Herbicide resistance: Abuse, lose

Abuse ‘em and you'll lose ‘em. University of California Cooperative Extension farm advisor for Merced and Madera counties and weed scientist Ron Vargas says it is that simple.

If growers continue to use one mode of action almost exclusively to control weeds, this mode of action will eventually be rendered useless, he says.

However, the growing concern about glyphosate resistance is that a growing number of weeds are more significant than simply herbicide resistance. A new and beneficial agricultural technology for row crop growers could go out the window with the resistance buildup.

Marestails has already been declared resistant to glyphosate in California and many other states. Ryegrass has too. Vargas said problems are surfacing with lambsquarter and pigweed showing signs of glyphosate-resistance in California as they have elsewhere. Vargas has collected seeds from lambsquarter and pigweed plants not phased by several glyphosate applications in cotton fields and almond orchards and sent them to the University of California, Davis to be grown out in a greenhouse and tested scientifically for resistance.

Vargas is telling field day visitors and everyone he can that he and his fellow University of California Cooperative Extension weed scientists are not yet ready to officially declare resistance in lambsquarter and pigweed as they have marestail. However, he leaves little doubt as to the direction it is heading unless producers start practicing better stewardship of unique and valuable technology.

He is not blaming the herbicide for the problem. He is blaming the herbicide user — the farmer — who understandably wants to take the easy and cheap way out and use glyphosate on every weed all the time.

The stakes are much higher than simply losing a herbicide. It would mean losing — needlessly — a valuable tool that has been a money saver to growers.

Others available

These transgenic, over-the-top crop herbicide resistance cropping systems “are too good to lose,” he told growers and pest control advisers at the annual cotton field day this year at the Westside Research and Extension Center in Five Points, Calif.

The loss could be inevitable if growers do not begin using registered herbicides with different modes of action than glyphosate. There is an arsenal of effective new and old herbicides that control unwanted vegetation with modes of action different than glyphosate.

Staple is excellent to control pigweed, said Vargas. He also cited Shark and Chateau as two other new herbicides that can be used as fallow bed weed control herbicides or post directed treatments.

When the Roundup Ready system came on the market, many growers abandoned pre-plant herbicides. Vargas said that is a mistake and many are now returning to the dinitroanaline herbicides for pre-emergence weed control due to the growing glyphosate resistance issue.

“And don't completely eliminate cultivation,” Vargas admonishes growers.

Although there is a big push and growing producer interest in conservation tillage, cultivation remains a part of Western agriculture partly to accommodate irrigation; reduce compaction in crop rotation and to hasten crop residue reduction.


However, herbicides are the primary go-to solution for controlling weeds and here are some of the weed control recommendations from the UC IPM program that offer different mode of action weed control.

For nightshade, tank mix applications of trifluralin (Treflan) with prometryn (Caparol), or pendimethalin (Prowl) with prometryn as a preplant incorporated treatment provides successful control when adequate soil moisture is present. A preplant treatment with the soil fumigant, metam-sodium, also can be effective. Well-timed post-directed treatments of carfentrazone (Shark), oxyfluorfen (Goal), and prometryn (Caparol) when nightshade is in the cotyledon to two-leaf stage also can be successful.

Annual morningglory does not cause a problem at cotton emergence because it normally germinates later in the season. A post-direct treatment of carfentrazone (Shark), prometryn (Caparol), or oxyfluorfen (Goal) to morningglory seedlings before it twines onto the cotton plant has provided control, especially when tank mixed with MSMA. Tank mix applications of prythiobac sodium (Staple) and MSMA, when applied up to the four- to six-leaf stage, have provided excellent control.

To give cotton a head start on yellow nutsedge, sweeps or other shallow cultivating tools can be used to dislodge emerging nutsedge growth before planting. After cotton emergence, the use of precision equipment to cultivate as closely as possible, and hand or mechanical thinning can also help reduce nutsedge competition. MSMA can be applied broadcast or as a directed spray. Metolachlor (Dual) can be applied either over-the-top or as a directed spray for pre-emergent control of nutsedge.

Directed sprays after the cotton plants have two or more leaves reduce crop injury. If these herbicides accidentally get on the growing point of the cotton plant, they will retard the plant's growth. Because nutsedge is sensitive to competition by shade, early chemical control will allow later shading from the cotton canopy to provide additional control. Rotation to alfalfa with multiple water run applications of EPTC, or to corn with the use of a thiocarbamate herbicide, has also significantly reduced both yellow and purple nutsedge infestations.

Prevention best

Prevention is the best method to control perennial grasses such as bermudagrass and johnsongrass. Dry fallow during summer when moisture is depleted has significantly reduced bermudagrass and johnsongrass. Cultivation can control these weeds between rows, but hand weeding is not effective. Sethoxydim (Poast), fluazifop-p-butyl (Fusilade), or clethodim (Prism) applied two to three times per season can provide season-long control. Glyphosate (Roundup) applied with a hooded sprayer to traditional cotton varieties will provide satisfactory control of bermudagrass.

The issue of resistance will only become more complicated with the new Roundup Flex cotton varieties which will be available on a limited basis this season with widespread availability next season.

Where the current Roundup Ready varieties only allow the application of glyphosate until the cotton plant's fourth true leaf, Roundup Flex cottons tolerate glyphosate throughout the season. The new varieties will carry a seven-day pre-harvest interval. This is expected to increase glyphosate use.

This will put more pressure on for weed resistance, especially since glyphosate is now so inexpensive. Vargas said there are more than 50 different glyphosate brands on the market with prices as low as $15 per gallon. This is considerably less expensive that other modes of action.

Coming is a new transgenic era of herbicide-resistant cottons called Liberty Link. Ignite is the name of the herbicide that can be used over the top of Liberty Link transgenic cotton.

The chemical name for Ignite is glufosinate ammonium, but do not get that confused with glyphosate, the chemical name for Roundup, Touchdown, and numerous generic products. These are very different herbicides and LibertyLink and Roundup Ready and Roundup Flex cotton varieties are not interchangeable.

Growers cannot spray glyphosate products over the top of Liberty Link varieties, and they cannot spray Ignite over-the-top of Roundup Ready varieties, or the result will be dead cotton plants.

Ignite is a non-selective contact herbicide that has activity on both grasses and broadleaf weeds, but its strength is annual broadleaf weeds.

Vargas said Ignite has worked well on field bindweed. With other products, Vargas said growers have come to expect to see field bindweed intertwined with the cotton plants at picking time.

“We did not see that with Ignite,” he said. Ignite provided 70 to 80 days of control of nightshade.

Again, Vargas warned that like the glyphosate-resistance, overuse or misuse of Ignite on Liberty Link could also lead to resistance problems with Ignite.

Complicating the glyphosate-resistance issue is the release of new Roundup Ready alfalfas, said Vargas. Weed developing glyphosate-resistance because of repeated use of Roundup will impact not only alfalfa, but the glyphosate-resistant cotton or corn that could follow in rotation.

Staving off resistance will be a challenge, but there are tools to meet the challenge is growers will use them, said Vargas.

e-mail: [email protected]

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.