Weeds resistant to common herbicides — dubbed superweeds — have spread over 60 million acres of U.S. farmland,1 and the economic and environmental devastation caused the weeds is growing, too. Worldwide, weeds have evolved resistance to 167 different herbicides, and herbicide-resistant weeds have been reported in 93 different crops in 70 countries.2
Scientists are now scrambling to understand how weeds are outsmarting these widely used agricultural chemicals, but it's often said that the spread is due to independent evolution via herbicide selection.
However, researchers writing in the journal Agriculture, Ecosystems & Environment believe herbicide-resistance gene flow may be primarily to blame, calling it an "underestimated and underappreciated" phenomenon.3
Herbicide resistance gene flow may be causing superweeds
Concerns surrounding herbicide resistance increased after the 1990s' introduction of genetically engineered (GE) "Roundup Ready" crops designed to resist glyphosate, the active ingredient in Roundup herbicide.
As farmers used an abundance of Roundup on their fields, neglected other weed control measures and continued to grow the same crops on the same land repeatedly (i.e., monoculture), superweeds spread and flourished.4
"Interest in the magnitude and consequences of intra- and interspecific herbicide resistance gene flow, particularly that mediated by pollen, increased in the mid-1990s with the introduction of herbicide-resistant (HR) transgenic crops," the researchers explained. "During that time, less attention was paid to the movement of HR alleles via pollen or seed among weed populations."5
Simulation models predict, however, that frequent widespread applications of glyphosate and other herbicides facilitate an increase in the frequency and movement of herbicide resistance traits. How this is occurring could be due to the flow of GE herbicide resistance from one plant to another, possibly via the transfer of pollen.
"Once a new HR weed biotype has been confirmed in a jurisdiction, how often have we been surprised at its rapid areawide expansion?" the researchers asked. It turns out, however:6
"In genotypic studies of HR weed populations, the contribution of gene flow to incidence of resistance is frequently similar or greater than that of independent evolution."
It's possible, for instance, that pollen carrying genes for herbicide resistance can fertilize a nearby weed, leading to offspring that carry the herbicide resistant trait.7
"Is it a surprise?" the Guardian noted. "On one level no, gene flow and hybridization are as old as plants themselves. Short of creating sterile male plants, it's simply impossible to stop crops releasing pollen to fertilize related neighbors."8
Farmers using more pesticides to fight herbicide resistance
Farmers are scrambling to find a solution for uncontrollable weeds that have outsmarted these man-made chemicals. The solution from agribusinesses entities is to introduce new GE crops designed to withstand not only glyphosate but also additional pesticides to kill off the weeds glyphosate leaves behind.
Enlist E3 soybeans, made by Corteva Agriscience, a division of DowDupont and seed company MS Technologies, is one of the latest, designed to tolerate glyphosate, glufosinate (another herbicide) and 2,4-Dichlorophenoxyacetic acid (2,4-D), one of the ingredients in Agent Orange, which was used to defoliate battlefields in the jungles of Vietnam, with horrendous consequences to the health of those exposed.
"The U.S. Department of Agriculture estimates that by 2020, the use of 2,4-D on America's farms could rise between 100 percent and 600 percent now that it has been approved as part of Enlist Duo," the National Resources Defense Council (NRDC) stated.9
This is concerning because the International Agency for Research on Cancer (IARC) ruled 2,4-D a possible human carcinogen in 2015, and there is concern it may increase the risk of Non-Hodgkin lymphoma and soft-tissue cancer known as sarcoma.
Another example is Roundup Ready Xtend cotton and soybeans, which are GE plants designed to tolerate both glyphosate and dicamba, a highly volatile herbicide known for drifting and damaging off-target crops, including trees.
It's not only weeds that are developing resistance to herbicides, either. Bacteria that have been exposed to Roundup, dicamba and 2,4-D develop resistance to antibiotics 100,000 times faster than the same bacteria that have never been exposed to the herbicides, University of Canterbury has found.10