Does glyphosate restrict crop mineral uptake?

Note: This post follows Extraordinary claims require extraordinary evidence about Don Huber’s alleged letter to the USDA that claims a never before seen “micro fungus” is endangering all of agriculture.
While claims about “micro-fungi” are too extraordinary to even consider until extraordinary proof is provided (and preferably replicated by another lab and peer reviewed), Don Huber’s claims that Roundup (specifically the active ingredient glyphosate) weakens crops by binding minerals in the soil seems to have at least some merit, at least enough to be taken seriously and examined further.
Over the years since Roundup Ready (RR) crops have been released, independent researchers have conducted many studies to determine whether there is a specific problem with some crop varieties with the RR gene, with all crops with the RR gene, or with glyphosate itself. Overall, the research shows that there may be some concern about glyphosate reducing availability of some minerals when the soil is deficient in those minerals. The research hasn’t found a problem with the RR gene itself.
It is important to note that the stack of peer reviewed papers indicating glyphosate to be a problem with disease or yield is much smaller than the stack indicating there is no problem. We must look at the entire body of evidence, not just cherry pick one or a few papers, in order to get a clear understanding of what’s really happening. Happily, extension experts from multiple universities have summarized the research for us, but if you want to look for yourself, PubMed is a great place to start.

Claims of interactions between glyphosate and minerals

In February of 2010, Dr. Huber appeared in an article by Martha Ostendorf titled Are We Shooting Ourselves In The Foot With A Silver Bullet? in No-Till Magazine along with Bob Streit, an agronomy consultant in Iowa. That article is no longer available from the No-Till Farmer website, but thankfully a Biofortified reader found another source (linked from the article title). Another article written by Huber at about the same time is Ag chemical and crop nutrient interactions. In these document, a lot of claims are made that aren’t consistent with the majority of peer reviewed research on the subject.
Since 2010, Dr. Huber has continued publicly claiming that glyphosate binds up minerals in the soil, making the minerals unavailable to crops and increasing susceptibility to disease (specifically fungal disease), thus decreasing yields. He spoke to the Innovative Farmers Association of Ontario in March 2010, one of many talks he’s given on this topic. In February 2011, he gave a talk in Des Moines at a seminar organized by the same Bob Streit and Amie Brandy. Dr. Huber has published some peer reviewed studies to back up his claims as well.
Dr. Huber is not the only scientist that has found interactions between glyphosate and minerals. Back in 2007, Barney Gordon published some research in an industry newsletter indicating that glyphosate treated soybeans may require manganese fertilizer for optimal yields: Manganese Nutrition of Glyphosate-Resistant and Conventional Soybeans. Of course, this research was used inappropriately as “evidence” that genetic engineering reduces yields, but that’s another story.
Dr. Gordon and Dr. Huber’s work has been used eagerly by fertilizer companies and organizations that promote fertilizers to encourage farmers to apply minerals to their crops. For example, see Glyphosate and Micronutrients by Jim Halbeisen of Growers Mineral Solutions and Missing Micro Nutrients by Larry Reichenberger of ProfitPro (who sells liquid fertilizer).
Dr. Huber has published directly in fertilizer promotion materials, such as the Fluid Journal (sponsored by the Fluid Fertilizer Foundation): What About Glyphosate-Induced Manganese Deficiency? Dr. Gordon’s Manganese Nutrition of Glyphosate-Resistant and Conventional Soybeans was published in Better Crops which is run by the International Plant Nutrition Institute which encourages use of a variety of fertilizers.

Response from extension

Understandably, farmers have been actively pursuing more information from extension agents as soon as they hear about a possible decrease in yields with glyphosate use. University extension has responded with multiple documents and presentations to help guide farmers using known research and by conducting additional research. Extension agents have a unique ability to bring research directly to farmers and other people near the university and can quickly conduct field tests to help farmers make science-based decisions.
In February of 2010, Iowa State University Extension produced a great overview of the research that includes analysis of some papers of which Dr. Huber was a co-author: Glyphosate-Manganese Interactions in Roundup Ready Soybean by Bob Hartzler, Extension Weed Specialist and Professor of Agronomy. He concludes that manganese uptake varies depending on which soybean variety is being used, not on whether or not the RR gene is present. He also concludes that while it is known that glyphosate will bind to soluble manganese, this is only a problem in manganese deficient soils.
In November of 2010, Bob Hartzler released Glyphosate Interactions with Micronutrients and Plant Disease, with the conclusion:

Due to the complexity of the processes that occur within the root zone, it is impossible to completely rule out negative effects of glyphosate on mineral nutrition or disease development in GR crops.  However, results from field research and our widespread experience with glyphosate on GR crops for over a decade do not indicate widespread negative impacts of glyphosate on these factors.

In April of 2010, University of Minnesota Extension put out a short commentary that also discussed Dr. Huber’s claims: Roundup and Manganese for Minnesota Soybeans. Extension agent George Rehm conducted experiments in Minnesota and found that additional manganese was not needed due to adequate manganese in Minnesota soils. The April commentary was actually a followup to a post about manganese from January of 2010, Magnesium In Minnesota, that attracted some critical commentary from none other than Bob Streit.

In January of 2011, Ohio State University Extension released a presentation (Flash needed) by Robert Mullen, extension specialist and associate professor, summarizing their work on this subject: Manganese / Glyphosate antagonism? Their research shows that applying manganese to soy does increase the concentration of manganese in plant tissues, but did not find that glyphosate caused decreases in yield or manganese. Adding manganese can cause yield increase or yield decrease depending on environment, specially soil type. They did find that soil type and pH causes significant differences in manganese uptake.

In February of 2011, Dr. Huber’s colleagues at Purdue University Extension put out a paper titled Glyphosate’s Impact on Field Crop Production and Disease Development that seems to be in direct response to the flurry of blog posts and “news” articles about Roundup that were spurred by Dr. Huber’s recent letter. While they don’t mention Dr. Huber directly, they do cite and express concern about articles that are credulous about Dr. Huber’s claims regarding glyphosate and plant and animal disease. They conclude:

Overall, the claims that glyphosate is having a widespread effect on plant health are largely unsubstantiated. To date, there is limited scientific research data that suggest that plant diseases have increased in GM crops due to the use of glyphosate. Most importantly, the impact of these interactions on yield has not been demonstrated. Therefore, we maintain our recommendations of judicious glyphosate use for weed control. We encourage crop producers, agribusiness personnel, and the general public to speak with University Extension personnel before making changes in crop production practices that are based on sensationalist claims instead of facts.

This isn’t the first time that Dr. Huber’s colleages have attempted to do damage control in response to “greatly exaggerated” reports by Dr. Huber about minerals and glyphosate. In April of 2010 Dr. Huber’s colleagues at Purdue University Extension released Glyphosate – Manganese Interactions and Impacts on Crop Production: The Controversy, referring interested persons to Iowa State University Extension. They state that high pH, high organic matter soils cause manganese to be less available to the crop whether or not glyphosate is present.
Update: Extension agents are still working to correct what they see as misinformation spread by Dr. Huber. Anne Dorrance, expert in soybean pathology and extension agent at Ohio State has a 14 March 2011 article in Ag Professional: Glyphosate Effects on Soybean Diseases. She directly assesses the claims that glyphosate use has increased incidence of disease, backed up with literature and her personal experience.
Have you seen any other extension or other articles by professional agronomists on this topic? Let us know and I’ll include them here.

Consider the data, not the source

I have read some claims that university researchers can not be trusted because many universities accept some grants from agricultural companies. Specifically, some bloggers have claimed that the Purdue extension agents’ scientific integrity is compromised, which is something that I think needs to be addressed, especially when it is clear that fertilizer companies and foundations are so eager to use Dr. Huber’s research. Potential conflicts of interest go every which way.
Purdue, like Iowa State and every other university, has strict standards of scientific and professional ethics. In addition, the amount of research funding granted by companies is small compared to funding from other sources. For example, at Iowa State, publicly available detailed reports of funding show that the research being conducted with corporate funding are far from the majority of funding and that most grants are extremely specific in scope. While there are isolated examples of inappropriate conduct of public universities regarding private companies or company interests, that is no reason to denounce every employee at every public university.
Instead of smearing the names of extension employees and researchers, we should examine the veracity of their work. We need to consider the data available. The identity of the source needs to be known in order to determine if a person has relevant expertise. We can look at the source to get a feeling for how much skepticism we need to apply. Go too far beyond that, and we get dangerously close to ad homs.

Anastasia Bodnar

Written by Anastasia Bodnar

Anastasia Bodnar serves as the Policy Director of Biology Fortified, Inc. She is a science communicator and multidisciplinary risk analyst with a career in federal service. She has a PhD in plant genetics and sustainable agriculture from Iowa State University.
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