Herbicide resistance was a fact of life long before the latest round of findings that certain weeds are resistant to glyphosate, one of the most widely used and safest herbicides ever introduced into world agriculture.

There are 315 weed biotypes validated as resistant worldwide to one or more of the 19 major classes of herbicides, according to the international Herbicide Resistance Action Committee (HRAC). Thirteen biotypes have been identified as resistant to glyphosate, which is ranked No. 6 in the ranking of herbicides which no longer control certain weeds due to resistance.

The fact is, 95 weeds are resistant to ALS inhibitors.

Nevertheless, the growing trend of glyphosate-resistant weeds is of concern to California Pest Control Advisors (PCAs) like Dale DeShane, a PCA with Supervised Control Service Inc. in Bakersfield, Calif.

Two of the more nettlesome glyphosate-resistant weeds are marestail and hairy fleabane. The entire weed spectrum has shifted, due largely to the onset of resistance in many areas, added DeShane.

Glyphosate-resistant marestail, also known as horseweed, was first identified in California in 2005 near Parlier. An irrigation district had been using glyphosate to control weeds in an area where other herbicides had been restricted due to groundwater contamination concerns. Regulations imposed by the California Department of Pesticide Regulation (DPR) in 2004 restricted the use of simazine, diuron, norflurazon and other compounds on approximately 2.4 million acres. As a result, the irrigation district had turned to glyphosate for control, and eventually marestail developed resistance as a result of repeated use of the compound. Since that time, glyphosate-resistant marestail has spread throughout California.

Marestail and hairy fleabane produce large numbers of seeds that are easily dispersed by wind and can travel long distances just on a breeze. A few escaped weeds can result in a much wider infestation of new weeds in a very short period of time.

As a result, marestail and hairy fleabane have become major weed pests for tree and vine growers, particularly in the southern San Joaquin Valley. It is spreading northward, largely from roadsides, irrigation ditches and other areas adjacent to orchards and fields treated repeatedly with glyphosate. These seed banks in close proximity to commercial agriculture have contributed to the rapid spread of both weed species. Researchers also attribute the increased infestation to several additional factors, including reduced weed control inputs in some orchards and vineyards, reduced use of pre-emergence herbicides, particularly in groundwater protection areas and improper timing of post-emergence herbicides.

Like many other PCAs, DeShane has switched tactics and herbicides to deal with the problem. He has reverted back to a preventative approach. One of his standard recommendations in areas with resistant marestail and hairy fleabane is to use a relatively new pre-emergence Chateau herbicide in a tank mix with Roundup and Prowl H₂0.

“That is a Cadillac treatment in almonds and pistachios, and it has worked very well,” he says. “In almonds we usually apply it in January. We use a 12-ounce rate of Chateau in a berm spray, and it cleans it up nicely. In the past, we've tried Gramoxone and Goal to get by a little less expensively, but that combination just didn't seem to give the kind of control we needed.”

Chateau's long residual activity means one application is usually sufficient for year-long control in almonds, according to DeShane. In pistachios, he splits the treatment, applying the first one in late fall and coming back in January to finish the job.

“In almonds, the leaves come off early so it's a little easier program than pistachios that tend to hang onto their leaves longer,” he says. “Some guys are using Shark and Goal, while others are using Rely. A lot are using Chateau across a variety of crops. It's nice to have options. The last thing we want to do is create another weed resistance problem out here.”

That could be a very real threat if any one herbicide is leaned on too heavily, according to Kurt Hembree, Fresno County University of California Cooperative Extension weed control farm advisor.

“The use of post-emergence herbicides in trees and vines increased greatly over the past 10 years. About 80 percent of that was directly related to the increased use of glyphosate. Undoubtedly, that is now why we're having problems.”

The same scenario could play out again if growers and PCAs aren't careful, according to Hembree. “There aren't that many new chemistries on the horizon when it comes to herbicides,” he says. “We need to be very careful to protect the ones we have by rotating classes of chemistry whenever possible, timing them properly and generally practicing good resistance management strategies.”

DeShane agrees with that assessment. “I think we were spoiled for a long time with how easy it was to clean up a field or an orchard with glyphosate,” he says. “It took a lot of years for the problem to develop, but we've definitely got it now. We sure don't want to see the same thing happen with these other herbicides.”

Chateau was first labeled in California on cotton. DeShane often uses it as a layby treatment. “We're using 2 ounces per acre on cotton, primarily for morningglory,” he says. “It's very effective. There's a little bit of an issue with phytotoxicity, but if you're careful with the sprayer, it's not a problem.”

Some cotton growers are using a 4-ounce rate for Chateau applications in fallow bed cotton which provides longer residual control throughout the winter. In some cases the higher rate, particularly around dairy land where weed pressure is higher, helps control tough weeds longer and more effectively.