roadfork's picture
Do GMO crops that have been engineered to resist herbicides, result in foods that contain either the herbicides themselves, or byproducts of the plants' metabolism of herbicides? If yes, can these herbicides interact in negative ways with human (or other animal) gut microorganisms?

A:Expert Answer

Food safety is an important part of the assessments that researchers conduct on herbicides. For your first question regarding herbicide residues, you might find it useful to look at a previous response on GMO Answers that addresses that question: Also, actual data on pesticide residues in the U.S. food supply are regularly collected by USDA Agricultural Marketing Service (AMS). The 2012 Annual Summary once again confirms that pesticide residues on foods are at levels below the tolerances established by the EPA and do not pose a safety concern. For more information, please visit the USDA AMS website.


Regarding your second question, the role of gut microbes in maintaining health is currently a hot topic in science and in the media. There is a lot of work being done in this area, and there is good evidence for a role of gut microbiota in some clinical diseases.


Recently, the German BfR, the scientific agency of the Federal Republic of Germany that is responsible for preparing expert reports and opinions on food, reported that it finalized a reevaluation of glyphosate, which stated, “A research project initiated by BfR and performed by the University of Veterinary Medicine in Hanover investigated the influence of a glyphosate containing herbicide on microbial metabolism and communities in ruminants. The results of this study are summarized in the draft suggesting that there is no negative impact on the microflora in the rumen. In particular, there was no indication that Clostridium bacteria might multiply under the influence of glyphosate.”


One critical factor in this study is that it uses a well-recognized technique that uses a mixed population of organisms under conditions designed to more closely simulate the complex matrix of feed and microbes than pure cultures grown in purified media. Previous studies have used pure cultures with purified media that don’t account for synergy in metabolism for mixed populations and may make them more susceptible to pH changes.

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Rickinreallife's picture

There have been repeated assertions that glyphosate resistant crops are "drenched", "doused", "sprayed with massive amounts" etc of glysophate. You can put the amount actually applied in some perspective. Using a Perdue University extension publication "Understanding Glysophate to Increase Performance" [], the highest rate of application recommended for three examples on page 5 of the document was for late season control of Ivyleaf Morningglory, at 60 oz./acre of the Roundup Weathermax formulation. The actual active ingredient in Roundup (i.e. glysophate) is for ease of calculation purposes about 1/2 the product by weight (it is usually a smaller percentage). There are 4840 square yards in an acre. At application rate of 60 oz / acre, that is 12/1000th of an ounce per square yard. Since glysophate is only 1/2 of the product, the amount of glysophate applied per sq. yd. would be 6/1000th of an ounce. You would be applying about an ounce of glysophate on an area of about 167 square.

Another way to look at it, this link is to an article that discusses typical corn seeding and harvest population rates for both dryland and irrigated corn in Nebraska [Row Spacing and Seeding Rate Recommendations for Corn in Nebraska --]. Note the average plant population at harvest is 29,000 plants / acre for irrigated, and 21,850 plants / acre for dryland corn. Take the sixty ounces of Roundup in the highest application rate from the Perdue study discussed above, divide that in half to reach 30 ounzes / acre active ingredient. Now, divide 30 oz by 29,000 plants to arrive at .001 oz / plant (or 1 oz per thousand plants) in irrigated corn assuming 100% of the application falls only on corn or in some manner makes its way into the corn crop even if not applied directly to the corn. Now suppose as much as 1/2 of the sprayed product falls on the growing corn or migrates to the corn (a high assumption), that is .0005 oz glysophate per plant. The numbers for dryland are .0013 ozs per plant (assuming 100% applied product is absorbed by corn) or .000565 ozs per plant (assuming 50 %).