The rise of glyphosate-resistant weeds has resulted in increased herbicide use, with farmers growing GM soya using more than those who didn’t adopt the technology.

That’s according to research carried out by a group of US scientists, published recently in Science Advances.

They also found that insect-resistant maize cut insecticide use.

See also: How glyphosate-resistant weeds will infest all US crops by 2020

The team at the University of Virginia looked at data from more than 5,000 soya bean farms and the same number of maize farms in the US between 1998 and 2011, in order to study the environmental impact of growing GM crops.

Since 2008, GM crops have accounted for more than 80% of maize and soya beans planted in the US.

Maize seeds are modified with two genes: one kills insects that eat the seed and one allows the seed to tolerate glyphosate. However, soya beans are modified with just one glyphosate-resistant gene.

The study, led by University of Virginia economist Federico Ciliberto, found maize growers who planted insect-resistant seeds used 11.2% less insecticide than those growing conventional maize

“Unsurprisingly, maize farmers who used the insect-resistant seeds used significantly less insecticide than farmers who did not use genetically modified maize,” he said.

These growers also saw a small benefit in herbicide use, using 1.3% less herbicide over the 13-year period.

However, in soya beans, those growing GM herbicide-resistant crops actually used 28% more herbicide than non-adopters rather than benefitting from a reduction.

Prof Ciliberto attributes this increase to the proliferation of glyphosate-resistant weeds.

“In the beginning, there was a reduction in herbicide use, but over time, the use of chemicals increased because farmers were having to add new chemicals as weeds developed a resistance to glyphosate,” Prof Ciliberto said.

He said the problem was less acute for maize farmers – who did not adopt the technology as quickly as soya bean farmers – but added they also now faced rising resistance.