Know which adjuvant to match with which herbicide? Not many people do, says John Nalewaja, weed scientist and adjuvant specialist at North Dakota State University.

Some herbicides come with adjuvants already in them, but others don't. And with more adjuvants coming on the market, opportunity for error grows.

A few herbicide companies provide performance data and a list of additives that will work with their chemicals.

"Companies want performance under many environments and don't want crop injury. So they have a label rate that should work over many cases or with many adjuvants," Nalewaja says.

But little data is available on many adjuvants. It's hard for a farmer to know whether an additive will work with a specific chemical, especially if herbicide rates are cut.

So what's a farmer to do?

1) First, know the different types of adjuvants and how they function, says George Kapusta, plant scientist at Southern Illinois University at Carbondale. They include: Surfactants. They break the surface tension of water droplets to allow the water to better cover the leaf surface.

The two main types of surfactants are non-ionic and silicone-based. Non-ionic surfactants have no charge. Silicone surfactants are more expensive and more effective, but very herbicide- and weed-specific. Whether a grower should choose a non-ionic or silicone-based surfactant depends on the herbicide chosen. But only a few herbicides, such as Gramoxone Extra (paraquat), need a surfactant alone.

Crop oil concentrates. They consist of 83% petroleum oil and 17% surfactant. The surfactant covers the leaf; the oil helps move the herbicide through the leaf's cuticle.

Vegetable oils. These adjuvants, also called crop-origin oils or seed oils, perform like crop oil concentrates but without the petroleum. They're 93% vegetable oil and 7% surfactant.

Nitrogen. This additive can be bought with a crop oil concentrate premix. Or a grower can buy it in the form of 28% UAN and add it to his herbicide. Very weed- and herbicide-specific, it is, none-the-less, applied by "almost everybody who applies post broadleaf herbicides," says Kapusta. UAN routinely improves velvetleaf control, he adds.

Surfactants with fertilizer premixes are also available, although products such as Gramoxone Extra "don't benefit greatly from additional nitrogen," Kapusta says. Adjuvants are most beneficial in stress environments - enhancing herbicide performance when it otherwise wouldn't be very high, Nalewaja says.

2) Then read herbicide product labels, he advises. Not all adjuvants react alike to every herbicide.

Highly water-soluble surfactants, for instance, work best with water-soluble herbicides. Less water-soluble surfactants generally work best with oil-soluble herbicides. And certain oil adjuvants may work better with one herbicide than another.

3) Check out university or company herbicide field trials. Then field-test some adjuvant-herbicide combinations and record their performance.

"Try a couple in the field, and if it's a good environment, cut the herbicide rate. If it's a bad environment, use the normal rate. After a few years of that type of testing, if they're all the same, start using the cheaper adjuvant."

4) Get a copy of the Compendium of Herbicide Adjuvants. It's a handy pocket guide organized by type of adjuvant. It lists product names, principal functioning agents used and use rates and gives various comments.

Compiled by Kapusta, the 50-page booklet costs $2. Write Kapusta at: Southern Illinois University, Mail Code 4415, Carbondale, IL 62901. Phone: 618-453-1782.