Recently, it has been confirmed that some populations of Palmer amaranth have developed resistance to ALS-inhibiting herbicides in eastern Georgia. Herbicides with this mode-of-action include Accent, Ally, Amplify, Beacon, Cadre, Classic, Exceed, Express, FirstRate, Harmony Extra, Peak, Permit, Pursuit, Python, Sempra, Scepter, Staple and Strongarm.

Herbicide resistance has been confirmed previously with goosegrass in 1992 (Treflan) and Italian ryegrass in 1995 (Hoelon).

The development of herbicide resistance in Georgia has been relatively slow in comparison to other states. However, herbicide resistance management should be considered before implementing any weed control strategy. Consequently, I would like to answer some of the more common questions about herbicide-resistant weeds and their management.

What is herbicide resistance?

Herbicide resistance is defined as the inherited ability of a weed or crop biotype to survive a herbicide application to which the original population was susceptible. There are two types of herbicide resistance - cross resistance and multiple resistance.

Cross resistance occurs when a weed biotype that has developed resistance to one herbicide also develops resistance to other herbicides with the same mode-of-action. Multiple resistance occurs when a weed biotype that has developed resistance to one herbicide also develops resistance to two or more chemically unrelated herbicides with different modes-of-action. What is a biotype?

A biotype is a group of plants within a species that has biological traits that are not common to the population as a whole.

How does herbicide resistance occur?

Some people believe that the overuse of herbicides results in the direct development of “super” or “franken” weeds. The truth of the matter is that these weeds already are present in a field at some level. Within any population of weeds, both herbicide-resistant and susceptible biotypes can be found. When the susceptible biotypes are removed from the population, the resistant biotypes flourish and become dominant. Although the genetic differences between the two biotypes are not well understood, herbicides are not known to cause these differences. When a herbicide does not perform satisfactorily, does this mean my weeds are resistant?

NO! NO! NO! There are many reasons why herbicides fail to work. Many factors including application rate, method, timing, weed stage of growth, moisture conditions, temperature, humidity and calibration have an influence on the performance of herbicides. All potential reasons for poor performance should be investigated before considering the possibility of resistance.

When should the possibility of herbicide resistance be considered?

Herbicide resistance should be suspected only when the following conditions exist: 1) all other causes of failure have been ruled out; 2) the same herbicides or herbicides with the same mode-of-action have been used year after year; 3) one weed that is normally controlled is not controlled while other weeds are; 4) healthy weeds are mixed with controlled weeds (same species); and 5) a patch of uncontrolled weeds is spreading.

What can growers do to prevent or delay the development of herbicide resistance?

Several strategies for the control or prevention of herbicide-resistant weeds can be implemented, including the following: 1) utilization of non-chemical control tactics such as cultivation and narrower row spacing; 2) rotating herbicides with different modes of action; 3) applying herbicides in tank-mixes or sequential applications using products with different modes of action; 4) rotating crops; 5) scouting fields on a regular basis to determine changes in population; 6) preventing suspected herbicide-resistant weeds from producing seed; and 7) cleaning tillage and harvesting equipment before moving from fields infested with resistant weeds to those that are not.

The development of resistance to chemicals by any organism is not an unnatural phenomenon. It is a mechanism of survival. Thus, weeds that develop resistance to herbicides should not be a surprise. We are fortunate in Georgia that this problem has not occurred at a faster pace or at higher level as it has in other states.

The recent confirmation of ALS-resistant Palmer amaranth should serve as a reminder to growers that herbicide resistance management should be an integral part of every weed control program. Additional information about herbicide resistance can be found on the University of Georgia Weed Science Web-Page (gaweed.com).