Scott Nolte, AgriLife Extension weed specialist, says 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

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.

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

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.

Confirming herbicide resistance

            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. 

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

Lonnie Jenschke is an Erath County extension agent.