D'Avignon warns that not all weeds use this particular resistance mechanism. "We have since screened a number of weeds," he says. "Some use a similar mechanism, but we also find that others use other mechanisms."
According to a recent article in the Journal of Agricultural and Food Chemistry describing work by scientists at Colorado State University, the U.S. Department of Agriculture and the University of Adelaide in Australia, certain variants of Palmer amaranth, or pigweed, for example, have become resistant by overproducing the EPSPS enzyme to the point that it cannot all be bound by glyphosate.
Gaming the system
The scientists used a simple trick to make resistant horseweed sensitive again. "If a plant that had been sprayed with glyphosate at room temperature was put into a warm, high-light greenhouse, we noticed much more rapid vacuole sequestration than if the plant was treated at room temperature and maintained at room temperature," d'Avignon says.
"As chemists we knew that many reactions are temperature dependent. What would happen, we wondered, if we put the plant in a cold environment rather than a warm one? Could we inhibit the flow of glyphosate into the vacuole?"
To find out, they cold-acclimated resistant horseweed at 10 degrees Celsius (about 50 degrees Fahrenheit) and then sprayed the plants with glyphosate. Sure enough, the plants succumbed to the herbicide.
The scientists also found the records of a field trial in Monsanto's archives where a field of resistant horseweed was sprayed with glyphosate in early spring. Although the trial had an unrelated purpose, the data from the trial showed that kill rates correlated with temperature under field conditions, just as they had under laboratory conditions.
These experiments suggest farmers might be able to stave off resistant horseweed by spraying in early spring, when the weathe
|Contact: Diana Lutz|
Washington University in St. Louis