That is a great question, and I wish I could give you a short answer, but a lot of work and scientific evaluation have gone into understanding what happens to glyphosate after it is applied to Roundup Ready crops like corn and to ensuring your family and mine have safe food to eat.
Most glyphosate applications in Roundup Ready corn occur before the corn kernels start to develop; therefore, very little glyphosate is present in the kernels, so residues in the kernels are low. Typical Roundup Ready corn grain residues are well below one part per million (ppm). Levels this low do not pose any health concerns. Studies of Roundup Ready corn have shown that the amount of glyphosate in the corn plant following a foliar application drops fairly quickly, due to wash-off of surface residues, dilution as the plant grows and distribution throughout the plant and roots.
Before glyphosate could be applied to Roundup Ready corn, that “over the top” use had to be approved by the US Environmental Protection Agency (EPA). The EPA must evaluate herbicides thoroughly before they can be marketed and used in the United States, to ensure that they meet the federal safety standards for protecting human health and the environment.
The process of registering an herbicide like glyphosate and Roundup-branded products is a scientific, legal and administrative procedure in which the EPA examines all the ingredients in the product; the crop on which it is to be used; and the amount, frequency and time of use.
Before allowing the use of an herbicide on food crops, the EPA sets a tolerance, or maximum residue limit. This tolerance is the amount of herbicide residue that is legally allowed to remain in or on each treated food commodity. In establishing the tolerance, the EPA must make a determination that the herbicide can be used with “reasonable certainty of no harm.” In making this determination, the EPA considers the toxicity of the herbicide and its breakdown products, how much of the herbicide is applied and how often and how much of the herbicide (i.e., the residue) remains in or on food.
The EPA requires companies seeking registrations on herbicides and other types of pesticides to conduct many different kinds of studies. The EPA uses the results of those studies in its evaluations to ensure that the product meets the federal safety standards.
The types of studies conducted to determine the maximum residue levels likely to result in or on food crops from registered uses are called crop field trial residue studies. These studies are conducted in multiple locations (for corn, 20 locations are required) that are representative of growing conditions in areas where the crop is grown and reflect maximum use rates, maximum number of applications and minimum duration after application that the crop may be harvested, as defined by the pesticide registration and label.
Residue samples are obtained immediately upon harvest of the food commodity. Residues expected on food, as consumed generally, are lower than the values measured in crop field trial residue studies because of variations in use practices (either not using the pesticide at all or using it in a manner not likely to produce maximum residues), degradation of residues between time of harvest and consumption and cooking and processing practices that break down residues.
As I mentioned earlier, in establishing the tolerance, the EPA must make a determination that the herbicide can be used with “reasonable certainty of no harm.” In order to make this safety determination for glyphosate-based products, the EPA considers how much glyphosate residue the use would contribute to the daily intake and then adds that amount to the amount of glyphosate residue consumed by all other possible routes of exposure, including on other foods, in drinking water, etc. This total consumption of glyphosate residues is then compared with the total acceptable daily intake (ADI) that has been established for glyphosate, based on toxicity studies that look at a variety of toxic effects, such as immediate or acute toxicity, effects on reproductive processes, cancer-causing and other long-term effects, etc. Just to be on the safe side, the EPA sets the ADI a minimum of 100 times lower than any dose level that showed any kind of toxicity in any study conducted. No more uses of product can be added once the ADI has been reached. The ADI of glyphosate, and many other herbicide-active ingredients, has been established by the US EPA and independently by regulatory authorities in different parts of the world, including the World Health Organization. Whether taken in as food or drink, these ADI levels are conservatively calculated based on animal models, crop residues and typical diets to account for daily exposures throughout our lives. Daily consumption of residues below the ADI is considered safe.
A risk assessment was conducted in May 2013 by EPA for glyphosate exposures through both food (agricultural products) and water, and it concluded that glyphosate exposure is no more than 13 percent of the ADI. Therefore, even when one takes the conservative approach that all fruits, vegetables and grains in the diet treated with glyphosate would have the maximum allowable residue levels remaining on those food crops when eaten, the use of glyphosate is well within what is considered to be safe.