U.S. farmers use glyphosate more widely and more intensively than any other weedkiller, says researcher Charles Benbrook in a paper published today in the journal Environmental Sciences Europe. Benbrook says growers applied nearly 250 million pounds (125.4 million kg) of the chemical in 2014.
That’s nearly 10 times the amount used two decades ago and mirrors the U.S. embrace of corn, soybeans and cotton hybrids genetically engineered to tolerate the herbicide. “The frequency and levels of glyphosate and residues in a variety of foods are increasing, and more refined dietary risk assessments should be carried out,” writes Benbrook, noting a WHO agency classified glyphosate as probably carcinogenic to humans in 2015.
Since GE crops came on the market, farmers have applied glyphosate, best known as the Monsanto weedkiller Roundup, more frequently and in higher doses, especially with the emergence of weed resistance, says Benbrook. Growers also added more herbicides to their weed-control programs.
“As a direct result, average herbicide use per hectare on land planted to GE-HT [genetically engineered herbicide tolerant] varieties has, on average, escalated steadily since the mid-1990s,” says the paper.
Farmers sprayed enough glyphosate in 2014 to average nearly 13 ounces per acre (1 kg per hectare) of cultivated cropland in the United States, and half a pound per acre (0.53 kg per hectare) for cropland worldwide, calculates Benbrook, who used a variety of sources to compile usage totals.
Benbrook told Ag Insider the drawback of wide adoption of GE crops that tolerated gylphosate was that “farmers bought in to the age-old, silver-bullet image and dropped virtually all other weed-management practices. This was a recipe for long-term problems, regardless of what herbicide emerged from the pack to dominate two-thirds of the market. Now, weed scientists are reminding farmers that the key to safe, sustainable weed management is using ‘many little hammers.’ Glyphosate, as used since the year 2000, has been the 20-pound sledgehammer of weed control.
“No one can predict where farmers will have to go to get weeds back under control, but … there will be collateral damage and large and sustained increases in weed-management costs,” Benbrook said.
Weed-control options for farmers include using a variety of herbicides, diversified crop rotations, and mechanical tillage to uproot and bury weeds and their seeds. For many growers, efficient herbicides are vital for low- and no-till cropping, which reduces erosion and production costs. Dow Chemical plans to sell GE seeds that tolerate a weedkiller that combines the herbicides glyphosate and 2,4-D.
Glyphosate will likely remain the top herbicide for years to come because of the many crop varieties that tolerate it and because it still controls dozens of weeds, says Benbrook. His paper says continued heavy use of the chemical means “interest will grow in quantifying ecological and human health impacts.”
For years, glyphosate was regarded as one of the least chronically toxic herbicides for mammals. Monsanto is contesting in court plans by a California state agency to include glyphosate on its list of known carcinogens based on the 2015 classification by the International Agency for Research on Cancer. Monsanto says the weedkiller is safe.
Supplemental tables in Benbrook’s study show the rise in glyphosate use and its effect on total herbicide use. U.S. soybean growers used a total of 2.02 pounds of herbicides per acre in 2014, up from 1.19 pounds in 1996, with glyphosate accounting for most of the growth. For cotton, herbicide use per acre rose 1 pound from 1996-2014 to total 2.89 pounds per acre, with glyphosate use up by 0.93 pound. In corn, glyphosate use rose by a quarter-pound per acre during the two-decade period while total herbicide use fell by half-a-pound per acre. Glyphosate was used on a fraction of corn, soybean and cotton land in 1996, but on almost all soybean and cotton land in 2014 and on 71 percent of corn land.
Benbrook, now a private consultant, wrote the paper, “Trends in glyphosate herbicide use in the United States and globally,” while a research professor at Washington State University. That post ended in May 2015.