Glyphosate has been called a once-in-a-century herbicide in part because it had been in use for an astonishing 25 years before there were any reports of resistance to it.
Glyphosate was first tested for herbicide use by John E. Franz of Monsanto in 1970. Franz found it is a “broad-spectrum,” or non-selective herbicide, capable of killing a wide variety of plant species.
Scientists later learned it works by disrupting a plant enzyme known as EPSPS. EPSPS plays an essential role in a metabolic pathway found only in micro-organisms and plants, called the shikimate biosynthetic pathway. This pathway, located in the chloroplasts in plants, is responsible for a third of the carbon the chloroplasts assemble into molecules. Shutting down the shikimate pathway kills the plant.
Due to the herbicide’s non-selective nature, glyphosate use in agriculture was limited at first to removing weeds from fields before crops were planted. But in1983, scientists found a gene in a soil bacterium that was surviving in a glyphosate manufacturing waste stream in Louisiana. The bacterium, designated CP4, had a different EPSPS that was insensitive to glyphosate. The scientists inserted the gene for the CP4 EPSPS enzyme into crop plants, giving them the ability to resist the effects of the herbicide.
In this way, herbicide-resistant soybean, cotton, corn, canola, alfalfa and sugarbeets were eventually developed. Beginning in 1996 with the introduction of Roundup Ready soybeans, glyphosate tolerant crops profoundly changed how many farmers managed weeds.
Farmers could spray glyphosate after the crops emerged, killing the weeds while leaving the crop plants intact. Moreover, they no longer had to till their fields to destroy weeds, saving time and fuel and avoiding soil compaction and top-soil loss.
This model of weed control was so successful that farmers came to rely increasingly on the new herbicide-plant combination, often abandoning older herbicides.
By 2006 more than 95 percent of all U.S. soybeans were glyphosate resistant and almost 70 percent of cotton was as well. The use of these transgenic crops — and the expiration of glyphosate patent protection – drove up sales of glyphosate, which became the best-selling herbicide in the world.
But in biology nothing is forever. With widespread herbicide use, weeds were able to develop selective resistance and these variants have multiplied.
The first glyphosate resistant weed was documented in 1997 and today 21 weed species are known to be resistant. Thirteen of those are found in the U. S., and of those 13, most can be found in row crops. They include Palmer pigweed, common waterhemp, Johnson grass, common and giant ragweed, kochia, goosegrass, annual ryegrass and horseweed.