Researcher studies where glyphosate goes

It’s easy to see what happens when you spray glyphosate on a field of Roundup Ready soybeans — the weeds curl up and die. Still, there’s more going on in that field than meets the eye, says Robert J. Kremer, USDA Agricultural Research Service scientist, Columbia.

Kremer told crop farmers attending the Northwest Missouri Corn Winter Meeting in St. Joseph that glyphosate also has an effect on the mysterious world of the rhizosphere, that narrow sliver of soil influenced by root secretions and associated soil microorganisms. You may not be familiar with the rhizosphere, but changes in the numbers and types of microbes living in that underground world can have a significant effect on how nutrients are cycled, and whether they are available to the plant.

Kremer is a scientist with the USDA’s Agricultural Research Service Cropping Systems and Water Quality Research Unit at the University of Missouri. Over the past decade or more, he has focused on finding where glyphosate goes in the plant, and what effects it has. His work has shown that glyphosate is released from Roundup Ready soybeans into the root zone, and there are consequences to the microbes that surround these roots.

Key Points

Agricultural Research Service researcher presents glyphosate study to corn growers.

Research focuses on where glyphosate goes in the soybean plant.

Research suggests an influence on plant’s rhizosphere and micronutrients.


He first began evaluating these effects in the mid-1990s, as the popularity of glyphosate-resistant crops began to build. One early finding: Colonization of roots with the fungal pathogen Fusarium was typically two to five times higher on RR soybeans treated with glyphosate, as compared with soybeans receiving a conventional herbicide (or no herbicide).

“We have continued to evaluate RR soybean and glyphosate treatments, and have found consistently high Fusarium root colonization in those plants,” he says.

Micronutrient impact

There’s also a question of whether shifts in the rhizosphere can have an effect on availability of micronutrients such as manganese (element symbol Mn). Recent studies, including research in cooperation with Brazilian researcher Luiz Zobiole, have found that there is a link. Kremer previously documented that glyphosate “leaks” from RR soybean roots, along with high concentrations of soluble carbohydrates and nitrogen-containing compounds.

“This strongly suggests an influence on the rhizosphere microbial communities,” he points out.

Mn has to be “reduced” by microbial action in the soil in order for plants to take it up and use it; the opposite chemical reaction is oxidation, which makes Mn unavailable to plants. Kremer found that the ratio of Mn-reducers to Mn-oxidizers was consistently lower in RR soybeans, especially when treated with glyphosate. “That suggests potentially low Mn availability to soybeans in these systems,” he says.

Further drilling down on this effect, Kremer found that bacterial colonies with Mn-oxidizing ability often produce a large amount of a molecule called EPS. Judging from other research, Kremer is now theorizing that EPS-based biofilms might be forming on roots of RR soybeans, which would boost Mn oxidation, thus lowering the amount of Mn available to plants.

“Good” microbes such as pseudomonads not only make Mn available to plants, but also help hold down the growth of pathogens such as Fusarium. It appears glyphosate shifts the microbial balance away from these “good” microbes.

New strategies

Now, for the hard part: figuring out how to manage differently, so these potential negatives in glyphosate-tolerant crops can be reduced or eliminated. It all starts with basics, Kremer points out, suggesting growers soil-test and make sure to correct any nutrients (including micros) that fall short.

After that, it gets more complicated. Kremer says scientists will need to develop a better understanding of the complex factors going on in the rhizosphere. They will need more study of factors such as root exudation and glyphosate release that interact with the microorganisms associated with roots.

Studies with Zobiole indicated that glyphosate affects the rhizosphere in both “first-generation” and “second-generation” Roundup Ready soybeans. It impacts the complex interactions of microbial groups, biochemical activity and root growth, which may lead to problems with plant growth and plant health, and perhaps keep a lid on yield.

“We’re looking to information that will help overcome any potential detrimental effects of glyphosate in glyphosate-resistant cropping systems,” Kremer says.

This article published in the March, 2011 edition of MISSOURI RURALIST.

All rights reserved. Copyright Farm Progress Cos. 2011.