Does GMO corn reduce the amount of herbicides used by farmers?

Expert response from Shawn Askew

Extension Specialist and Associate Professor, Department of Plant Pathology, Physiology, and Weed Science, Virginia Polytechnic Institute and State University

Tuesday, 23/09/2014 23:57

This question is too general for a simple answer, so a brief discussion is in order. Before the advent of GM corn, farmers were reliant on a host of different herbicide-active ingredients to manage various weed populations in corn. Base programs would have typically included triazine and chloroacetamide herbicides, each applied at one or more pounds active ingredient per acre (lb ai/A), followed by an early postemergence treatment for grasses, broadleaf weeds or sedges, as appropriate.

 

Based on this scenario, use of glyphosate in GM corn would have resulted in half as much herbicide applied. In areas where a triazine and/or chloroacetamide preemergence herbicide were not used, and these were rare, there are possibilities where only ALS-inhibiting herbicides were used postemergence that would have resulted in a total active ingredient load of less than 0.1 lb. ai/A and considerably less than glyphosate.

 

Likewise, for the many corn growers today that must use triazine and chloroacetamide herbicides preemergence, followed by glyphosate postemergence, the total active ingredient load has either remained steady compared with non-GM corn or increased. It all depends on which herbicides were used before and after the change to GM corn. The overall effect of glyphosate on herbicide use in corn would be anyone's guess. Whether the net change in herbicide use after GM corn introduction is greater or less, my guess would be that the difference is very small.

 

The most significant change brought about by GM corn is a reduction in the amount of tillage needed for weed control. Glyphosate enabled growers to reduce their reliance on tillage while maintaining levels of weed control as good as or better than those of conventionally tilled corn. The control of weeds in reduced-tillage corn was usually too expensive and required too many herbicides before the advent of glyphosate-resistant corn.

 

Another missing component in most discussions related to herbicide “amount” is the relative biological activity of one herbicide compared with another. Some herbicides are applied at very low rates but have extremely high biological activity. There is currently no way to quantify biological activity in terms that would be meaningful for this discussion, but suffice it to say that simply knowing the total active-ingredient load per acre is not necessarily a meaningful assessment of an herbicide’s impact on the environment.