by Sally Colby

Since glyphosate is a hot button issue for both consumers and producers, American Hort brought together several experts in the field to discuss the topic. American Hort represents the entire horticulture industry and has been following the glyphosate debate in the interest of dispersing the science behind the glyphosate issue.

Dr. Scott Senseman, Department of Plant Science, University of Tennessee, has done extensive work with pesticide residue. He explained herbicide activity from glyphosate was first observed in 1971, when it was completely non-selective and highly effective for burn-down applications; especially for no-till operations.

“One of the things we’re fighting is herbicide resistance,” said Senseman, “and glyphosate is no different. We’ve had issues with glyphosate resistance in crop systems as well as some very specific weed problems; palmer amaranth being one.” Senseman added that as of fall 2018, agronomists have listed 42 weed species resistant to glyphosate.

However, despite the resistance issues, glyphosate has many positives. Because of its absorptivity, glyphosate doesn’t leach, there’s low potential for movement and surface runoff, and it’s degraded rapidly by soil microorganisms.

“A combination of rapid degradation and no movement minimizes exposure to water sources,” said Senseman. “I would contend that in our soil samples, regardless of the rate we apply, all the materials we apply through glyphosate have dissipated and broken down into CO2 at that point.”

However, IARC (International Agency for Research on Cancer) places glyphosate in group 2A, which means ‘probably carcinogenic to humans’. “One of the things important to note is what group 2A as a probable human carcinogen means in the context of the report,” said Senseman. “This category is used when there is limited evidence of carcinogenity in humans and sufficient evidence of carcinogenity in experimental animals.” Senseman explained that limited evidence means that a positive association has been observed between exposure to the agent and cancer, but other explanations for the observations could not be ruled out.

Senseman noted that other probable human carcinogens on the IARC report include red meat, indoor emissions from burning wood, high-temperature frying and late-night work shifts. “The known human carcinogen list has some things that are a lot worse,” he said. “Processed meat, all alcoholic beverages, sunlight, engine exhaust and outdoor pollution.”

Christine Olinger, senior chemist in the office of pesticide programs at EPA, works on risk assessment and policy development. Olinger supervised the EPA team that prepared the most recent human health risk assessment for glyphosate.

“The office of pesticide programs is responsible for registering all pesticides in the U.S. as well as studying pesticide tolerances,” said Olinger. “We’re in our second round of our reevaluation program.”

Olinger said work on glyphosate began in 2009, and in December of 2017, EPA initiated draft risk assessment. Each step of the re-evaluation program allows public comment, and the EPA received more than 200,000 comments on glyphosate risk assessment. Most comments were the result of mass mailing campaigns, but many comments came from growers, farmers, the pesticide industry and other interest groups.

Although some interesting information was presented, the group concluded that there was nothing to cause them to change their risk assessment. “We identified no human health risk concerns,” said Olinger. “There were some ecological risks, mainly to non-target plants, so we’re proposing to modify labels to include spray drift language.”

Olinger explained the steps involved in risk assessment. The first, hazard assessment, examines the adverse effects of a pesticide coupled with a dose response assessment — how much would it take to cause an adverse effect? “Alongside of that, we are doing an exposure assessment for all registered uses of glyphosate,” she said. “We’re looking at exposures of food, people handling the pesticide, people entering treated areas after application. Then the information is combined for risk characterization, which provides a quantitative estimate of risk, a description of confidence level in the assessment and any uncertainties.”

With respect to non-cancer effects, Olinger said very few hazards were observed. “Our laws require us to consider any special sensitivity to infants and children,” she said. “We have several studies that show they are not more sensitive to glyphosate, and there’s no evidence that it affects the nervous system or the immune system.” Olinger added that glyphosate can potentially be inhaled or absorbed through the skin, but no adverse effects were seen when laboratory animals were exposed close to the limit dose — the highest dose at which we can reasonably test and still reflect normal metabolism.

“Studies show that glyphosate is excreted very rapidly from the body,” said Olinger. “Usually within 24 hours, and only a very small amount is absorbed (in tissue). We also have studies with human skin that show a very small amount gets through the skin — less than 1% of what’s applied.”

Olinger said all studies were conducted on laboratory animals, and on select species most sensitive to adverse effects. However, since the results don’t reflect the response in humans, studies assume humans will be 10 times more sensitive than the most sensitive species.

“Because there’s variation among the human population, we’re going to apply another safety factor,” said Olinger. “These safety factors are standard across the government with respect to making sure we have an appropriate margin of safety. Our laws require us to assume children are up to 10 times more sensitive than adults unless we have reliable data to show otherwise.” Olinger added that several studies showed children are not more sensitive to glyphosate.

Three types of studies were examined when determining the ‘not likely’ classification. “Epidemiological studies look at human populations and the potential environmental exposures and coming up with statistical correlation,” said Olinger. “We concluded that there’s no evidence of various types of tumors, including leukemia and Hodgkin’s lymphoma. The main study we looked at was the agricultural health study, which is conducted by the National Institutes of Health, and they’re looking at about 90,000 farmers and their spouses; looking at pesticide use and all health outcomes — not just cancer.”

In regard to animal carcinogenicity studies, Olinger explained that the EPA normally receives two cancer studies when registering food use pesticides. However, they looked at 15 studies and also the literature, and some studies showed no tumors at all. Scientists observed tumors in animals that were dosed at very high levels, above the limit dose, but EPA doesn’t believe that is a realistic comparison for any human cancer risk.

The third study type is genotoxicity, which looks at potential for damage to genetic material. Both in vitro (test tube) and laboratory animals (in vivo) studies showed no genetic mutations.

Olinger said after careful study, the EPA reached the same conclusions as many other regulatory authorities — that glyphosate is not likely to be carcinogenic. She added that many individual countries, the EU, Canada, Australia, New Zealand and Brazil all agree with the EPA’s findings.

“Overall, we concluded that there is no meaningful risk to humans when glyphosate is used in accordance with the product labels,” said Olinger. “We’ve also concluded that glyphosate is not likely to be carcinogenic to humans.”