Scientists now know that the increase in dissolved reactive phosphorus (DRP) runoff that’s been plaguing the western Lake Erie basin is mostly coming from farms located in the Maumee watershed.

But the question they’re still trying to answer is — why?

Christopher Spiese, a chemist at Ohio Northern University, says a combination of factors is causing the problem. But at the Conservation Tillage and Technology Conference held earlier this year in Ada, Ohio, he focused in on one specific practice that’s changed over the last few decades — the use of glyphosate.

Coincidence or Causation?

Lake Erie’s troubles with phosphorus (P) aren’t new. In fact, the U.S. and Canada have been trying to clean up the amount of P in the lake for more than 40 years now, as the two governments created the Great Lakes Water Quality agreement in 1972 to focus on reducing the amount of P in the Great Lakes.

Since then, total P in Lake Erie has decreased significantly, Spiese says. DRP loads were also coming down, but started increasing in the mid 1990s.

At that same time, glyphosate use took off with the advent of herbicide-tolerant crops. As of 2014, Spiese says, more than 95% of soybeans and over half of corn in the U.S. are Roundup Ready, and those percentages probably haven’t changed over the last 2 years.

“These crops that are able to grow in the presence of glyphosate have really kind of started to take over, to the point where we’re washed…