Although herbicide resistance has become a very serious and growing problem in many of the southern and central states, Arizona is one of only 4 states in which no cases of herbicide resistance have been documented. In Arizona, this is a problem most commonly associated with insects rather than weeds. It is often misunderstood.

Weeds inherit characteristics that determine how they respond to herbicides. When we conduct herbicide trials we often list response for individual weed species as control, partial control or no control or as a percent control. When there is no control we say that a species is tolerant to that particular herbicide in the area and under the conditions that our tests were conducted. On the other hand, when weeds are controlled, we say they are susceptible to the herbicide. It is when weeds change from susceptible to tolerant that we say they have developed resistance to that particular herbicide.

The use of selective herbicides began on a large scale in the US in the late 1950’s and early 1960’s. The first case of weed resistance was documented in 1968 in Washington State and involved the resistance of common groundsel to Atrazine and Simazine. Over the next 25 years, 110 weed species were reported to be resistant. The International Survey of Herbicide Resistant Weeds reported 185 resistant species in 2008 and 200 resistant species in 2011.

The states bordering Arizona have all documented at least one weed species with herbicide resistance. 24 resistant biotypes were documented in California, 4 in Colorado, 2 in New Mexico and 1 in Utah. The closest incidence to the Arizona low deserts has been the resistance of littleseed canarygrass (phalaris minor) to the ACCase inhibitors, Sethoxydim, Clethodim and Fluazifop in the Imperial Valley. This was documented in 2001.

Weed populations may look uniform, but in fact they are very diverse on a genetic level. Resistance to a particular herbicide occurs when there are naturally occurring variant individuals in a weed species population that are tolerant to the herbicides effects. Two principals that are important to understand are 1) the resistant variants are not caused by the herbicide but occur naturally in the weed population and 2) individual weeds do not change to become resistant, rather the weed population changes to become immune over time. What happens is that if the same herbicide is used continuously for a prolonged period of time, the susceptible biotypes will die out while the resistant biotypes will survive and reproduce, passing on their resistance to the next generation.

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.