Improve Herbicide Action
W. Carroll Johnson, III
Herbicide adjuvants are defined as additional materials added to a spray tank that improve herbicide efficacy. Confusion abounds; knowing the basic differences among the adjuvants, which to use, and whether herbicide adjuvants are needed. There are two main reasons for this confusion. One reason is the lack of standardized terminology among agriculturists regarding adjuvants. This makes it difficult to know what is what. Another reason for the confusion is the incredible number of proprietary brands of spray adjuvants. There is no confusion in the universal truth: herbicide adjuvants are a critical factor that may directly affect herbicide efficacy.
Herbicide adjuvants can be grouped into five broad groups.
1. Compatibility agents – used to help pesticides applied in combination overcome chemical or physical incompatibility
2. Drift retardants – alter the spray droplets to reduce drift
3. Suspension aids – help non-water soluble pesticides stay in suspension
4. Spray buffers – alter pH of spray water
5. Surface-active agents – help spray droplets spread, stick, or penetrate the leaf cuticle.
The commonly encountered spray adjuvants used in food plots will be spray buffers and surface-active agents. These are used mainly with postemergence herbicides and a few are used with soil-applied herbicides.
Spray buffer. Minerals (calcium, magnesium, potassium, and sodium) are present in water as salts in varying concentrations. The degree to which they are present is commonly called water hardness. Water with dissolved minerals < 50 ppm is soft-water; 50 – 100 ppm is medium hard-water; >100 ppm is hard-water.
Some herbicides are formulated as a salt to improve their stability and handling properties. Examples are Slay® (ammonium salt of imazethapyr) and Roundup® (potassium salt of glyphosate). When these herbicides are added to hard-water, the large amount of dissolved minerals in the water will bind with the parent herbicide molecule and form an insoluble salt. This reduces herbicide efficacy.
Spray buffers help negate the adverse effects of water hardness. A common spray buffer is ammonium sulfate (AMS). Sprayable grade AMS reduces hardwater antagonism of certain herbicides in two ways. First, the sulfate portion of AMS combines with the hardwater minerals in solution, reducing mineral interference with the herbicide parent molecule. Second, AMS is an acidifier that alters the pH of the spray water such that the herbicide parent molecule is in a chemical state that is efficiently transported through the leaf cuticle. AMS does not replace the need for a surface-active agent. They are often used together. AMS will be found at agrichemical/fertilizer dealers. Sprayable grade AMS is available as a dry material (mixed at a rate of 17 lbs./100 gal. of spray water) or a liquid concentrate (1 to 2 gal./100 gal. or spray water).
Surface-active agents. The term “surfactant” is derived from “surface-active agent”. A common type is a non-ionic surfactant (non-ionic refers to the surfactant having a chemically neutral charge). A non-ionic surfactant will have two distinct components; a water soluble portion and an oil soluble portion. The proportion of the water-soluble component to the oil-soluble component affects the utility of the surfactant with different types of herbicides. Most surfactants tend to be weighted toward the water soluble portion and these perform better with water soluble herbicides. This is an example of the need to follow herbicide and surfactant use instructions since substituting spray adjuvants does not always work.
A waxy leaf cuticle repels water and causes beading of the spray droplets on the leaf surface. In extreme cases, spray droplets can bounce or roll off a leaf, greatly reducing herbicide efficacy. Non-ionic surfactants reduce the surface tension of spray droplets allowing the droplets to spread over a larger leaf surface area (Figures 1 and 2). By spreading over a larger area, more of the leaf surface is in contact with the herbicide. In addition, surfactants tend to add “sticking” properties to the sprayed herbicide, somewhat protecting the herbicide from wash-off.
Another type of surface-active agent commonly used with herbicides is a crop oil concentrate (COC). A COC is a blend of paraffin-based petroleum oil (80-90%) and a surfactant (10-20%). The paraffin-based oil facilitates herbicide penetration through the leaf cuticle, while the surfactant portion increases the sticking-spreading properties on the leaf surface. COC are commonly used with postemergence graminicides.
There are some herbicides (example – Slay®) that allow using either a non-ionic surfactant or a COC. In these cases, the COC tends to make the herbicide more active compared to using a non-ionic surfactant. This may be useful for controlling some weeds that are marginally sensitive to the herbicide or if weeds are a bit too large. However, forage injury from Slay® tends to be greater when a COC is used, compared to using a non-ionic surfactant.
Using Surface Water for Herbicide Spraying. Surface water (water from streams, creeks, ponds, or lakes) has significant amounts of dissolved solids and organic particulate matter (soil, clay, algae, and decayed vegetation). These dissolved particles in spray water will decrease the activity of herbicides, particularly Roundup®. In addition, the dissolved particulate matter will accelerate the wear on spray pumps and nozzles, not to mention plugging the spray tip orifice. Antagonism caused by particulate matter cannot be corrected by adding ammonium sulfate or any other adjuvant. Always use clean water in a sprayer. Using potable water will ensure clean spray water.
It is worth noting that whenever filling a spray tank directly from a garden hose, use extreme caution to prevent back siphoning from the spray tank, through the garden hose, and back into the potable water source. In addition, herbicide spills next to a well can contaminate ground water. These potential problems can be avoided by filling the spray tank from a portable nurse tank, away from the water source.
If the volume and accessibility of clean spray water is a complicating issue, a possible strategy is to replace spray tips with those having a smaller orifice. This will reduce sprayer output (gallons per acre) and increase the acreage covered per tank. It should be noted that the sprayer needs to be re-calibrated to determine the new output and then revise the herbicide dilution.