Both of these copulating frogs are genetically male, but the larger one on the bottom has been completely feminized by atrazine exposure and produces viable eggs. ((Tyrone Hayes/ABC))

One of the most common weed killers in the world, atrazine, can chemically castrate male frogs, turning them into females that lay eggs, say U.S. researchers.

Atrazine continues to be used on cornfields in Canada, although it is no longer approved for use in Europe. The U.S. Environmental Protection Agency announced last year it would launch a new scientific evaluation of atrazine's effect on humans.

The gender change occurred at a concentration of atrazine half of the Health Canada guideline for drinking water.

Prof. Tyrone Hayes of the University of California, Berkeley, and colleagues reported their findings in this week's Proceedings of the National Academy of Sciences.

"Atrazine-exposed males were both demasculinized [chemically castrated] and completely feminized as adults," said Hayes.

Atrazine is widely used by farmers around the world as a herbicide, particularly in production of corn, sorghum and sugar cane.

Earlier studies have found that the chemical feminized zebra fish and leopard frogs, and caused a significant decline in sperm production in male salmon and caiman lizards.

"Atrazine exposure is highly correlated with low sperm count, poor semen quality and impaired fertility in humans," said Hayes and colleagues.

While previous studies have shown atrazine adversely affects amphibian larval development, this latest study of African clawed frogs shows the process can go even further, said Hayes.

"Before, we knew we got fewer males than we should have, and we got hermaphrodites. Now, we have clearly shown that many of these animals are sex-reversed males," said Hayes.

Genetic males become functioning females

Hayes and colleagues compared 40 male control frogs with 40 male frogs reared from hatchlings until full sexual maturity, in atrazine concentrations similar to those experienced year-round in areas where the chemical is found.

Of the male frogs exposed to atrazine, 90 per cent had low testosterone levels, decreased breeding gland size, feminized laryngeal development, suppressed mating behaviour, reduced sperm production and decreased fertility.

The remaining atrazine-exposed male frogs developed into functional females.

"Ten per cent of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs," the researchers wrote.

The larvae that developed from those eggs were all male.

"Atrazine has caused a hormonal imbalance that has made them develop into the wrong sex, in terms of their genetic constitution," said Hayes.

Atrazine level restricted in drinking water

The Health Canada maximum acceptable concentration for atrazine in drinking water is 0.005 milligrams per litre of water, or five parts per billion.

Hayes and colleagues exposed frogs to water contaminated with 2.5 parts per billion of atrazine.

Health Canada concluded an environmental assessment of atrazine in 2007. Its Pest Management Regulatory Agency "determined that the use of atrazine on corn for weed control does not entail an unacceptable risk to the environment."

In Australia, environmental groups have been concerned about the adequacy of testing of Australian waterways for chemicals such as atrazine.

The National Health and Medical Research Council recently released revised guidelines that restrict the level of atrazine in drinking water to 20 parts per billion.

Whether the effects found in frogs translate to humans is far from clear.

Frogs have thin skin that can absorb chemicals easily and they literally bathe in the polluted water.

With files from CBC News