The emergence of resistant weeds
In the 1990s, glyphosate-resistant weeds were not known to exist. By 2014, glyphosate-resistant weeds were dominating herbicide resistance research. At that time, 23 glyphosate-resistant species were found in 18 countries. "Resistance develops after a weed population is subjected to intense selection pressure in the form of repeated use of a single herbicide."
According to weed expert Ian Heap, who completed his PhD in 1988 on the resistance of annual ryegrass (Ryegrass) to multiple herbicides -- Australia's first herbicide-resistant weed -- by 2014 ryegrass was "the worst herbicide-resistant weed in the world", Present in "12 countries, 11 operational sites, 9 cropping schemes" and affecting "more than 2 million hectares". Annual ryegrass has been known to be resistant to herbicides since 1982. The first documented case of L.rigidum resistant to glyphosate was reported in Australia near Orange, New South Wales in 1996. In 2006, farmers' associations reported 107 biotypes out of 63 herbicide resistant weed species. Canada identified its first resistant weed, giant ragweed, in 2009, when 15 weeds were identified as resistant to glyphosate. As of 2010, 700 to 10 million acres (28 to 4 million hectares) of soil in the United States were infested with herbicide-resistant weeds, or about 5 percent of the 170 million acres planted with corn, soybeans and cotton. The hardest hit are 22 states. In 2012, Charles Benbrook reported that the American Weed Science Society lists 22 herbherbicide resistant species, with more than 5.7 x 106 hectares (14 x 106 acres) infested by GR weeds, And Dow Agricultural Sciences conducted a survey and reported a figure of about 40 x 106 hectares (100 x 106 acres). The International Herbicide Resistance Weed Survey Database lists species that are resistant to glyphosate.
To counter the resistant weeds, farmers are weeding by hand, using tractors to turn the soil between crops and using herbicides other than glyphosate.
Monsanto scientists found that some resistant weeds have up to 160 extra copies of a gene called EPSPS, an enzyme that destroys glyphosate.
On 15 March 2017, the European Chemicals Agency (ECHA) published the recommendations of the ECHA Risk Assessment Committee (RAC) on the risk assessment of glyphosate. Their recommendations maintain glyphosate's current classification as a substance that causes severe eye damage and is toxic to aquatic life. However, RAC found no evidence that glyphosate is a carcinogen, mutagenic, toxic to reproduction, or toxic to specific organs.
