AMS Boosts Late Weed Control

With planting delays pushing back glyphosate applications, many growers will be treating bigger weeds, which are tougher to control. Water conditioners are a type of herbicide adjuvant that can boost performance, but how do you choose the best one?

Researchers say the ammonium sulfate (AMS) content is key.

Glyphosate and other herbicide labels recommend at least 8.5 pounds (lbs.) of actual ammonium sulfate for every 100 gallons of spray solution. Harder water or more difficult-to-control weeds may require up to 17 lbs. But many premixed water-conditioning adjuvants on the market today deliver less than half the minimum recommended rate.

According to Mark Bernards, weed specialist with University of Nebraska, these adjuvants just don’t have enough AMS to consistently improve weed control. In many cases, performance is no better than glyphosate alone.

In a recent University of Nebraska trial, two out of seven AMS alternatives were as effective as AMS when added to glyphosate for velvetleaf control. The other five resulted in weed control that was equal to — or lower than — weed control from glyphosate alone.

Likewise, the best velvetleaf control in a 2005 Kansas State University study came from glyphosate plus liquid AMS, followed closely by three adjuvants with high AMS content. The remaining six adjuvants had low or no AMS content and resulted in control roughly equal to or less than glyphosate alone.

Bernards says glufosinate has responded similarly to the addition of AMS and low-AMS substitutes.

The take-home seems simple enough: Make sure your herbicide adjuvant contains the minimum recommended rate of AMS. But according to Bernards, few aspects of glyphosate-based weed control systems have generated as much confusion as the role of AMS.

“Shortly after glyphosate was commercialized as Roundup herbicide, farmers and scientists recognized that its ability to kill weeds was reduced when it was mixed with hard water, which was typical for many of the areas where it was being used,” Bernards says.

The problem was antagonism. Negatively charged glyphosate ions bind to positively charged calcium, iron, and magnesium ions — three minerals commonly found in hard water.

“After many years of testing with many different materials, most researchers concluded that AMS was the most consistently effective and economical approach to overcoming hard water antagonism,” says Bernards. If you add enough ammonium sulfate to the spray tank, then the hard water ions (calcium, iron, and magnesium) precipitate from the solution as sulfate salts, and the glyphosate ions bind to the ammonium ions instead of the hard water minerals.

“It’s more commonly used as a nitrogen fertilizer, but all of a sudden ammonium sulfate got a lot of attention for improving weed control,” says Dean Collamer, agronomist with Honeywell. “The hitch was that many granular grades were designed for large-volume fertilizer application — not for dissolving in herbicide spray tanks.”

To alleviate handling problems, suppliers came out with a profusion of AMS pre-mixes made with less AMS and more alternative ingredients, like defoaming agents and surfactants.

“To make sure that you’re using the best adjuvant with your glyphosate spray, ask your supplier for formulations that deliver the minimum required dose of AMS,” advises Bernards. “Alternatively, you can apply a dry sprayable grade of ammonium sulfate directly to your tank.”

Collamer says adjuvant formulators now have access to grades of ammonium sulfate that are free of impurities, easier to dissolve, and more stable in solution. Honeywell, for example, produces Sulf-N PRO soluble ammonium sulfate specifically for liquid applications.

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