When recently asked what weed I thought was the most troublesome in peanuts this year, it didn't take me long to reply - pigweed. Consequently, I've begun to refer to the 2000 crop year as "The Year of the Pigweed." Since pigweeds usually are some of the easiest weed species to control, I've received many questions about the lack of control this year.
As we all know, Georgia was not blessed with abundant rainfall this year. Soil-applied herbicides such as Prowl, Sonalan, Dual or Frontier require rainfall or moisture to move the herbicide into the upper two inches of soil, where most weeds germinate.
These herbicides often are mechanically incorporated to improve consistency and reduce the dependence on moisture for activation. However, even mechanically incorporated herbicides need some moisture to be effective. If you look back at the rainfall records for May, when a majority of the peanuts in Georgia were planted, some locations were deficient of rain by as much as four inches. So, the bottom line is that soil-applied herbicides don't work well in severe drought conditions.
In addition to the influence of dry weather on soil-applied herbicides, drought also can have a profound effect on the performance of postemergence herbicides. Several unique things occur in plants that grow in drought conditions.
Plants develop thicker cuticles (the waxy layer on a leaf) and experience reductions in various physiological processes such as translocation and herbicide absorption/retention. These changes within plants help them to survive the effects of the drought and, unfortunately, make them more difficult to control with postemergence herbicides.
Another reason for the poor control of pigweed this year has to do with the timeliness of postemergence herbicide applications. I wish I had a dollar for every telephone call I received asking for recommendations to control pigweed that was 12 inches tall in height or taller.
With the exception of Pursuit or Cadre, which many peanut producers are unwilling to use because of the 18-month cotton rotation restriction, large pigweeds (larger than six inches) cannot be adequately controlled in peanuts. There is a finite time when postemergence herbicides can be applied and optimum control can be expected.
To illustrate this point, research has shown that Blazer applied to Palmer amaranth at three to five inches tall, seven to nine inches tall and 11 to 13 inches tall provides 99 percent, 57 percent and 48 percent control, respectively. Timeliness of application is the most critical factor for the success of postemergence herbicides.
A fourth reason for the lack of pigweed control this year may be related to the species of pigweed that is most readily found. There are many species of pigweed that grow in Georgia and throughout the Southeast, including redroot (Amaranthus retroflexus), slender (Amaranthus gracilis), smooth (Amaranthus hybridus), spiny (Amaranthus spinosus) and Palmer (Amaranthus palmeri).
By far, the most common pigweed that I've observed in the field is Palmer, a.k.a. Palmer amaranth.
At least two research trials have shown that Palmer amaranth is much more difficult to control than the other species of pig-weed. Although this differential species response often is not addressed on the labels of most herbicides, I suspect that some changes will have to be made in the future. By the way, you can brush up on your pigweed identification by checking out the following Website from Iowa State University:http://www.weeds.iastate.edu/weed-id/waterhemp/default.htm.
I also would like to address the issue of herbicide resistance. When herbicide performance is poor, and the other factors that influence performance have been ruled out as potential causes, herbicide resistance might be suspected.
Palmer amaranth resistance both to the DNA (Prowl, Sonalan, Treflan) and ALS (Pursuit, Cadre, Classic) herbicides has been documented in South Carolina. It has not yet been confirmed in Georgia. Is it possible?
Given the long-term use of the DNA herbicides and the quick adoption of the ALS herbicides in Georgia it certainly is possible. However, until it can be confirmed positively through extensive testing, I'm reluctant and unwilling to cry wolf at this time.
For whatever reason pigweed was not controlled this year, you can only plan and hope for a better next year. Mother Nature has control over the rain, but you can control the other factors that contribute to the success of a weed management program, including proper herbicide incorporation techniques, rate selection, timeliness of application and the use of herbicides with different modes of action, to name a few. As always, good weed hunting!