Herbicide Resistance – Prevention Starts In The Tank

Never underestimate the power of the right tankmix, says Patrick Tranel, a University of Illinois weed scientist who was part of a team that conducted a landmark study on factors that stem herbicide resistance development.

At its heart, the study looked at whether it was possible to predict if resistant populations would take shape in specific fields, based on both the farmer’s decisions and natural influences.

Of all the factors the project compared, a tankmix strategy ranked at the top in terms of minimizing resistance. Farmers who used tankmix programs early on in the study period were 80% less likely to run up against resistance 4 years later.

This was a “thinking outside the box” project because researchers did not conduct classic plot comparisons. Instead, they looked at commercial fields that either did or didn’t have clear evidence of glyphosate-resistant waterhemp. From there, they examined farmers’ practices and other influences.

As the study evolved, they saw that tankmix combinations were far more effective in slowing resistance.

“At best, rotating herbicides (from season to season) buys you maybe 3 additional applications” of a material that has run into resistance issues, says Tranel.

While the Illinois researchers based their study on glyphosate resistance in waterhemp, the principals apply to other weeds and herbicides, Tranel points out.

Where farmers used tank mixes, they did a better job of holding resistance at bay. The results also were fairly predictable. Where tankmixes weren’t deployed, it was easy to forecast that resistant populations were on their way.

This project has its roots in 2006 in the first field in Illinois where glyphosate-resistant waterhemp was identified. The location became an ongoing test site and as time progressed the scientists saw more infested fields through that part of the state.

“It was remarkable how one field would be overrun with waterhemp while a field just across the road was very clean,” Tranel remembers. “That begged the question: what’s the difference – what were farmers doing that led to such different outcomes? That was the origin of the study and the idea that maybe we could predict outcomes.”

Luckily, a custom applicator worked many of the farms in the area, and he shared data on herbicide selections for specific fields going back a couple of years before resistant waterhemp hit the radar screen.

“With typical studies, you start with a hypothesis and try to prove it with replicated plots and different types of treatments, then quantify results,” Tranel says. “But in this case we already had the results. It was simply a matter of backtracking to determine what led to those outcomes.”

Team members acted much like epidemiologists tracking down the source of a disease outbreak – to determine why some people got sick and others didn’t.

The applicator gave them a goldmine of data, including:

  • A sampling of about 100 fields within a 400-square-mile area.
  • Details about how often glyphosate was used in a given field.
  • Frequency of herbicide and crop rotations.
  • The number of herbicidesapplied annually.
  • Specifics on tankmix choices.
  • Whether a preemergence was applied.

Team members visited fields and evaluated soil types and geographic features, like whether flooding might deposit heavy numbers of resistant waterhemp seed in certain locations. They also collected soil samples and grew out weed seed in the greenhouse to determine if resistance was already present.

Of all the factors considered, management came out as the most important influence in stalling resistance, and tankmixing strategies figured into the management category.

“The role of management (in resistance management) was good news because it means that as a farmer you are in control,” Tranel says. “Frankly, I was a little surprised. I figured that if you had a water course through a field and the acreage often flooded, then resistant waterhemp would become established in the field, regardless of what you did. It’s called waterhemp for a reason, so you might expect it in association with low areas that are prone to flooding.

“What we found – even on farms prone to flooding – was that you can delay resistance evolution if you do the right things.”

Whether or not a farmer tankmixed herbicides in 2004 through 2006 greatly influenced outcomes, he adds.

“Fields receiving mixes of herbicides were less likely to develop resistance issues by 2010. If a farmer, on average, used 2.5 herbicides per tankmix in 2004-2006, he was about 80% less likely to have glyphosate resistance in 2010.”

Those combinations, he quickly adds, were across all herbicide applications, including preemergence sprays.

“Maybe these were combinations of a triazine-type material and metolachlor or maybe the farmer also included a burndown herbicide, like 2,4-D, then came back in the season with an over-the-top material. In soybeans, that might be glyphosate with a PPO inhibitor. In corn, he would have had even more herbicide options.”

The mode of action (MOA) analysis wasn’t “glyphosate specific,” he emphasizes. “We didn’t treat glyphosate special. We simply looked at how many MOAs were in the tank, altogether.”

Tranel quickly adds that studies by other groups also have shown that herbicide rotation isn’t particularly effective. “What we did was to demonstrate this in real fields managed by real farmers, not in a small-plot research setting.”

Editor’s Note…