One of the most common weed killers in the world, atrazine, causes chemical castration in frogs and could be contributing to a worldwide decline in amphibian populations, a study published Monday showed.

Researchers 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.

Ninety percent of the male frogs exposed to atrazine had low testosterone levels, decreased breeding gland size, feminized laryngeal development, suppressed mating behavior, reduced sperm production and decreased fertility.

And an alarming finding of the study was that the remaining 10 percent of atrazine-exposed male frogs developed into females that copulated with males and produced eggs.

The larvae that developed from those eggs were all male, according to the study by researchers at the University of California at Berkeley, which was published in the Proceedings of the National Academy of Sciences (PNAS).

Earlier studies have found that atrazine 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," the study said.

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

According to the Washington-based Natural Resources Defense Council, the chemical herbicide has been banned in the European Union, although advocates for atrazine, who say the weed-killer increases crop yields, say only some European countries have banned it.

The US Environmental Protection Agency said four years ago that there was insufficient data to determine whether the chemical herbicide affects amphibian development and refused to ban the chemical.

Around 80 million pounds of atrazine are applied annually to crop fields in the United States alone, and half a million pounds of the herbicide fall to earth in rainfall in the United States, including in areas hundreds of miles from the farmland where it was originally applied, the study says.

"Atrazine can be transported more than 1,000 kilometers (620 miles) from the point of application via rainfall and, as a result, contaminates otherwise pristine habitats, even in remote areas where it is not used," the study says.