Atrazine is the most widely-used weed-killer in the United States. It is also one of the most controversial. Studies have linked atrazine to environmental damage and adverse health effects, including cancer. While the European Union banned its use nearly ten years ago, it is still approved for use in U.S. corn, sugarcane and sorghum fields.

Jamie Jamison produces corn, wheat and soybeans on his 500-hectare farm. To boost production, he uses genetically modified seeds. To control weeds, he uses Atrazine. “It gives us good long-season control and allows us to have a good crop," Jamison said.

Jamison takes precautions when he sprays atrazine on his fields. He knows the wind could take the pesticide to unintended areas and kill other crops, or pollute waterways. But he's convinced its benefits outweigh the risks.

But opposition to Atrazine is growing. “I think the most convincing evidence of effects on humans is actually the ones on birth defects data, and also reports on effects on male reproductive fitness, poor sperm quality and low sperm mobility," said Jennifer Sass, a senior scientist at the Natural Resources Defense Council in Washington, a private environmental group.

Sass says critics are not convinced by the safety studies done by Atrazine's leading manufacturer, the Swiss giant, Syngenta. “I think Syngenta has been able to use its lobbying and financial weight to keep this argument going on for so long,” Sass said.

Although Atrazine's product label says it is toxic to aquatic invertebrates, Tim Pastoor, the company's principle scientist, says Syngenta's studies prove Atrazine is safe, if used properly. “As principal scientist with Syngenta I take it very seriously," Pastoor said.

Pastoor says U.S. farmers annually apply more than 35 million kilograms of Atrazine on their fields. He concedes it doesn't always stay there.

“On occasion, Atrazine gets into the water, but in such low amounts that it will not harm human or environmental health,” Pastoor said.

Fish pathologist Vicki Blazer, with the U.S. Geological Survey, studies fish mortality and mutations and collects evidence on water pollutants. She has found fish with compromised immune systems. She blames that on Atrazine and other farm runoff.

“... the spreading of poultry litter and then runoff during storms, plus herbicides such as Atrazine and other pesticides we are finding,” Blazer said.

The U.S. Environmental Protection Agency declined to comment for this report. But EPA's website acknowledges public concerns about Atrazine in drinking water. “Europe has made a very reasonable and very scientific decision that people shouldn’t drink pesticides in their drinking water,” Sass said.

Syngenta continues to defend the safety of Atrazine when it's used responsibly. U.S. farmers like Jamie Jamison, who appreciates the fewer weeds and bigger harvests, plan to continue using it.