Expert response from Marian Bleeke, P.h.D.
Global Residue and Exposure Strategy Lead, Bayer Crop Science
Friday, 17/04/2015 14:30
There are two main mechanisms that have been used in the modification of crops to make them tolerant to herbicides. One is the insertion of a gene that produces a modified version of the target site of the herbicide, so that the plant can still function in the presence of the herbicide. A second mechanism for conferring tolerance in a crop is the insertion of a gene that produces an enzyme that metabolizes the herbicide, so that it is rapidly detoxified in the plant and becomes herbicidally inactive.
Glyphosate-tolerant corn and soybeans utilize the first mechanism. Glyphosate inhibits an enzyme called 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). The EPSPS enzyme is present in plants and microorganisms but not in animals and is part of the aromatic amino acid biosynthetic pathway. Glyphosate-tolerant corn and soybeans contain a version of EPSPS that is derived from a microorganism and can still function in the presence of glyphosate. There is no change in the way that glyphosate is metabolized in these crops, compared with conventional corn and soybeans.
In most plants, glyphosate undergoes minimal metabolism. There is some conversion to the breakdown product aminomethylphosphonic acid (AMPA), but most of the glyphosate remains unchanged. This is the situation with glyphosate-tolerant corn. Soybeans (both conventional and glyphosate tolerant) inherently have a greater ability to metabolize glyphosate to AMPA. Thus, pesticide residues in glyphosate-tolerant corn commodities are composed primarily of glyphosate, whereas residues in glyphosate-tolerant soybean commodities are a mixture of glyphosate and AMPA.
Glyphosate-containing herbicides are formulated to allow uptake of the glyphosate through the surface of the leaves, and thus into the targeted weeds. Once inside, glyphosate can be translocated to other parts of the plant, particularly to actively growing regions. This is what makes it an effective herbicide, as it can inhibit growth throughout the plant, including the roots.
The distribution and the magnitude of glyphosate and AMPA residues in glyphosate-tolerant corn and soybean plants depend on the growth stage of the corn and soybeans at the time of the glyphosate application to the weeds in the field. Most applications are made starting at the time of planting and throughout the vegetative growth stages, prior to the start of the development of the seeds or grain. Much of the glyphosate remains in the foliar part of the plants, particularly when applied early. Some is also translocated into the seeds or grain as they develop. Applications of glyphosate can also be made to glyphosate-tolerant corn and soybeans shortly before harvest to manage late-season weeds. Because the corn and soybean plants are no longer actively growing, the glyphosate remains primarily on the plant surfaces. At this stage, corn grain and soybean seeds are well covered by husks and pods, which largely protect them from the spray, although small amounts of glyphosate may reach the grain and seeds.
The levels of glyphosate and AMPA residues occurring in all food and feed commodities in glyphosate-tolerant corn and soybeans (corn forage, grain and stover; soybean forage, hay and seed) were determined in field trials using the maximum application rates allowed at the herbicide label and were reviewed by the Environmental Protection Agency (EPA) prior to approval of these uses. EPA determined that the potential dietary exposure from these uses, combined with all other uses of glyphosate in other crops, was well within the total intake level that it established as an allowable level for safe use.
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