Scientists in America are warning that weeds resistant to a herbicide used on GM crops might pose a serious threat to farmers.
A team from the southern weed research unit at Mississippi State University have found 15 species of weed resistant to glyphosate, including ryegrass, bindweed and goosegrass.
They say farmers could be giving the weeds an advantage over other plants which are killed by the chemical, and this may mean they multiply faster in certain conditions as a result.
It is not known whether some strains of the weeds are naturally resistant, or have evolved to survive against the herbicide, but scientists say that ignoring the problem could result in the world losing the use of a very effective weed killer.
The use of glyphosate has soared since GM crops were modified to resist it, from 5,000 tonnes in 1995, to over 30,000 tonnes in 2002.
