Richard Zollinger

NDSU Extension Weed Specialist

Postemergence herbicide effectiveness depends on spray droplet retention and herbicide absorption by weed foliage. Adjuvants and spray water quality influence POST herbicide efficacy. Adjuvants are not important to preemergence herbicides because retention and absorption by foliage does not occur.

Spray adjuvants consist of surfactants, oils and fertilizers. The most effective adjuvant will vary with each herbicide, and the need for an adjuvant will vary with environment, weeds present, and herbicide used. Adjuvant use should follow label directions and be used with caution as they may increase injury to crops or reduce weed control. An adjuvant may increase weed control from one herbicide but not from another. Comparisons of adjuvants should be made at marginal control levels to determine the effectiveness of adjuvants for specific herbicides, sprays, water and weeds. Effective adjuvants will enhance herbicides at reduced rates and provide consistent results under adverse conditions. However, reduced rates exempt herbicide manufacturers from liability for nonperformance.

Surfactants are used at 0.12 to 0.5% v/v (1 to 4 pt/100 gal of spray solution). Surfactant rate depends on the amount of active ingredient in the surfactant and other factors such as plant species and herbicides. The main function of a surfactant is to increase spray retention, but surfactants also function in herbicide absorption. When a range of surfactant rates is given, the high rate is for use with low rates of the herbicide, drought stress, tolerant weeds, or when the surfactant contains less than 50% active ingredient. Surfactants vary widely in chemical composition and in their effect on spray retention and herbicide absorption.

Silicone surfactants reduce spray droplet surface tension allowing the liquid to run into stomata on leaves ("stomatal flooding"). This allows entry into plants differently than adjuvants that aid in absorption through the leaf cuticle. Rapid entry of spray solution into leaf stomata from use of silicone surfactants does not always result in improved weed control. Silicone surfactants are weed and herbicide specific just like other adjuvants.

Oils generally are used at 1% v/v (1 gal/100 gal of spray solution) or at 1 to 2 pt/A depending on herbicide and oil. Oil additives function to increase herbicide absorption and spray retention. Oil adjuvants are petroleum, vegetable, or methylated vegetable or seed oils (MSOs) plus an emulsifier for dispersion in water. The emulsifier, the oil class (petroleum, vegetable, etc.), and the specific type of oil in a class all influence effectiveness of an oil adjuvant. MSOs have been especially effective with Accent, Pursuit, Raptor, and Poast but generally are equal to or better than the petroleum oils with most herbicides (except Cobra). Vegetable oils (non MSO type) are usually equal to petroleum oils. Results vary when comparing specific adjuvants, even within a class of adjuvants.

Fertilizers containing ammonium nitrogen have increased the effectiveness of Accent, Basagran, glyphosate, Poast, Pursuit, Ultra Blazer, and 2,4-D amine. Fertilizer applied with other herbicides may reduce weed control or cause crop injury. Fertilizers should be used with herbicides only as indicated on the label or where experience has proven acceptability.

AMS at 8.5 to 17 lb/100 gal spray volume (2%) has enhanced weed control with glyphosate. Enhancement of glyphosate is most pronounced when spray water contains relatively large quantities of certain ions, such as calcium, sodium, and magnesium. AMS may contain contaminants which may not dissolve and then plug nozzles. AMS should be dissolved in a small amount of water and filtered to prevent nozzle plugging. Commercial solutions of AMS are available.

AMS at 2% is adequate to overcome severe salt antagonism. AMS at 0.5% has adequately overcome antagonism of glyphosate from 300 ppm calcium. Ammonium ions also are involved in herbicide absorption and have enhanced phytotoxicity of many herbicides in the absence of salts in the spray carrier. Herbicide enhancement by nitrogen compounds appears most pronounced to certain species like velvetleaf or sunflower.

AMS enhances phytotoxicity and overcomes antagonism from salts of Poast, glyphosate, and 2,4-D amine. Liquid 28% UAN fertilizer is effective in enhancing weed control from many POST herbicides and overcoming sodium but not calcium antagonism of glyphosate. Sodium bicarbonate antagonism of Poast is overcome by 28% UAN, ammonium nitrate, and AMS. AMS or 28% UAN does not preclude the need for a surfactant. Adjuvants vary in enhancement of herbicide action. The precise salt concentration in water which causes a visible loss in weed control is difficult to establish because weed control also is influenced by many other factors.

Some water pH modifiers are used to lower (acidify) spray solution pH because many insecticides and some fungicides breakdown under basic conditions (high water pH). Most solutions are not high or low enough in pH for important herbicide breakdown in the spray tank. pH reducing adjuvants (example: LI-700) are sometimes recommended for use with herbicides because of greater absorption of weak acid type herbicides when the spray solution is acidic. However, low pH is not essential to optimize herbicide absorption. Many herbicides are formulated as various salts which are absorbed as readily as the acid. Salts in the spray water may antagonize these formulated salt herbicides. In theory, acid conditions would convert the herbicide to an acid and overcome salt antagonism. However, herbicides in the acid form are less water soluble than in salt form. Formation of herbicide acid with pH modifiers may precipitate and plug nozzles when solubility is exceeded, such as with high rates in low water volumes. Antagonism of herbicide efficacy by spray solution salts can be overcome without lowering pH by adding AMS or, for some herbicides, 28% liquid nitrogen fertilizer.

Basic blend adjuvants are non-oil and are different from additives which lower spray solution pH. These increase water pH which increases water solubility of certain herbicides, such as Accent, Pursuit, Raptor, and UpBeet. For example, Accent solubility at water pH of 5 is 360 mg/L, at 7 is 12,200 mg/L, and at 8 is 39,200 mg/L. Basic blend adjuvants also reduce precipitation problems with Betamix/Betanex/Betamix Progress plus UpBeet at low rates by increasing water pH. They contain nitrogen fertilizer to overcome antagonistic salts; a surfactant to aid in spray retention, spray deposition, and herbicide absorption; and a buffer to increase pH.

Research has shown that basic blend adjuvants enhance weed control from Accent, Pursuit, and Raptor similar to MSO type adjuvants. They may be used in those situations where oil adjuvants are restricted. For example, dicamba labels restrict oil adjuvants when used alone or in tank-mix with Accent on corn. Quad 7 is less expensive at field use rates than MSO type adjuvants. Quad 7 used at 1% v/v (1 gal/100 gal of water) and costs approximately $0.75/A at 5 GPA or $1.50/A at 10 GPA. MSO type adjuvants cost approximately $2.50 to $3.00/A at the 1.5 pt/A rate.

Antagonism of glyphosate by calcium in a spray solution was overcome by sulfuric but not nitric acid, indicating that the sulfate ion was important, not the acid hydrogen ion. The importance of the sulfate explains the effectiveness of the ammonium sulfate, and not 28% UAN, in overcoming calcium antagonism of glyphosate. Other herbicides which become acid at a higher pH than glyphosate may more realistically benefit from a reduced pH as has been shown for Poast. However, Poast does not require a low pH for efficacy. pH of 4 has overcome sodium antagonism of Poast, but nitrogen fertilizer or AMS also will overcome sodium antagonism of Poast without lowering the pH. The ammonium ion provided by these fertilizers is apparently the important ion.

Assert 2.5S is formulated as a sulfate ester, is water soluble only at a low pH, and contains chemicals to keep the pH low. The amount of acidifier in the formulation may be inadequate when rates are low in certain highly alkaline waters or in a mixture with other herbicides or fertilizers that raise pH. Thus, precipitate problems have occurred occasionally with Assert in North Dakota. The "solution" to the problem is a lowering of the spray solution pH by a strong acid, like sulfuric or hydrochloric acid. Sulfuric acid is available at most dealers and is sold as a potato vine desiccant. Hydrochloric acid, as muriatic acid, is available in most hardware stores.

In summary, adjuvants that are designed specifically to reduce pH generally are not required for herbicide efficacy. The type of acid or components of buffering agents and the specific herbicide all need to be considered before using pH modifying agents.

Commercial adjuvants differ in effectiveness with herbicides. Data from the table below are from experiments conducted from 1992 through 1995 compared commercial adjuvants with Roundup (glyphosate with surfactant) or Honcho (glyphosate without surfactant) at various locations in North Dakota. Data was included only when a differential in control occurred among adjuvant treatments. In some experiments, all treatments gave similar control, probably because of a more humid and favorable environment for glyphosate uptake and translocation. Roundup/Honcho was applied at lower than labeled rates (2.7 to 4 fl oz/A) so that control would not be complete and differences were much greater at some locations than others. All adjuvants enhanced glyphosate (Roundup and Honcho), but some were more effective than others. The last four commercial adjuvants listed in the table are believed to contain ammonium sulfate (ingredients are often a trade secret) and were more effective than the surfactants as a group. The adjuvants differed in effectiveness across locations, possibly from variable spray water quality and environmental conditions at treatment. The results are averaged over various locations and may not represent adjuvant effectiveness for all situations. However, adjuvants differ in effectiveness and users should compare several products for their specific conditions or select one of the more effective adjuvants from the list.

Commercial adjuvant effect on Roundup/Honcho (glyphosate) phytotoxicity to selected grass and broadleaf plants^a,b,.