Renewable Farming

Expansion of cover crops stimulates search for “burndown” alternative to glyphosate

Cover crop benefits to soil biology keep multiplying with new evidence: More species means a widening spectrum of beneficial soil organisms and helpful insects to combat pests and pathogens.

However, “terminating” cover crops with glyphosate imposes another exposure to a non-selective herbicide which is a well-established biocide against specific beneficial soil bacteria. 

We’ve experimented with various fertility products “intensified” with WakeUP to burn down cereal rye, with mixed success. And we’re exploring rumors of an Australian-patented “natural” burndown which may show promise of helping. Using non-glyphosate, non-glufosinate “conventional” herbicides in a crop program requires careful management to avoid carryover which stunts fall-planted covers.

Meanwhile, a new research paper on glyphosate has emerged, reporting certain bacterial and protozoa response to glyphosate foliar-applied on barley, which is sometimes used as a cover crop. The researchers observed no substantial net response in the total of bacterial populations after application of glyphosate. They also saw a temporary rise in certain fast-growing bacteria, apparently feeding on dying roots of barley killed by glyphosate. The paper is titled, “Stimulation of bacteria and protists in rhizosphere of glyphosate-treated barley.”  Authors are Valentina Imparato, Susana S. Santos, Anders Johansen, Stefan Geisen and Anne Winding. It’s in the journal of Applied Soil Ecology.  You can link to the study here.

Our concern is that readers may presume that this carefully documented study indicates “no bacterial impact at all” with glyphosate (along with its adjuvants).  In fact, years of microbiology research with glyphosate has confirmed substantial impacts of glyphosate on beneficial soil bacteria. One of the most widely recognized researchers in this realm is Dr. Robert Kremer, whose work as USDA/ARS scientist at the University of Missouri built the foundations of understanding on this subject.  We asked Dr. Kremer for comments on the new study.  His reply:
Dr. Robert Kremer

Hi Jerry,

I had just read this paper as well.
There are many factors that need to be considered.
The research in Imperato et al. is focused on barley; most of the other glyphosate work focuses on soybean, corn, or canola – conventional vs transgenic. The rhizosphere microbial community will differ between all plant species.
The focus was on bacteria only, and protists (protozoa) although culturable bacteria (fcfu and scfu) and molecular markers (16S rDNA) were observed, there is no specific categorization of genus/species of bacterial groups. We are only presented with the differences in composition based on different molecular profiles (or fingerprints) detected.
The other assumption that you bring up, with which I have a concern, is that glyphosate is assumed to be a general bactericide. This is not true based on my work. I’ve found that glyphosate suppresses only certain bacterial groups, which all seem to be beneficial. These include fluorescent pseudomonads, Mn-reducing bacteria, and rhizobia. So this means that many other opportunistic, and possibly detrimental, bacteria can proliferate in the absence or depletion of the beneficials, and perhaps increase in abundance and composition, as the new publication indicates. For example, in the Kremer et al. 2009 paper we found an increase in Mn-oxidizers with glyphosate, and many of these were Agrobacterium, which were lower in rhizospheres of non-glyphosate treated plants.
The release of high amounts of readily available C from roots of plants treated with glyphosate was thought to stimulate fast-growing bacteria, which I agree with; however, I associated this also with increased Fusarium on roots as these fungi are known to preferentially grow on simple carbohydrates as well – they did not report on this. Fast-growing bacteria could replace many of the beneficial bacteria as mentioned above, but this is not reported.
Finally, it’s interesting that the protists were not further classified as bacterial predators, saprophytes, etc. That information would have been very useful in determining whether there were impacts on the final abundance and composition of the bacterial community.
Overall, I liked the paper – it brings in an area not investigated previously, by looking at the protists, we are beginning to see potential overall impacts of glyphosate (and RR crops) on the soil food web, and by their conclusions, those impacts could be positive or negative.
Thanks for your message and maybe we’ll meet again at some meeting this winter. Have a good Thanksgiving.
One comprehensive report on “Glyphosate and Plant-Microbe Interactions” is available from a 2010 presentation by Dr. Kremer in Indiana. His PowerPoint slides are available at this link in PDF form.

Dr. Don Huber has also been tireless in helping farmers understand the long-term, cumulative impact of glyphosate on soil organisms. Yet, much of the hard data has been shrugged off by producers.

We asked Don for a brief observation on the paper.  He noted in a return message:

You only see what you are looking for.  Total numbers may or may not change, but there can be a large difference in constituents.  The techniques would not detect the 12-fold drop in Mn reducers and 10-fold [increase] in Mn oxidizers. It only showed that the population of protists seemed similar.  Also, desiccation application [of barley] is going to go primarily to the seed at that stage of growth and not to the rot system and exudates.


This fall, I asked several of our neighbors who raise Roundup Ready corn how their corn turned out, and what specific thing they did for the best yield response.  Two of them instantly said, “We had another 10 to 15 bushels of corn where we sprayed fungicide twice.” 

My followup question was, “Do you remember 10 or 15 years ago when you weren’t getting benefits from fungicide — you didn’t even think about needing it?  What do you suppose is happening in our soils, and why is fungus now such a significant problem?

Neither of them had a theory.