This article appeared in the October 2001 issue of Vegetable Production & Marketing News,
edited by Frank J. Dainello, Ph.D., and produced by Extension Horticulture,
Texas Cooperative Extension, The Texas A&M University System, College Station, Texas.

Make The Most Of Your Herbicide Applications

This article by Will Stockwin appeared in "AVG," March, 1996.

Typically, growers donít realize theyíre seeing herbicide-resistant weeds in a field until theyíre a significant problem.

"The rule of thumb is that resistance isnít really noticed until 30 percent of the plants (weeds) are surviving herbicide applications," says University of California-Davis weed ecologist Joe Di Tomaso. "Below that threshold, growers will blame persistent weeds on some factor that might have caused poor herbicide application, or just about anything else they can think of other than resistance."

The two primary conditions contributing to resistance development are long residuals in the soil, and repeated use over large areas. Di Tomaso encourages growers to begin thinking more about, and looking harder for, herbicide resistance because, he says, the number of weeds developing the characteristic world-wide is increasing at an exponential rate.

"The first instance of verified herbicide resistance happened in 1968 with atrazine," he says. "Then there was a 19-year gap until the next case. Today there are between 130 and 140 species of weeds resistant to herbicides, and about half of those are triazine resistance." In 16 of 18 western states right now there are 22 known weed species that show herbicide resistance, including kochia, Russian thistle, foxtail, redroot pigweed, and common chickweed.

Of particular concern for California growers is the increasing amount of resistance developing to the popular herbicide family known as ALS (acetolactate synthase) inhibitors. The ALS family includes Pursuit, Upbeet, and Matrix among others. Current research indicates that the usual 25-year period of herbicide-resistance development can be cut to five years with continuous use of ALS inhibitors. In one case, four different weeds developed resistance to the ALS inhibitor Londax in just three years.

ALS inhibitors are susceptible to resistance because they have only a single site of action in the plant. Essentially, the herbicide blocks the synthesis of important amino acids by binding to one specific site on a protein molecule. If there is a mutation in that protein at a very low frequency within the weed population, and the mutation occurs at the specific site the herbicide binds to, the weed develops absolute resistance.

That doesnít mean that single-site-of-action herbicides cause resistance. "They only select for resistant strains," emphasizes Di Tomaso. "The mutations in the binding sites may have been there for thousands of years. Take what happened with Londax (a sulfonyurea compound used in rice) for example. When it came out, it was so good that in a couple of years 98 percent of rice growers were using it. That kind of intense selective pressure is why resistance showed up in three years. Titus (an ALS inhibitor currently permitted in California under a conditional-use permit), which is being highly touted as the herbicide of the Ď90s for tomato growers, could go the same way if growers use it continuously."

Di Tomaso says the problem isnít just that growers tend to use the same herbicide over and over, but that they tend to use the same family of herbicides continuously.

"Right now, there are approximately 15 compounds in the ALS inhibitor group, registered for different crops, that are all slightly different but work exactly the same way," he says. "So even though a grower may be conscientiously rotating his crops, he may not be rotating his herbicides. The names may change, but heís still using the same chemistry with the same site of action in the weed, year after, crop after crop."

Detoxification is the second most common way weeds develop resistance.

"The weeds develop mechanisms by which they break down a herbicide before it can accumulate at the site of action," explains Di Tomaso. "Some crop plants -- corn with atrazine for example -- have the same ability. This results in an increased level of tolerance, meaning the grower has to go with a higher application rate to attain the kill effect he needs."

"New herbicides are becoming available less and less frequently, and so when we do get one, we have to start using a resistance strategy right away," Di Tomaso says. "That means going with a multiple-herbicide approach, in combination with some mechanical control methods. Itís really a lesson everyone in agriculture should have learned by now. Otherwise, three to five years down the road, resistance has developed, and thereís little you can do about it."

Reduce Herbicide Resistance

Strategies for reducing the selective pressures that ultimately cause herbicide resistance include:

  • Rotate crops and herbicides, with the herbicide rotating to a chemical that affects a different action site in the weed.

  • Avoid or at least minimize use of long-residual herbicides with single-action sites.

  • Use the lowest application rates whenever possible.

  • Use other forms of weed control, such as cultivation, whenever feasible.

  • Minimize the number of applications made each year.

  • Eradicate any areas of persistent weed survivors with a hoe before they can produce seed, removing the plant material as completely as possible from the field.