But choosing among 4,000 unregulated products is no easy task.

An adjuvant could help you trim herbicide costs, improve weed control and broaden the window of application. On the other hand, you could shell out your hard-earned cash on an adjuvant and not reap any of these benefits.

"Picking the adjuvant that complements your herbicide isn't a simple task," acknowledges Dennis Berglund, a crop consultant with Centrol Crop Consulting, Twin Valley, MN. He adds, "If there was a practical book written on the subject, I'd be the first in line to buy it."

Even so, Berglund believes that if you learn how to use adjuvants as a management tool, you can stretch your herbicide dollars without compromising weed control. The key is understanding the types of adjuvants available, the companies that are reputable providers and how to use their products successfully in the field.

Which adjuvant do you need? An adjuvant is any substance added to the spray tank, apart from the herbicide, that improves its weed-control performance. Adjuvants fit into three basic categories: activators, spray modifiers and utility modifiers. Farmers most often use activators in their weed-control programs, as specified by herbicide product labels. Activator adjuvants include nonionic surfactants, fertilizer solutions, petroleum-based crop oil and methylated vegetable oil, commonly referred to as methylated seed oil or MSO.

Adjuvants truly fit the philosophy of you get what you pay for. An inexpensive surfactant may cost only $0.25/acre, whereas a superior MSO may reach $3/acre. The difference in their performance can be just as dramatic, says Richard Zollinger, North Dakota State University extension weed specialist. He offers one example as proof: In North Dakota, Accent herbicide used at its full rate in corn, along with a $0.25 surfactant, costs about $20.25/acre. A half rate of Accent along with a superior $2.50 adjuvant runs only $12.50/acre. Zollinger says that, with care, farmers can use the lower-cost approach, secure weed control comparable to the full-rate results, and pocket the $7.75 difference.

"Our research and experience show a superior adjuvant often will provide adequate weed control with reduced herbicide rates," he confirms.

Zollinger mentions another potential benefit from this approach to adjuvant use. Both the current political administration and the general public continue to express a preference for less pesticide use in the food chain.

However, Zollinger cautions farmers against blindly shaving rates. Farmers must evaluate where a reduced herbicide rate combined with an adjuvant makes sense and offers the most savings. They also must be willing to shoulder responsibility for applications that fail.

Because no regulatory agency oversees the development and marketing of the 4,000-plus adjuvants available, some of these products work and some don't, Zollinger explains. Growers must evaluate university research results, ask their local ag chemical dealers for recommendations and perhaps use trial and error to identify which adjuvants fit their needs. Zollinger adds, "Be aware that weed pressure, heat and humidity can also impact adjuvant performance from year to year."

Start on a small scale. Minimize any gamble by using the reduced rate with an adjuvant approach on only a few acres, Berglund says. He recommends selecting fields with a minimum to moderate level of weed pressure. Plus, you need to have a backup plan ready in case the first spray application gives less than stellar control results.

In 1997 brothers Kelly and Perry Skaurud of Gary, MN, began using herbicide micro-rates with an MSO on 1,800 acres of sugar beets. That year they used an 11-in. banded application and two cultivations. "We decided if it didn't work, we'd come back in and spray the second time with a conventional rate," Kelly says. "But we were so happy with the results, we haven't used full conventional herbicide rates since."

However, the brothers have experimented with different application approaches. In 1998 they broadcast micro-rates and the MSO. That allowed them to eliminate all cultivations. But in 1999, due to a drop in sugar beet prices, they returned to the 11-in. banded approach followed by two cultivations. Their total investment per season for three banded applications and two cultivations is approximately $30/acre, about half the cost of the broadcast approach.

"We've gained better weed control at a lower cost and less injury to the crop, and we have more application flexibility," Kelly says. "We used to be able to spray only in the late afternoons and early evenings, but now we can go pretty much throughout the day."

Bob Herzfeld, adjuvant product manager for Agriliance, agrees that spray flexibility is an important payoff. But, like Zollinger, Herzfeld says temperature and humidity can cause problems. He explains that, of the three most commonly used adjuvants, MSOs tend to be the fastest acting and the most efficacious, petroleum-based crop oil concentrates rank second, and nonionic surfactants rank third. Herzfeld encourages growers to ratchet down their selection of those three products, depending on the weather of a given day, to prevent crop injury. "If it's 85 degrees with 75% humidity, you're headed for trouble with an MSO," he says. "Under those conditions, switch to a nonionic surfactant to minimize any crop response."

The role of water. The quality of spray water and how it interacts with herbicides and adjuvants are neglected aspects of the weed-control equation, Herzfeld says. He tells farmers to start with clean water and to test all water sources to identify the elements they contain. For instance, well water may have a high iron content. City water probably contains chlorine. These elements can antagonize the adjuvant and impact its performance, whereas the right water can improve performance.

For example, Herzfeld says that tankmixes including sulfonylurea and imidazolinone chemistries, plus an adjuvant, prefer a neutral to high pH water, but glyphosate prefers an acid-based water. The solution is to use buffering agents, also called utility modifiers, to change the water quality to the degree that is most compatible with the herbicide and adjuvant used.

One last note about water. Herzfeld points out that in April spray water is usually cold and doesn't contribute to good mixing results. "You want water that's at least 50 degrees for mixing purposes," he notes. Herbicides that don't mix well can cause nozzle plugging. Many growers use nurse tanks that hold nothing but water to raise the temperature and sidestep this problem.

Manufacturers need to help. Zollinger advises growers to consult herbicide manufacturers about adjuvants that complement their products. Dupont, for one, offers an agricultural bulletin that lists the adjuvants approved for use with its row crop and cereal herbicides. Zollinger hopes other companies will follow suit with specific recommendations. "Most herbicide labels are ambiguous about the adjuvant to be used, and that needs to change," he says.

Herzfeld encourages growers to consult adjuvant manufacturers for their research results. He says cost is also a good guideline on value. "Sometimes there's only a dime's difference between a good-quality adjuvant and a bad one, so make the better investment," Herzfeld says. "Don't cheap out."

For more information, check out a good adjuvant home page at Southern Illinois University's site at www.siu.edu/~weeds/adjuvants/ frames-index.htm.