What is in this article?:
- How to protect new technologies from resistant weeds
- Out of options?
Weed scientists at the University of Arkansas System Division of Agriculture have created quite a stir with news that through experimentation in the greenhouse, researchers selected for a population of pigweed — Palmer amaranth — that is tolerant to the herbicide dicamba at a field rate. This pigweed population did not evolve resistance in the field, but there is much we can learn from the artificial selection that took place in the greenhouse.
The researchers exposed three generations of pigweed to sub-lethal doses of dicamba, a recipe for resistance development. We started with pigweed collected from the field that was susceptible to dicamba. By the third generation, we were able to select for seedlings capable of surviving an application at one-time rate that should’ve provided effective control.
This type of selection experiment with pigweed was done previously with glyphosate in Arkansas and with other weeds and other herbicides in Australia.
Dicamba is, of course, the active ingredient for use on Xtend crops, one of the emerging technologies for weed control in soybeans and cotton. This research was conducted only to examine the possibility of the development of resistance in the future — rest assured, these greenhouse-developed pigweed will remain tightly controlled. We believe this research applies not only to dicamba but also to all herbicide technologies and, therefore, is of great concern.
This research doesn’t mean that pigweed will have evolved dicamba resistance within three years of using this technology, but it shows the need to continue using residual herbicides, multiple effective modes of action, prevention of pigweed escapes and other tactics.
The results of these experiments would also apply to other emerging technologies such as Enlist soybeans, cotton and corn. With both dicamba and 2,4-D effective at control when applied to small pigweed, we must make every effort to protect these technologies.
This past year we have seen the emergence of numerous populations of Palmer amaranth (pigweed) that are resistant to the PPO (Group 14) class of chemistry. This group of herbicides includes important soybean products such as Valor and Flexstar, and has been increasingly relied upon since the development of glyphosate-resistant pigweed in the late 2000s. Some of these populations have now been found to be resistant to as many as three formerly effective modes of action. This leaves few options for growers going into the 2016 season.
With this in mind, the thought of protecting these new technologies is of the utmost interest to those of us who make herbicide recommendations for a living. The problem is we are starting to run out of options. For example, in these three-way resistant fields, Liberty is our last effective post-emergence applied option. Even though there are a couple of residual options still available, if the field is to stay in soybeans, producers will need to or have to grow LibertyLink. But as we’ve seen in the greenhouse with dicamba, focusing solely on Liberty is not a long-term solution to a recurring problem.