Weed resistance to glyphosate has increased fivefold since 2007 and is developing at a rate of one new weed species each year, William Reville of University College Cork told delegates at CropWorld.

Ten weed species in the USA are now resistant to Roundup, he revealed, with a further 10 species worldwide also affected.

“In the USA, species such as common ragweed, horseweed, Johnson grass, waterhemp and pigweed are resistant. Some 11m hectares are affected by this development.”

While Roundup Ready crops have been grown in the USA since GM soya was introduced in 1996, the resistance has developed by natural selection, he confirmed.

“It’s not surprising. Glyphosate was discovered in 1970 and has become the single most used herbicide in the USA. It has great benefits – two of which are that it is the least toxic and the least persistent of all herbicides.”

The discovery of a mutant gene which doesn’t contain the enzyme inhibited by glyphosate allowed major cash crops to be genetically engineered to contain this gene, simplifying weed control and cultivation practices across a huge area.

“It was a very smart technology and uptake in America, China and India was rapid. But a different approach may now have to be taken, as resistance is a reality.”

An alternative would be to genetically engineer crops that are resistant to two or more herbicides, Prof Reville suggested. “But the concern is that these are mostly older herbicides, none of which are as safe as glyphosate.”

Furthermore, herbicide development has slowed since GM was introduced, he added. “There’s only a limited number of herbicides available and natural selection is a factor with all of them. Traditional breeding methods may yet prove to be better.”