About 20 per cent more frogs were female among tadpoles exposed to the herbicide. (CBC)

A popular weed killer sprayed on cornfields across North America turns male frogs into females even at low levels, Ottawa biologists have found.

Vance Trudeau, a University of Ottawa biologist, and his PhD student Valérie Langlois raised leopard frog tadpoles over a spring and summer in outdoor tanks containing the herbicide atrazine. The low levels of the herbicide were only 12.5 per cent higher (1.8 micrograms per litre) than the maximum levels (1.6 micrograms per litre) found in the Raisin River in Cornwall, Ont., where the tadpoles were captured.

The study, published in the Nov. 19 issue of the journal Environmental Health Perspectives, found that 20 per cent more frogs were female after being exposed to atrazine, compared with frogs that weren't exposed to the herbicide. In addition, many male frogs exposed to atrazine had formed egg cells in their testicles.

"Animals were switching from male to female," Trudeau said Thursday.

Vance Trudeau hopes his study will persuade the government to lower allowable limits for atrazine. (CBC)

Fewer of the animals had also transformed from tadpoles into frogs.

He said that is of great concern because frogs are a key species in the ecosystem. They are important for controlling insects and are a key source of food for fish and some birds.

Trudeau acknowledged that his findings are controversial since other studies have shown atrazine to be harmless to frogs, including some funded by the manufacturer of the herbicide.

However, he said he hoped his results would lead the Canadian government to re-evaluate its guidelines and lower the acceptable level of the herbicide. He would not go as far as to recommend banning it, as some European countries have already done.