The next time you admire a weed-free field of Roundup Ready soybeans treated with Roundup, bear in mind that the herbicide does more than just kill weeds.

University of Missouri (MU) researchers discovered that glyphosate (Roundup’s active ingredient) herbicide applications changed the microbial composition of soil in the field. Pat Donald, MU plant pathologist, and Robert Kremer, an MU soil scientist and USDA-ARS microbiologist, conducted experiments from 1997 through 2000 at two Missouri locations, which revealed that Roundup Ready soybeans treated with glyphosate at recommended rates had more Fusarium on roots within one week of application than did soybeans that did not receive glyphosate.

So what? Fusarium fungi are almost always present in soybean fields. But at elevated levels, they can clip yields through such diseases as sudden death syndrome (SDS) and other root rots, Donald says.

“Although soil Fusarium populations varied among locations, glyphosate significantly increased numbers at each location,” Kremer says. “There is a natural ebb and flow, but with Roundup Ready beans treated with glyphosate, there was always a spike in the levels of the fungi studied.”

The study coincided with findings by Missouri farmers regarding disease in Roundup Ready varieties. “There were several complaints that surfaced about sudden death syndrome that they had not seen with conventional varieties,” Kremer says.

However, Donald and Kremer emphasize that glyphosate did not reduce soybean yields in their studies. Yet, they add that the potential yield impact of high soil Fusarium levels from continuous glyphosate use requires more study.

“We need to look at it more and see whether there’s a buildup of the organism from year to year,” Kremer says.

In contrast, Harvey Glick, head of biotechnology stewardship for Monsanto, says numerous studies show that Roundup does not seriously impact soil microbe populations around roots.

“I know that Fusarium is an important component of sudden death syndrome, which is a real problem in beans,” Glick says. “But just because you have Fusarium doesn’t mean you'll have an expression of the disease. Fusarium is always present in the soil. When you spray Roundup on weeds, scientists speculate that the Fusarium species from the roots of dead weeds may become available to colonize the roots of living soybean plants. However, this does not mean you will have the disease present.”

Farmers also have been using the Roundup Ready production system for five years over 60 million hectares, Glick says. “Any concerns with populations of soil microbes or disease would have manifested itself by now,” he adds.

More study needed. The Missouri researchers claim that little study of the relationship between soil microbes and transgenic plants has been done.

“The tests are often limited to small soil insects and earthworms,” Kremer says. “We think there’s been an oversight.”

Though the increased Fusarium levels did not reduce yields, the MU researchers express concern about continuous glyphosate use in Roundup Ready soybeans.

Seasonal Fusarium increases result when farmers apply glyphosate on Roundup Ready soybeans. Normally, these levels do not carry over into the next year. However, repeated glyphosate applications in consecutive growing seasons could cause Fusarium levels to snowball.

“It may be a good idea to rotate out of Roundup Ready soybeans periodically,” Kremer says. “Otherwise, there may be a future ecological consequence if Fusarium builds up.”