Donna Lee's take-home message is this: glyphosate resistance in weeds is a “real problem heading right toward us. A proactive approach is needed to sustain glyphosate-tolerant technology. It's a great thing, and we need to keep it around.

“We'll have to become more and more cognizant in order to keep it viable,” said Lee, a Louisiana Extension agent in East Carroll Parish. “There are no new products to take glyphosate's place. If we end up with (ubiquitous) resistance to glyphosate, we are in for major problems.”

Lee, who spoke at the 2006 Louisiana Cotton Forum in Delhi, La., on Feb. 7, provided the following definition of herbicide resistance: The inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wide biotype.

“I think the key word in that segment is ‘inherited.’ Everyone needs to understand that the resistance ability is already in the plant. Most weed scientists are of the opinion that the chemicals used don't cause mutations in genes leading to resistance.”

Lee's preferred definition of herbicide tolerance is: The inherent ability of the plant to survive and reproduce after a herbicide treatment at a normal rate. “Simply put, this weed won't be killed by a specific herbicide.”

A biotype is naturally occurring plant within a given population that differs slightly in genetic make-up. “You can have two weeds of the same species and one is slightly different genetically. You cannot look at the weeds side-by-side and tell any difference.”

When a herbicide is used repeatedly and continuously, susceptible plants are killed while resistant plants survive to reproduce without competition from susceptible plants. Once that happens, “resistant plants can run rampant. For example, in California, producers are having some problems with weeds resistant to Londax. In Louisiana, we've got duck salad developing resistance to that product too.”

“Herbicide cross resistance” involves a biotype that has developed resistance to one herbicide and is then resistant to other herbicides using the same mode of action. Lee pointed to another example in California. There, “littleseed canarygrass has developed resistance to a several products.”

A plant with “multiple resistance” means a biotype has developed resistance to two or more chemically unrelated herbicides with different modes of action. “The term ‘mode of action’ simply refers to how the chemical kills the plant.”

There are cultural practices conducive to developing resistance. Among them:

  • Shifting away from multiple crops to mono-crop systems. “That isn't good if we want to keep weeds from becoming resistant.”
  • Reduced/no-till systems that lead to continuous or repeated applications of single herbicides with a single, or the same, mode of action.
  • Herbicide-tolerant crops.
  • High and/or low herbicide rates applied relative to the amount needed for control.

Bob Wilson at the University of Nebraska has studied Roundup Ready cropping systems for quite a while. “In one study, he found weeds didn't develop resistance to Roundup UltraMax when applied at 26 ounces per acre. However, when applied at 13 ounces per acre, he ran into a weed shift to common lambsquarter. That doesn't mean it's resistant but it does take more Roundup to kill the lambsquarter. He also noted that plots not treated with Roundup shifted predominantly to grasses after seven years.”

The time required for resistance to develop depends on selection pressure, herbicide rotation patterns, weed species, germination, naturally occurring resistance, vigor and environmental conditions. Resistance typically doesn't happen overnight and should be suspected when:

  • Other causes of herbicide failures have been ruled out.
  • The herbicide(s) with the same mode of action have been used year after year.
  • One weed normally controlled isn't while others are.
  • Healthy weeds are mixed with controlled weeds of the same species.
  • A patch of uncontrolled weeds is spreading.

Escaped weeds aren't always resistant. Improper herbicide application is common and “probably the most frequent thing we do wrong. Spraying an improper rate is also common, as are improper timing and not calibrating a sprayer properly.”

Misidentification of a weed is also a problem. “If you're not completely sure of a weed's identification, take it to a crop consultant or Extension agent. Let someone help you name the weed. All herbicides don't kill all weeds.”

One of the largest causes of herbicide failure is weed size. “If the label states a product will control weeds 2 inches to 4 inches tall, then that's when you need to put it out. Don't wait until the weed is 10 inches tall. You won't be happy with the control.”

Strategies for controlling and preventing herbicide resistance include cultivation, hoeing and rotating modes of action.

“If you're going from Roundup to Touchdown, you aren't rotating your mode of action. You're simply rotating a glyphosate product.

If you have questions about mode of action, the label contains information about each chemical.

“You may want to consider tank-mixes, or look at some of the old standards like atrazine. Rotating Roundup Ready soybeans with Roundup Ready cotton with Roundup Ready corn increases our usage of glyphosate. We need to add something in with it.

“No glyphosate-resistant weeds are currently confirmed in Louisiana. However, some suspected weeds (including pigweeds) are being investigated.”

With the coming introduction of Roundup Ready Flex cotton, even more glyphosate will be added to the mix. This increases Lee's worries.

“Resistant marestail will arrive in Louisiana with a little warning. It's already in Arkansas, Mississippi and Missouri and will, most likely, be in Louisiana (soon). Glyphosate-resistant pigweed will not give us a warning. We're growing our own. The pigweed isn't geographically mobile.”