Don’t let the last weed standing reproduce

When weeds begin showing resistance, it’s not a case of the herbicide changing the weed, it’s a simple “survival of the fittest” case, according to a Texas A&M AgriLife Extension Service specialist.

            Scott Nolte, Ph.D., AgriLife Extension weed specialist, College Station, said the common misconception is the herbicides cause changes. Instead, it’s mainly the inherited ability of a species to survive.

“The problem is these resistant plants become parents because they were not killed out,” Nolte said. “It’s more of a selection, because they were left.”

Building resistant weeds

            Target-site mutation is the most common cause of herbicide resistance, he said. This is where a change at the target site prevents the herbicide from binding or otherwise disabling the action, thus preventing herbicidal activity.

            “It’s like a lock and key and someone changed the lock, so the key no longer works,” Nolte explained.

            Repeated use of herbicides with a single site of action promotes selection for resistant weeds. It kills out the susceptible biotypes and leaves only the resistant biotypes. And a single resistant weed multiplies over and over.

            The resistant weeds documented so far in Texas are perennial ryegrass, barnyard grass, Palmer amaranth, kochia, Johnsongrass, tall waterhemp, common sunflower and marestail.

            Nolte said weeds that are prolific seed producers develop resistance first because they have a high germination rate and reproduce quickly. Extended germination periods are also a problem.

            “If they are highly susceptible to an herbicide and everything around is killed, there is nothing for these surviving weeds to cross-pollinate with,” Nolte said. “So, they can only reproduce the resistant biotype.”

Resistant weeds multiply

            One pigweed can leave 100,000 seeds to soil, he said.

            “If only 10,000 of those germinate and 98% are killed by the herbicide, then 200 seeds are still surviving,” Nolte said. 

            Producers have to get into the practice of rotating modes of action in their herbicide programs, he said.

            “I know the weeds are still growing. And, sometimes it doesn’t pencil out to keep treating,” Nolte said. “But I can’t tell you to do nothing. We have to use what we do have to slow them down.”

            The problem often occurs when producers use herbicides that have a single mode of action, are very effective and have a long soil residual, high-use rate and high-use frequency.

            “For instance, with Roundup it was effective, it was cheap, but ‘nix the mix’ was the downfall, because it was alone,” Nolte said. “Just like dicamba now, we don’t want to encourage single use.”

            Cultural practices can favor development of resistance, he said. Some issues include growing the same crop year after year, not tilling or cultivating, using herbicides with the same mode of action and not using soil herbicides.

Weed management plan

            “You have to rotate the things that work, otherwise they eventually won’t work.”

            Another consideration, Nolte said, is equipment.

            “How many of you change your spray nozzles? They get clogged and damaged, which can affect the rate being applied,” he said. “Also, poor agitation prevents uniform herbicide applications. Or maybe the boom is not set at the right height.”

            Application rates and weed size calculations can be off, Nolte said. Applying herbicides to weeds larger than what the label specifies, or at a rate below the recommendation, can cause unacceptable control. Soil conditions, weather conditions, stressed plants and other factors can result in poor herbicide control.

            “Sometimes a new flush of weeds comes up after the initial herbicide application, or dense stands prevents the herbicide from getting down into the canopy to get good control,” Nolte said.

Confirming herbicide resistance

            “But if you suspect resistance, you have live plants next to dead plants – good control of most weeds, but not a few, take action,” he said. “Be aware of the weeds on the list that are resistant and start taking steps early to make sure you don’t have to spend the money later.

            “If you suspect resistance and want a population tested, you can contact me, and I will test it.”

            If resistance is confirmed, Nolte said, immediate steps to take are: 

            – Eliminate the resistant weed population to limit or prevent seed set and shed. 

            – Don’t use the same herbicides and don’t let the plants go to seed. 

            – Prevent movement of the resistant population to other fields by cleaning all equipment.

            – Implement a weed-management strategy to prevent future occurrences of resistant weeds.

Start clean, utilize all tools

            Cultural practices can be used to eliminate the resistant weeds, including delaying planting or using a non-selective herbicide, promoting crop competitiveness, scouting fields for weed population shifts, using certified seed or possibly employing crop rotation. 

            “We have to look at things we have available. Tillage, even if it is only occasionally in no-till or minimum till systems, is an option.”

            He said a good weed control program includes four steps: start clean, pre-plant and post-emergence weed management and use remedial control options.

            “We don’t have new herbicides in the pipeline, so we have to be diligent with the tools we have,” Nolte said.