Most postemergence herbicide labels recommend use of a spray adjuvant. Spray adjuvants are any nonpesticidal material added to a herbicide to improve the effectiveness of the herbicide. The most commonly used adjuvants are nonionic surfactants and crop oil concentrates. Table 11 lists the types of adjuvants recommended with various postemergence herbicides.

A surfactant reduces the surface tension between the leaf surface and the spray droplet and allows the spray droplet to spread out and cover a greater area of the leaf surface. This effect results in greater absorption of the herbicide into the leaf and a greater probability of killing the weed. Use of surfactants may also increase crop injury with some herbicides.

Brands of surfactants may differ in their concentration of active ingredients (referred to as surface-active agents). The concentration of surface-active agents should be listed on the label. A good agricultural surfactant should contain at least 75 percent surface-active agents.

Recently, several silicone-based surfactants have entered the market. These surfactants have excellent wetting properties but generally are more expensive than regular nonionic surfactants. Only a limited amount of research has been conducted comparing herbicide efficacy with silicone surfactants to that with nonionic surfactants. Research to date generally shows that silicone surfactants work well but are not sufficiently better than nonionic surfactants to justify the extra cost.

Crop oil concentrates are a mixture of 80 to 85 percent nonphytotoxic petroleum- based oil plus 15 to 20 percent emulsifier (surfactant). The function of crop oil concentrates is to reduce surface tension between leaf surfaces and spray droplets, to promote herbicide penetration into leaves, and to prolong droplet drying time which allows more herbicide to be absorbed. Use of crop oil concentrates may increase crop injury with some herbicides.

Vegetable oil concentrates are similar to crop oil concentrates. A vegetable oil concentrate normally contains 80 to 85 percent once-refined soybean or cottonseed oil plus 15 to 20 percent emulsifier. The function of a vegetable oil concentrate is basically the same as that of a crop oil concentrate. Research has shown that the effectiveness of vegetable oil concentrates may equal that of crop oil concentrates but is rarely greater. Some herbicide labels allow substitution of a vegetable oil in place of a crop oil concentrate, while other labels prohibit use of vegetable oils (Table 11).

Some postemergence herbicide labels specify the use of only nonionic surfactants, some specify the use of only crop oil concentrates, and some specify the use of either. The best herbicide performance will be achieved when the label directions for adjuvant usage are followed.

There are several types and numerous brand names of adjuvants on the market. In most cases, the buyer cannot determine the exact composition of the particular adjuvant. In addition, because there are so many brands available, neither university scientists nor herbicide manufacturers can evaluate all adjuvant- herbicide combinations. Without this unbiased evaluation, adjuvant manufacturers are free to make whatever claims they wish for their particular brands.

Be careful not to be taken in by incredible claims or flashy demonstrations. If the sales representative shows you data on a particular adjuvant, make sure legitimate comparisons are being made. As a general rule, if the claim sounds too good to be true, it probably is not true. Be suspicious of anyone claiming that an adjuvant allows use of greatly reduced herbicide rates, improves activity of soil-applied herbicides, improves water infiltration into soil, controls insects or diseases, or breaks soil hardpans.

Here are some guidelines from the American Soybean Association for the use of spray adjuvants with postemergence herbicides:

1. Follow label directions for adjuvant usage. If the label specifically states to not use an adjuvant, then do not use one. If the label does not mention the use of an adjuvant, think twice before using one. It may or may not improve performance and it might result in unacceptable crop injury.

2. Whenever possible, use the exact adjuvant(s) specified on the label. When the specific brand is not available, ensure that the substituted product has the same general characteristics. In the case of surfactants, select one that is of the same type specified on the label (usually nonionic) and has the same percentage of surface-active agents as specified on the label. For oils, select one that contains the type oil specified on the label (petroleum-based or vegetable oil) and has the same amount of emulsifier.

3. If the label specifies only the type adjuvant but not the brand, buy a reputable brand from a reputable dealer.

4. Be careful when mixing adjuvants. When more than one adjuvant is added to the spray tank, there could be an unexpected interaction.

5. Be careful with adjuvant rates. Adding more than is recommended on the label is not always better, and adding less may give poor results.