When weeds break through your herbicide, it’s easy to blame the product, rate or application timing. However, weed-control experts Greg Dahl and Joe Ikley suggest the real culprits might be something else altogether: the water in the tank and the adjuvant — or lack thereof — mixed into it.
Dahl, a retired research manager at WinField United, says hard data tells the story best. After reviewing thousands of university trials, he found
that skipping the herbicide’s required adjuvant is an invitation for weed-control failure.
“We saw a 30% to 90% reduction in weed control when the adjuvant was left out,” Dahl says. “That’s a pretty big sting.”
But simply using an adjuvant isn’t enough. The trials showed that using the wrong class of adjuvant in the tank can slash performance by up to 50%. Even settling for a “good enough” product over a premium version can result in a 25% drop in efficacy. Notes Dahl: In the world of weed control, “close enough” often isn’t.
Solutions That Work
Hard water in the spray tank is often a contributor to poor weed control. When spray water is loaded with calcium and magnesium cations, that can create a hostile environment for weak-acid herbicides like glyphosate. These minerals physically bind to the herbicide, forming particles the plant cannot absorb.
“Plants don’t eat rocks,” says Ikley, a weed specialist at North Dakota State University Extension.
To counter this, he and Dahl recommend using ammonium sulfate (AMS) as a dedicated water conditioner. Dahl’s research indicates that in cases of extreme hardness, adding at least 8.5 lbs. of AMS per 100 gallons is necessary to fully neutralize the water and restore herbicide efficacy.
However, the battle isn’t just in the tank; it’s often on weed leaves. Ikley points out that certain weeds, like velvetleaf, actually secrete their own calcium crystals onto their leaf surfaces.
“Several of our weed species actually need higher AMS rates because of the crystals on the surface of that leaf,” Ikley explains. “We have to account for that interaction on the weed surface, not just in the water.”
While “all-in-one” adjuvant blends offer convenience, Ikley and Dahl urge caution. To hit a specific price point or fit multiple ingredients into the jug, manufacturers sometimes compromise on the water-conditioning component. University trials consistently show that very few “convenience” products can outperform the gold-standard combination of AMS plus a nonionic surfactant, Ikley and Dahl say.
Conditioners Aren’t All Created Equal
Dahl and Ikley both note that water conditioners and “all-in-one” adjuvants vary widely in performance. To get everything into a gallon and hit a certain price point, they say manufacturers sometimes compromise and come up short on one of the functions, often the one for water conditioning.
Ikley says university trials show only a handful of conditioners outperform the standard combination of AMS plus a nonionic surfactant.
“A few of the water conditioners do quite well,” he reports. “The rest don’t perform as well as AMS plus surfactant.”
Before they select or change the adjuvant used, Dahl and Ikley tell farmers to test their water, and ask the laboratory to report the results to them in parts per million.
“Some labs report in grains, and then you’ve got to do more math,” Dahl says.
Ikley adds a practical tip: always run the water for a few minutes before taking a sample to ensure you aren’t testing stagnant residue from the lines.
Ultimately, adjuvants are a valuable safety net for herbicide performance and weed control. As Dahl puts it, “You can raise herbicide rates until you can’t anymore. When you need help, the adjuvants can help.”
