Herbicide-Resistant Weeds in Mississippi
Weed resistance is defined by the Weed Science Society of America (WSSA) as the inherited ability of a plant to survive and reproduce after exposure to a dose of herbicide normally lethal to the wild type. Repeated application(s) of the same herbicide or a different herbicide with similar mode of action on the same field has contributed to the widespread occurrence of resistance to herbicides in several weed species around the world, in the U.S., and in Mississippi. Weed management programs must not be solely dependent on herbicides in order to be economically sustainable. In general, a combination of the following strategies is recommended:
1. Use residual herbicides;
2. Practice crop rotation;
3. Rotate herbicides with different modes of action;
4. Tank-mix herbicides with different modes of action at full recommended rates;
5. Avoid sequential applications of the same herbicide continually;
6. Utilize tillage, cultivation, and other cultural practices wherever and whenever feasible;
7. Clean equipment thoroughly before and after each use; and
8. Control weeds postharvest to reduce soil seedbank.
A comprehensive list of herbicide-resistant weeds found in Mississippi is maintained annually in Weed Control Guidelines for Mississippi (Publication 1532). Although weeds with resistance to herbicide modes of action such as ALS-inhibitors and photosystem II-inhibitors are somewhat common in Mississippi, glyphosate resistance poses the most severe threat to Mississippi agriculture. To date, seven weed species (horseweed, Italian ryegrass, Palmer amaranth, johnsongrass, waterhemp, giant ragweed, and goosegrass) have evolved resistance to glyphosate in Mississippi. The most prevalent and economically damaging of these are glyphosate-resistant Palmer amaranth and Italian ryegrass.
Glyphosate-resistant weeds require producers to change their farming practices and/or herbicide programs. Research has shown that glyphosate-resistant Palmer amaranth and Italian ryegrass can be controlled with residual herbicides and timely applications of alternative chemistries. For detailed information on managing these species, see Herbicide Programs for Managing Glyphosate/ALS-Resistant Palmer amaranth in Mississippi Soybean (Information Sheet 1352), Cotton (Information Sheet 1354), or Corn (Information Sheet 1353), and Herbicide Programs for Managing Glyphosate-Resistant Italian Ryegrass in Mississippi (Information Sheet 1359).
A major concern of many producers is the location of glyphosate-resistant weeds in relation to their farms. In an effort to aid Mississippi producers in their decision making process, statewide surveys were conducted to determine where glyphosate-resistant Palmer amaranth and Italian ryegrass were present in the state. The following maps were generated, based on scientific experiments, to inform producers of where these problems have been documented in Mississippi. Since the initial surveys in 2009 for glyphosate-resistant Italian ryegrass and in 2010 for glyphosate-resistant Palmer amaranth, the maps are updated annually as more counties become infested with these weeds.
ALS-Resistant Palmer Amaranth
Special thanks to the Mississippi Soybean Promotion Board and the Mississippi Corn Promotion Board for providing funding for this project. Special thanks to Robin Bond and Tom Allen in assisting in the collection of samples. For further information or to report herbicide failures contact Tom Eubank or Jason Bond.