Application of Glyphosate at Low Rates Contributes to HR Weeds
GR weeds result from over-reliance on glyphosate for weed control in GR crops such as soybeans. This is an accepted fact.
Dr. Jason Norsworthy, Associate Professor in the Dept. Of Crop, Soil, and Environmental Sciences at the University of Arkansas, recently published an article entitled “Repeated sublethal rates of glyphosate lead to decreased sensitivity in Palmer amaranth” in Crop Management. The results presented in this article provide additional information on the development of GR pigweed.
According to Dr. Norsworthy, there are populations of GR Palmer pigweed that have a high level of resistance to glyphosate applied at the 0.75 lb/acre recommended rate.
So, how did this happen? Possible scenarios are:
● Application of lower-than-recommended rates, which can lead to herbicide resistance in weeds;
● Delaying application of glyphosate in order to control more weeds with a single application, which results in glyphosate being applied to some weeds that are larger than the size recommended on the product label; and
● Improper or incomplete spray coverage [e.g., spray volume too low, spray not reaching targeted weed(s)], which can result in targeted plants receiving less than the recommended rate.
The objective of this experiment was to determine if GR Palmer pigweed could be selected by applying glyphosate to plants at rates lower than the recommended rate of 0.75 lb/acre.
Seeds from a population of Palmer pigweed known to be highly sensitive to glyphosate were collected.
After four cycles of spraying the pigweed with sublethal rates, plants grown from seed harvested from these plants required 47% and 114% more glyphosate to achieve 50% and 95% kill, respectively, compared to plants grown from seed of the original population.
Applying the recommended field rate of 0.75 lb/acre resulted in complete kill of plants from both populations. Thus, plants grown from both seed sets were susceptible to glyphosate applied at the recommended rate. However, this research demonstrates that sublethal rates of glyphosate will select for less sensitivity to glyphosate in succeeding generations of Palmer pigweed.
It is quite possible that this selection for less sensitivity to glyphosate from using sublethal rates will increase over a period longer than that used in this study. Thus, it is probable that this is a contributing factor to the widespread resistance of weeds to glyphosate in midsouthern soybean fields.
The take-home message from this research is this:
● Using reduced rates of glyphosate (and other herbicides as well) on Palmer pigweed (and probably other weeds) should not be practiced.
● All recommended/labeled practices for herbicide application should be followed to prevent the three scenarios listed above that will lead to the application of sublethal rates of herbicides to targeted weeds.
The Plant Management Network has posted two new “Focus on Soybean” webcasts entitled “Herbicide Resistance Management in Soybean” and “Root-knot Nematodes: A Threat to Southern Soybean Production”. These and other soybean information webcasts can be viewed at http://www.plantmanagementnetwork.org/fos.
Posted by Larry G. Heatherly, May 2012, larryheatherly@bellsouth.net