What is in this article?:
- University of Arizona greenhouse assays confirm Palmer amaranth resistance to glyphosate (Roundup) herbicide in cotton in a Buckeye, Ariz., field.
- For the 2013 cotton season, UA weed scientist Bill McCloskey urges Arizona growers to follow new production practices to reduce the spread of resistance.
Bill McCloskey, University of Arizona Extension weed scientist, confirms Palmer amaranth resistance to glyphosate (Roundup) herbicide in cotton in a Buckeye, Ariz., field. 2008 file photo.
Adopt a “zero-tolerance” attitude toward any pigweed which escapes treatments in the field or growing on the field margins and nearby drainage areas. This will reduce gene spread into other fields.
“Scout the fields after each herbicide application. If a pigweed escape is found, cut it down,” McCloskey said.
Current crop rotation practiced in the West due to year-round growing conditions helps reduce resistance risks. Crop rotation provides improved herbicide effectiveness through the use of multiple technologies. One of the best rotation options is alfalfa. “Conventional alfalfa is a good rotation choice with cotton in a herbicide-resistant environment,” McCloskey said. “Alfalfa is cut and mowed which reduces the pigweed population.”
Water-run herbicides can help suppress pigweed emergence in alfalfa. Herbicides for winter annual weed control can provide pigweed control in the spring.
The cotton picker is a vehicle to spread resistance by seed. Clean the picker before entering a field. If using a custom harvest service, ask the operator where the picker came from and if it was cleaned afterwards. If not, clean it before entering a new field.
Pigweed seeds are extremely small.
Growers should use existing tools in the herbicide toolbox to control weeds. McCloskey says there is no “magical herbicide with a new mode of action in the pipeline.”
“If a new mode of action was discovered today it would take more than 10 years to bring it to market. The herbicide tools we have now are all we have. We have to make existing herbicides work for a long time.”
One herbicide in Arizona growers’ toolbox is Staple LX. The herbicide was used in cotton fields in the Southeast once glyphosate resistance to pigweed developed. Three years later, pigweed developed a resistance to Staple LX.
McCloskey is unsure if Staple LZ-pigweed resistance could occur in Arizona.
In McCloskey’s greenhouse resistance trials, Staple LX was tested for resistance in the Buckeye and Glendale samples. None was found.
McCloskey shared several points on when to suspect herbicide resistance:
1 – When other causes of herbicide failure are ruled out.
2 – The same herbicide(s) with the same mode of action(s) are used year after year.
3 – One weed species normally controlled is not controlled while other weed species are controlled.
4 – Healthy weeds in the field are mixed with dead weeds of the same species.
5 – A single-species weed patch of uncontrolled plants is spreading.