GM weedkiller use increases

A NEW study reveals that while US pesticide use dropped during the three first years of commercial GM crop cultivation, it has increased sharply thereafter.

GM maize, soybeans and cotton have led to a 55,000 tonnes increased in pesticide use since 1996, according to the study published by the Northwest Science and Environmental Policy Center.

The study, conducted by Charles Benbrook, a former Executive Director of the Board on Agriculture of the US National Academy of Science, therefore concludes the biotech industry‘s claims that GM crops help reduce the use of pesticides are unfounded.

But the study differentiates between herbicide tolerant crops (HT) and crops genetically engineered to express the bacterial toxin Bacillus thuringiensis (Bt), which is toxic to many insects, and it shows that on the latter crops pesticide use has dropped.

Bt crops have helped reduce insecticide use by 7,000 tonnes from 1996, while herbicide use on HT crops has increased by 62,000 tonnes.

The overall pesticide use has risen by about 4.1% on the US GM acreage, according to the study.

Reliance on a single herbicide, glyphosate, as the primary method for managing weeds on millions of acres planted to HT varieties is said to be the main factor that has led to the need to apply more herbicides per acre to achieve the same level of weed control.

Average application rates of glyphosate in HT weed management systems have jumped sharply in the last few years, the reports says, because of:

  • The spread of glyphosate-tolerant or resistant marestail (also known as horseweed)

  • Shifts in the composition of weed communities

  • Substantial price reductions and volume-based marketing incentives from competing manufacturers of glyphosate-based herbicides

The study predicts that for the foreseeable future, HT crops will increase pesticide use more than Bt crops will reduce it, in part because HT crops are grown on a much larger area than Btcrops.

The study is based on official US Department of Agriculture data on pesticide use over 670m acres of GM maize, soya and cotton.

The study is available for download here: