The problem of herbicide-resistant weeds is among “the most important issues agriculture faces,” according to Roy Vidrine.

“Many years ago, we began stressing the use of pre-emergence herbicides,” said the LSU AgCenter professor at the Louisiana Soybean Association annual meeting in Alexandria, La. “Lots of products were developed back in the 1960s and 1970s.

“Some of you are also old enough to remember post-directed treatments. When those were popular seems a long, long time ago.

“Since the mid-1990s, pretty much all we've seen is this,” said Vidrine, pointing to the word “glyphosate” on a projection screen.

Currently, in many cases, researchers are focused on tests that “have gone back to pre-emergence weed control. That leads up to the two points I want to emphasize: weed competition and weed resistance.”

Pre-emergence weed control is useful and helps add insurance where there are weed competition problems, especially early season. Such weeds can cause yield losses early and they're often hidden.

“Pre-emergence herbicides assist in preventing weed resistance by adding another mode of action to a grower's program. In some areas of the Mid-South, there have been problems with resistant johnsongrass. Often, it's uncertain whether there's actual resistance or a lack of control. Many times, it takes a bit of research to make that determination.”

More recently, a similar scenario has played out in Mid-South pigweeds. However, despite suspicious plants in the state, glyphosate-resistant pigweeds are yet to be confirmed in Louisiana.

“Do we have resistance or different varieties? Is size a factor? Recent samples that we've checked have turned out to be uncontrolled but not due to resistance.

“We do have, and have had, resistant weeds in Louisiana. Among them are barnyardgrass, cocklebur, and johnsongrass. We need to avoid resistance, delay it, as long as possible.”

There are two types of resistance: cross and multiple. “Cross resistance can occur when a weed biotype has gained resistance to more than 1 herbicide with the same mode of action, which can be in the same or different herbicide families (imidazolinones as well as sulfonylureas, for example). Multiple resistance occurs when a weed biotype has developed tolerance to more than one herbicide brought about by different selection pressures (different modes of action).”

Worldwide there are 284 different herbicide-resistant weeds. The problem first surfaced in Washington in the late 1960s with common groundsel in nursery crops. Since then, the list of herbicide-resistant weeds has grown dramatically.

The ability of a weed to produce an abundance of seed is a trait that favors resistance.

“That's also true with how the seeds are dispersed through wind, water, animals, etc. For instance, some pigweeds are tremendous producers of seed. That means there's a greater chance of resistance and other states are already seeing resistant pigweed popping up.”

What are some things that promote resistance?

  • When only one mode of action is used in herbicide treatments.
  • When a herbicide is used more than once a season.
  • When a herbicide is used in consecutive seasons.
  • When no other control strategy is employed, whether mechanical or through other modes of action.

“There are eight species we're currently looking at (with possible resistance) including everything from horseweed to grasses to ragweed and pigweed. We can suspect resistance when, among other things, a herbicide failure has been ruled out and when one weed isn't controlled while others around it are.”

In the United States, there is already a resistant horseweed problem in nine states. Vidrine says more are coming and preparation is paramount.

“We need to be ahead of the curve, not behind it. It's much more expensive controlling resistant weeds than to keep them from becoming resistant to begin with.”

As far as control strategies, “use rotation, rotation, rotation. That's not only for modes of action but also crops. Rotate as much as possible.

“This resistance situation is real and it'll be in Louisiana. That's due to the limited chemistries we're using currently. We need to keep it away from our front door as much as possible. This is an investment we need to make — just like a nutrition program, or picking varieties properly.”

Resistance is something landowners and growers certainly don't want. “So invest in a resistance program,” said Vidrine. “The sooner you do that, the cheaper it'll be in the long run.”