23 November 2001

Glyphosate resistance is showing a worldwide rise

New products, new advice

and new problems were

debated in detail at last

weeks 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 $2.5bn 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 $2.5bn," said Andrew Cairns, of Natal University, Pietermaritzburg.

Resistance has also been found in ryegrass in New South Wales and Western Australia, a grassweed 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, its 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 dont see that the shifts or 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." &#42