A chemical compound used to stimulate weight gain in cows may be contaminating aquatic ecosystems in the United States and disrupting the reproductive processes of fish, according to a new study in the journal Nature Communications.

Cattle farmers have been using the chemical compound trenbolone acetate (TBA) for decades. The hormone has a potentially toxic byproduct, 17-alpha-trenbolone, but earlier research had seemed to show that the chemical breaks down and becomes harmless when exposed to sunlight. But it turns out that when the chemicals wind up in rivers and streams, they transform back into 17-alpha-trenbolone when it gets dark, according to new Nature Communications study.

“As goes the compound, [so] goes the risk. In the sunlight the compound goes away, therefore the risk has also gone away,” said lead study author Adam Ward, assistant professor at Indiana University School of Public and Environmental Affairs. “What our study shows is that chemically that’s not true.”

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The full extent of the damage that may be caused by the hormone isn’t entirely clear. The hormone 17-alpha-trenbolone leaves cows through their manure and moves into rivers, streams and other bodies of water where it can disturb fish and other water dwellers. The hormone resembles testosterone in its effects, but it’s ten times as strong. It also has been shown to reverse the sex of fish and reduce their rates of reproduction. “We’re releasing this into the environment at levels that are potentially problematic for the ecosystem,” said Ward. “If you’re an amphibian, a fish, a minnow, you spend your whole life being bathed in this sort of low dose of testosterone.”

The implications of the research challenge the way regulators approach risk management in water systems, Ward said. Typical water management programs assess the risk that individual substances pose to the water supply. Managing 17-alpha-trenbolone and all of its related compounds requires considering “potency of mixtures,” Ward said, not just the potency of individual compounds. And trenbolone acetate is far from the only endocrine disruptor affecting aquatic life in lakes and streams across the country, according to research from the U.S. Geological Survey. Vinclozolin, a fungicide, and insecticides like DDT and carbaryl have also led to similar changes.

“The prevailing wisdom on risk management is incomplete,” he said. “We’ve got tens of thousands of compounds that we produce and use in this country every year, but we don’t know what happens to them into the environment in complicated systems.”

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Write to Justin Worland at justin.worland@time.com.