Owen.

That puts a strong selection pressure on the weed population.  In one
case in the eastern corn belt, horseweed became resistant to glyphosate after
just three years of growing glyphosate tolerant crops.  A desire to show
spotlessly clean fields to non-farming landlords adds to the problem, he
said.

"Growers do not look at the economics of weed escapes.  They view GM
crops as an excellent chance to go out late season with the sprayer just to
clean the crop up.  This aesthetic weed management is a real problem for us."