Soybean growers who rely on glyphosate for weed control may improve their results this spring by including a simple but often overlooked weed-fighting ally in their spray tank: ammonium sulfate (AMS).

AMS, a fertilizer ingredient, is labeled as an adjuvant by many manufacturers for use with glyphosate. AMS makes two important contributions to help glyphosate whip weeds, says Bob Herzfeld, adjuvant business manager for Minneapolis-based Agriliance, LLC.

“The ammonium ions act like the point of an arrow to pierce tough weeds such as velvetleaf, lambsquarters, waterhemp and foxtail,” he says. “It helps improve the glyphosate coverage of weed leaf surfaces and plant uptake.”

The sulfate ions, he adds, work to neutralize calcium, iron and magnesium carbonates that may be present in the tank due to hard water, plant matter or dirt. The sulfate ions help prevent carbonates from interacting with glyphosate and compromising weed control.

Growers who pair AMS with glyphosate can expect up to $10/acre return on their investment (ROI), according to Southern Illinois University (SIU) research.

The ROI will vary depending upon the type of weeds present, population levels and form of AMS used, says Bryan Young, SIU weed control specialist. Young manages an online informational tool, Compendium of Herbicide Adjuvants. The site, which is updated annually, details nearly 450 adjuvant entries from 36 companies and is located at www.herbicide-adjuvants.com.

“AMS is cheap insurance,” Young says. “You will almost always see improved performance from pairing it with glyphosate, especially on velvetleaf.”

Pure AMS is available from retailers in a variety of formulations. Costs range between 25-50¢/acre for dry, spray-grade AMS, the cheapest form available. It's used at 8.5 lbs./100 gal. for most conditions or at 17 lbs./100 gal. of carrier when tough weeds, adverse environmental conditions or hard water are present.

Unfortunately, spray-grade AMS is not user friendly. It's sold in hefty 51-lb. sacks that are difficult to handle and store. However, user-friendly options are available. Liquid AMS runs slightly higher in cost at 40¢-$1/acre but is easy to handle, load and mix, making it a good value for the money invested.

Growers who are experiencing breaks in their weed control typically tell Young they plan to boost their glyphosate usage rates from 1 to 1.5 quarts/acre. “I try to show them the benefits of using the same labeled rate of glyphosate they've been using all along by adding AMS to boost weed control and consistency of control,” he says. “It's a cheaper alternative to adding more glyphosate, and they will likely get good results.”

In addition to pure AMS, growers can opt to purchase AMS-replacement products, typically adjuvants that contain AMS along with other value-added products such as defoaming agents and surfactants. AMS-replacement products range widely in their performance and cost $1-2.50/acre.

“You pay for the convenience with a lot of these (AMS replacement) products,” says Mark Loux, extension weed specialist at Ohio State University.

Mike Owen, extension weed specialist for Iowa State University, says including AMS with glyphosate is a standard weed-control practice for Iowa soybean growers. He attributes much of the widespread acceptance of AMS to custom herbicide application practices that predominate in the state.

Loux says the majority of Ohio soybean growers use AMS or a replacement product. “It's particularly good under cool conditions for spring burndown and for fall applications for dandelion control,” he says.

However, in the very most southern soybean producing states, growers don't necessarily reap such benefits from AMS. “They typically have higher humidity, which is good for herbicide activity,” Young explains. He adds that some weed control specialists there are adamant against AMS usage, because they say it promotes corrosive activity in the tank.

“AMS does create skepticism among some growers, because you're basically telling them to add fertilizer to their spray tank,” says Andy Walsh, agronomy department manager for Kettle Lakes Co-op, Random Lake, WI. “We have coached our customers over the years that it's important to the performance of the glyphosate, and I think that's helped our success.”

David Harms adds that some glyphosate formulations available now already include adjuvants and/or surfactants, so growers may not need to add AMS to the tank. “Simply read the label so you know,” advises Harms, a certified professional agronomist for Crop Pro-Tech, based in Bloomington, IL.

Young says, however, that every glyphosate label he's aware of allows for the addition of AMS, regardless of the surfactant load in the glyphosate.

Hard water and water pH are two factors Herzfeld says impact water quality more than many people realize. AMS helps condition water affected by hardness or pH to boost glyphosate performance and minimizes the impact of impurities. Hardness levels vary depending primarily on how much calcium carbonate is dissolved in the water. Other minerals and properties also contribute to water hardness, including magnesium sulfates, chlorides and iron. However, dissolved calcium carbonate is typically considered the major contributing factor causing hard water. Hardness is commonly defined as spray solution water that contains dissolved hardness minerals above 17 grains/gal. (GPG) or 300 ppm.

“We've found that glyphosate performs best when used with water that's not hard,” Herzfeld says.

A measure of acidity in water, pH is a logarithmic scale from zero to 14, with seven being neutral. Below seven, a substance is defined as being acidic, while levels above seven are said to be alkaline.

Herzfeld says glyphosate performs best in water with a pH of about 5.5. Of the 700-plus Midwest water samples Agriliance tested in 2003, Herzfeld reports, “80% of the samples we tested were more than 7.8.”

For a couple of dollars, growers can test their water sources and know how they stack up in hardness and pH, then take corrective steps, if needed. Or they also can send water samples to Agriliance for a free test and recommendations. “It's important to test all your water sources, because they can vary even if they're only a mile apart,” Herzfeld says.

Harms agrees. “Often, you can improve the efficacy of glyphosate by simply correcting the hardness or pH of your water,” he says.

Evaluate Replacement Products

Manufacturers have identified AMS-replacement products as a market opportunity. Some replacement products provide a valuable service to soybean growers and custom applicators, while others do not.

“Sometimes it's difficult to know what's real and what's a joke,” says Bryan Young, Southern Illinois University weed control specialist.

Bob Herzfeld, adjuvant business manager for Minneapolis-based Agriliance LLC, agrees, saying, “A lot of these aren't good AMS-replacement products; or they're used in the wrong water.”

Young encourages growers who opt to use an AMS replacement product to choose carefully.

“Manufacturers don't tend to develop these replacement products based on worst-case scenarios,” he says. “They usually fit typical or average conditions.” He adds that such products also don't usually offer the complete range of benefits traditional AMS provides. “Replacements often only condition the water, and that may be it,” he says.

While not all AMS-replacement products perform well, some do a good job. Andy Walsh, agronomy department manager for Kettle Lakes Co-op, Random Lake, WI, used Alliance on 85% of the acreage he sprayed with glyphosate in 2003. He says its price of $2.50/acre is considerable, considering the cost of spray-grade AMS, but the payoff is worth the investment. “It was a bad year statewide for lambsquarters, but we had no sprayer complaints — zero — on those soybean fields where we sprayed with the Alliance,” he says.

One of Walsh's customers, Gary Held, says his soybean fields averaged 36 bu./acre while most area growers harvested 20-25 bu./acre.

“I work for an area farm equipment company, so I was able to get a good feel for the yields in the area,” he says. The Sheboygan County grower believes his better-than-average yields for a dry year resulted from no competition between weeds and soybeans for what little moisture was available. Held adds, “I had neighbors who cut their costs and therefore cut their yields.”

Walsh encourages soybean growers to buy inputs based on appropriate value and not lowest cost. That includes AMS. To evaluate glyphosate with AMS or an AMS-type replacement product, Walsh says to “give it a difficult job to do — those beans with weeds that are on the edge of the label relative to size. Most products available will handle the easy problems. Give it the tough stuff and then evaluate your results.”