I’ll start by addressing your second comment, on how Roundup Ready crops work. Glyphosate acts in plants by inhibiting an enzyme that is required for plants to synthesize certain amino acids. All Roundup Ready crops contain a gene that codes for a version of the enzyme that is not inhibited by glyphosate, allowing the plants to synthesize the amino acids even when they have been sprayed with glyphosate. (One crop, Roundup Ready canola, also metabolizes glyphosate to its primary crop metabolite, AMPA.) Thus, when the field is sprayed with glyphosate-containing herbicides at the approved application rates and timings, the crop is not affected by the glyphosate.
In order to answer your question about whether residues are higher in Roundup Ready crops, I will assume you are asking about glyphosate use on conventional vs. Roundup Ready crops. So let’s look at how glyphosate is used in conventional crops. Before the introduction of Roundup Ready crops, there were four primary ways in which glyphosate could be used in crop production:
1. At planting, before emergence of the crop.
2. Directed spray applications, between crop rows or around trees or vines, avoiding direct application to the crop.
3. After harvest, to clean the fields
4. Applied shortly before harvest, after the crop is mature and is not susceptible to damage by glyphosate
The first three uses result in no or minimal residues, as the crop either is not present or is avoided during spraying. Preharvest applications can result in higher residues.
Roundup Ready crops add a fifth way for glyphosate use in crop production—that is, while the crop is growing. The amount remaining in the harvested food or animal feed commodities at harvest depends on the crop and on when the application was made.
All uses of glyphosate, whether in conventional or Roundup Ready crops, need to be reviewed and approved (in the U.S.) by the EPA. The EPA sets a tolerance, or maximum residue level, for each of the raw agricultural commodities (grain, forage, straw, etc.) from the treated crop. The tolerance is based on the maximum expected residues in the unprocessed raw commodity when the crop is treated at the approved labeled rate. The tolerance will be approved only if the resulting dietary exposure from foods made from these commodities falls within the safe level already set for glyphosate by the EPA.
In some instances, tolerances for crop commodities were already established for conventional uses that were sufficient to cover the residues in the commodities from Roundup Ready crops, so new tolerances did not need to be set. This was the case with Roundup Ready soybeans. There was an existing EPA tolerance for soybeans based on preharvest uses, and the tolerance did not have to be increased for Roundup Ready soybeans. In other instances, such as Roundup Ready sugarbeets, tolerances needed to be increased. But, regardless of the use, the tolerance needs to meet the same standard, which is the total dietary exposure of glyphosate from all food, and water sources must be within the acceptable dose level already established by EPA. Currently, using highly conservative assumptions (that 100 percent of all approved crops are treated with glyphosate, at the maximum rates, resulting in residues at the maximum tolerance level, and that the residues in the crop commodity all end up in the consumed food), and looking at separate subpopulations (infants, children, men, women, etc.), the EPA has concluded that dietary exposure is at most only 13 percent of the acceptable dose level.
Now, to get back to your original question: Do Roundup Ready foods usually have higher glyphosate residues? This is dependent on the crop and application timing. It is critical to highlight that the actual levels of glyphosate in food we consume, and resulting exposure, are much lower than the conservative levels estimated by the EPA. First, the residues in the starting raw commodities are lower than the levels assumed in the EPA estimation, because not all crops are treated with glyphosate and the average residues in treated crops are lower than the tolerance. But more importantly, much of the glyphosate is removed during processing. Most of the foods derived from Roundup Ready crops are processed prior to consumption, which removes much of the glyphosate initially present in the raw commodity. For example, glyphosate is very water soluble and is removed during processes such as crystallization of sugar from sugar beets, isolation of protein from soybeans or production of cornstarch from corn grain. It does not mix with oil, so there are negligible glyphosate residues in vegetable oil from treated crops such as soybeans, canola and corn. Thus, while the residues of glyphosate in a crop commodity may be higher in a Roundup Ready crop, much of the residues are removed during processing.
In summary, glyphosate residues can occur from uses in both conventional and Roundup Ready crops. All uses are evaluated in the same way by regulatory agencies such as the EPA. Even conservative estimates of residues in raw commodities indicate that the exposure to glyphosate in foods is well below the level established by the EPA, and that actual residues present in the food we eat are much lower, due to reductions of glyphosate during processing of raw commodities (whether from conventional or Roundup Ready crops) into food.
US EPA Federal Register / Vol. 78, No. 84 / Wednesday, May 1, 2013 / Rules and Regulations 25396-25401, http://www.gpo.gov/fdsys/pkg/FR-2013-05-01/pdf/2013-10316.pdf.