A widely used herbicide can interfere with animals’ sexual development, according to a growing body of research. Applied to agricultural fields, residential lawns, and elsewhere to kill weeds, atrazine is often found in low levels in streams, lakes, and drinking water in the U.S. and Australia, the world’s two biggest users.
A study published in August in the journal Reproduction, Fertility and Development found that atrazine impaired genital development of tammar wallabies, an Australian marsupial in the same family as kangaroos. When researchers gave female wallabies drinking water with the herbicide throughout their pregnancy and while nursing, their male offspring developed shorter, smaller penises.
The research provides more evidence that atrazine “messes around with the way that our hormones are balanced within our bodies,” says study author Andrew Pask, a geneticist at the University of Melbourne. He’s not just referring to wallabies, but to all mammals—including humans.
The study is the first to explore atrazine’s impacts on marsupials, which give birth to underdeveloped babies that then mature in their mother’s pouch. Marsupials’ genital development occurs entirely after birth, meaning they’re more likely to be influenced by external factors such as chemical pollution, says Jennifer Graves, a marsupial researcher at La Trobe University, in Melbourne, who was not involved in this study. Because of this, the animals are likely to more clearly show effects of atrazine that might be happening more subtly in placental mammals, Graves says.
Switzerland-based Syngenta, the largest producer of atrazine, insists the product is safe. Syngenta has conducted its own studies on atrazine’s impacts and says “there is no consistent or compelling evidence to conclude that atrazine adversely affects wildlife at environmentally relevant concentrations,” said company spokesperson Chris Tutino in an email.
The concentration of atrazine in Pask’s study is higher than what wallabies would likely encounter in the environment, Tutino says, calling it “an implausible exposure scenario.” Pask disagrees, citing evidence of high atrazine spikes in Australian streams. Though the highest documented concentration recorded in Australian waters is about one-eighth that used in this study, animals can be exposed to additional atrazine by eating plants sprayed with the chemical, Pask says.
Atrazine is the second-most used herbicide in the U.S., where 70 million pounds are sprayed annually on crops—mainly corn, sugarcane, and sorghum. Australia, the chemical’s second-largest market, uses six million pounds each year. The European Union, for its part, banned atrazine in 2004 over lack of evidence of the chemical’s safety and concerns about regulating its concentration in watersheds.
In the U.S., atrazine is the most commonly detected pesticide in tap water, according to the Environmental Working Group, a nonprofit that advocates for reduced pesticide use and stronger drinking water standards. A 2010 Natural Resources Defense Council analysis of 153 drinking water systems through the U.S. found over a third had spikes in atrazine above the U.S. Environmental Protection Agency’s (EPA) limit of three parts per billion. Some systems in Ohio, Illinois, and Indiana had spikes above 10 parts per billion in their drinking water.
Atrazine tends to increase the conversion of testosterone to estrogen, Pask says, which can push an animal’s hormonal profile toward developing female traits. Research suggests it can disrupt the endocrine system, the body’s network of glands and hormones that governs everything from sex organ development to neural function.
Because the endocrine system is similar across all mammals, studies like these have potential implications for humans too, says Jennifer Freeman, an environmental toxicologist at Purdue University who was not involved in the wallaby study.
While there hasn’t been extensive research on atrazine’s human health effects, a 2013 study of boys born in Texas linked high prenatal atrazine exposure to genital malformations, including small penises, and a study the year before showed that women in areas with heavy atrazine use were more likely to have preterm births. A 2003 review from the U.S. government’s Agency for Toxic Substances and Disease Registry found that “maternal exposure to atrazine in drinking water has been associated with low fetal weight and heart, urinary, and limb defects.”
Even at levels below the U.S. drinking water standard, atrazine has been shown to have harmful effects on animals. A 2002 study found abnormal genital formations in male frogs exposed to water with just one part per billion of atrazine. A 2010 study showed atrazine could alter the development of reproductive organs in male frogs at 2.5 parts per billion. Most of those frogs had low testosterone, abnormal sperm count, and decreased fertility. One in ten of the previously male frogs turned into fully-functional females.
In a previous study led by Pask, published in 2019, his group found that mice exposed to a level of atrazine considered to have “no observed effect” on humans led to weight gain and decreased sperm count in the animals.
Water treatment can bring down atrazine levels for humans, but animals can encounter it at much higher concentrations in the wild. In Australian streams near farmland, atrazine has been measured at concentrations as high as 53 parts per million (53,000 parts per billion) following the spraying season.
In the farmlands of southwest Australia, where tammar wallabies live, streams and ponds with atrazine runoff “are some of the only places where there’s a permanent water source for those animals,” says Pask. “So they will go large distances to drink from those contaminated water sources, unfortunately.”
Tammar wallabies can ingest atrazine when they eat plants that have been sprayed or when they drink contaminated water. Pask and his colleagues gave 20 pregnant wallabies normal water and another 20 water with added atrazine, at 450 parts per million (450,000 parts per billion)—a high concentration, Pask says, but still a level that wallabies could plausibly encounter in the wild. The females freely drank the water through pregnancy, birth, and lactation.
Pask’s team wanted to see if and how the genitals of male offspring, known as joeys, born to the females ingesting atrazine might develop differently. Penis development is a good indicator of an animal’s overall hormonal balance during gestation because it’s easily affected by hormonal changes, Pask says.
The joeys born to mothers drinking contaminated water had penises that were 20 percent shorter and significantly thinner than the control group, and genes responsible for normal testicular function were also altered. These results suggest there was a hormonal imbalance during early embryonic development, according to the paper.
“It really was very surprising that we could see such dramatic effects,” Pask says.
Haitian Lu, North American product safety head for Syngenta, disputes the findings, saying it is "highly unlikely" that atrazine could dissolve in water at the high levels cited in the study. Pask insists the chemical fully dissolved at the level reported, but added that if the atrazine solubility were lower, as Lu suggests, the fact that baby wallabies developed abnormalities would indicate the chemical is even more potent and damaging.
Male tammar wallabies take two years to reach breeding age, so researchers don’t yet know if or how the atrazine exposure will affect breeding, but Pask says he expects smaller penises will make successful insemination more difficult.
“These are really dramatic effects that they’re reporting,” says Jennifer Sass, a senior scientist and toxicologist at the Natural Resources Defense Council. Finding significant developmental abnormalities in wallabies, even at a high dose, is “just chilling,” Sass says, in part because the chemical is so widely used and its effects on mammalian development are not comprehensively understood.
Tutino disagrees, though, saying that “nearly 7,000 studies have concluded that atrazine is safe for humans and the environment at relevant exposure levels.”
Tammar wallabies are currently considered a species of “least concern” by the International Union for Conservation of Nature but have been under increasing threat as their grassland habitats are replaced by farms and ranches. “Any additional pressures that you put on [a] species that make it more difficult for it to reproduce can have profound impacts,” Pask says.
The next step of his research, Pask says, is to do further studies with lower amounts of atrazine to determine the minimum dose at which problems arise. He says he hopes this research can inform when and how pesticides are sprayed across Australia’s rural farmland—where atrazine is used most heavily, and where many marsupials live.
“At the very least,” Pask says, “we need to consider that in some of the marsupial management plans for some of these really endangered species.”