Is a herbicide-resistant weed coming soon to a field near you?

That’s a question farmers and ranchers in the West may ask themselves as they read reports of weeds that are resistant to multiple herbicide modes of action in the Southeast, Southwest, and now in the Midwest states.

Actually, the movement of weeds from one region to another can be a more pressing problem than many growers realize. To be so stationary, weeds sure do get around, according to weed scientists who track the rise and fall of weed pests in the U.S.

“The majority of weeds currently found in the United States did not originate here, but managed to hitchhike here from other countries,” says the text of the new Continuing Education Course, Weed Resistance Management in Agronomic Row Crops & Trees, Nuts & Vines.

The course, developed by the editors at Farm Press, is sponsored by Monsanto.

“An estimated two-thirds of all weeds now growing in the U.S. are non-native,” the course says.

One of the most problematic weeds – Palmer amaranth or pigweed – was first found in the desert Southwest, which would make it feel right at home in Arizona and California.

The purpose of the new CEU is to give growers an overview of important management practices that can help avoid or delay the development of herbicide-resistant weeds, such as pigweed, which has biotypes that are resistant to glyphosate and other herbicides across the Southeast and into Texas and New Mexico.

More recently glyphosate-resistant Palmer amaranth has been found in Iowa, Illinois and Michigan along with biotypes of a related species, common waterhemp, that are also glyphosate-resistant.

The course begins with a brief review of common weed types and herbicides, followed by factors that can influence the evolution of resistance in weeds, and methods for potentially delaying its occurrence in agronomic row crops and permanent crops.

(While weed resistance management guidelines may be introduced and discussed under a specific annual or perennial crop heading, many of the WRM techniques have cross-crop applicability.)

Other topics discussed in this training:

Herbicidal Modes of Action

Weed Resistance Detection and Strategies

“Weeds exhibit a tremendous amount of genetic variation and biotypes, permitting survival under a number of adverse situations,” the course says. “This trait also predisposes them to develop populations with resistance to herbicides.”

Weed seeds frequently are moved easily from place to place, some facilitating their spread via fuzzy fibers or barbs that help them easily attach to animals or equipment, or “parachutes” or “wings” that speed them on the wind.

Some weed seeds are spread through the feces of animals, or birds that consume them in one field and deposit them miles away.

In California and Arizona, the irrigation and reclamation canal systems are responsible for the widespread distribution of all types of weed seeds, including herbicide resistant biotypes.

Weeds can be physically removed by cultivation or by hand hoeing; however, this can be tedious, expensive, and time-consuming.

The post–World War II proliferation of herbicide discoveries changed farming forever. Herbicides were introduced that were effective, inexpensive, persistent, broad-spectrum, and easy to apply.

As effective as herbicides have become and with the advent of herbicide tolerant crops, the practice of relying too heavily on just one or two herbicides has led to the development of herbicide resistance in weed populations. The importance of integrated weed management has never been greater.

The integrated weed management approach relies on a combination of weed-control measures: cultural controls whenever possible, mechanical control, and use of herbicides when weed populations threaten to cause economically significant damage.

The course concludes with a 20-question exam in which users must score 70 percent to earn credit.

The course is accredited in California, Arizona, Idaho, Maine, Maryland, New Jersey, New Mexico, Oregon, Pennsylvania, Tennessee, Virginia, West Virginia, Wyoming, as well as ASA’s Certified Crop Adviser program.

To view the course materials and sign up, visit www.pentonag.com.

There are now over 12,000 licensed professionals and farmers registered on the portal, and to date, more than 70,000 courses have been completed for credit hours/units on more than 27 courses.