Choosing the right herbicide formulation
Experts often stress the value of proper weed identification. This makes it possible to determine the best cultural and chemical strategies for control. However, an additional step — choosing the best formulation for the intended use — receives relatively scant attention.
There are several reasons why you should consider the effects of herbicide formulation.
Formulations may vary in their effectiveness on weeds.
Formulation may affect the tolerance of turf and ornamentals to the herbicide.
Some formulations are more costly than others.
Formulation affects drift potential.
Ease of application and compatibility with your application equipment vary according to formulation.
At its most basic, formulation means an active ingredient (the chemical that actually controls the weeds) plus a carrier, such as water or oil for liquid chemicals, or clay with granular formulations. However, herbicide formulations frequently contain other additives to enhance mixing and application and improve adhesion and uptake by weeds. In addition, some formulations include not just one, but two, three or even four active ingredients combined into one product. Let's look at some of the ways that formulations vary and how this affects your choice. As you'll see, every option involves a tradeoff.
Prepackaged combinations of two or more herbicides are becoming more common. While these pre-mixed products clearly have advantages, one issue to consider is cost. You may be able to purchase the various active ingredients separately and then combine them in the spray tank for less money. Another advantage of buying active ingredients separately is that you can choose the specific rates of each component. With a prepackaged mix, you are locked into one ratio of the active ingredients.
You must balance the added flexibility of mixing ingredients yourself against the convenience of prepackaged combinations. Especially when treating small areas, it is easier to transport and measure out one product than to carry two or three products that you have to mix together.
Esters and amines
With certain herbicides, manufacturers can synthesize the active ingredient with different “side groups.” This can alter its weed-control properties. One example is 2,4-D, a common component of broad-leaf herbicides for turf. The term “2,4-D” stands for 2,4-dichlorophenoxy acetic acid. The acid portion of the molecule can be modified by adding different chemical groups. If an isooctyl group is added to the acid, the herbicide becomes an ester. Adding a dimethylamine group converts the acid to an amine. Why is this important? Because ester formulations move more easily through the waxy cuticle covering weed leaves. This enhances herbicide uptake and provides better weed control, compared with amines.
Why, then, would you choose an amine formulation of 2,4-D if the ester controls weeds better? The major reason is safety to ornamentals near the application site. Ester forms are more volatile — they vaporize more readily. These vapors may drift and injure nearby ornamentals. Therefore, amines may be a better choice in summer when higher temperatures increase herbicide volatilization.
Ester formulations are typically a better option in winter or early spring. Temperatures are cooler and deciduous trees and shrubs have not yet leafed out. This gives you a chance to treat hard-to-kill weeds like wild garlic with the more potent ester formulations without undue danger of injury to ornamentals.
Another chemical factor that affects herbicide activity is the occurrence of different isomers. Isomers are versions of a chemical compound put together in slightly different ways. Your two hands make a good analogy — both consist of the same components, such as fingers, thumb, palm, etc., but they are put together in different ways: one is a left hand, one is a right hand. There's no way to rotate or flip a right hand to make it a left hand, and vice versa. Isomers are similar in that they consist of the same elements, but are put together in slightly different ways. Generally, one isomer of a herbicide is active in controlling weeds, while the other has little or no activity as a herbicide.
Herbicide-manufacturing processes often result in a mixture of isomers. However, Aventis, which makes fenoxaprop (Acclaim Extra), learned how to manufacture only the active form of the fenoxaprop-ethyl molecule. This allowed a nearly 50-percent decrease in fenoxaprop rates because the formulation now consists almost entirely of the active isomer. This change does not alter the spectrum of weed control; only the amount of active ingredient and application rate change.
We probably will see more of this in the manufacturing of herbicides. If a manufacturer is able to create a herbicide with more of the active isomer, it will likely phase out the old formulation and offer only the newer product.
Formulating with surfactants
Manufacturers often add materials to herbicides to change their characteristics. One example is the surfactant chemistry for glyphosate. Monsanto changed the surfactant found in traditional Roundup to create Roundup Pro. The new surfactant apparently increases the absorption rate and improves rainfastness, resulting in better weed control.
These benefits come with a potential disadvantage. Along with increasing absorption in weedy species, this surfactant change may increase the potential for injury to desirable plants should drift occur.
When a post-emergence herbicide is formulated with a surfactant, it's not necessary for you to add one when you mix the product for application. However, other products lack a surfactant in the formulation. Thus, you may need to purchase the adjuvant separately.
Make sure you thoroughly read product labels to determine whether a surfactant is recommended. Whenever you compare prices of herbicides, be sure to include the additional cost of any surfactants (or any other adjuvant) that you'll need to use.
Granular vs. liquid
Certain herbicides are available in both granular and sprayable form. The type you use may affect the degree of weed control.
Dithiopyr (Dow's Dimension) is more volatile in the liquid form than in granular form. To compensate for volatility losses (and leaf adsorption), labeled application rates for the active ingredient are higher for the liquid than for the granular product. Rohm and Haas is planning to release a sprayable (wettable powder) formulation of dithiopyr that resolves these issues.
This phenomenon does not occur with all herbicides, however. In a study I conducted comparing granular and liquid formulations of metolachlor (Syngents's Pennant), I saw no difference in weed control between the two products when I used them at the same rates.
- Leaf absorption
Another difference between granular and sprayable forms is the extent of leaf absorption. Granular formulations exhibit relatively low foliar absorption because most of the applied granules fall through the leaf canopy to the soil below. The reason granular post-emergence herbicide labels typically state that some wetness should be present during application is so that the granules will stick better to the moist leaves. Applications when moisture is absent are generally ineffective.
By contrast, sprayed products achieve good coverage and adhere better to the foliage, providing relatively good weed control.
A disadvantage of sprayable formulations is increased potential for phytotoxicity. Generally, granular forms are safer for over-the-top applications than sprayable products containing the same herbicide. One example is oxadiazon (Aventis's Ronstar). You can apply Ronstar G to a wide range of cool and warm-season turfgrasses, as well as many woody ornamentals. The sprayable formulation of oxadiazon, by contrast, is used primarily on certain dormant warm-season turfgrasses and a limited number of ornamentals. This difference is due to the greater potential for burning of leaves with the sprayable type.
Oxyfluorfen is a herbicide that has both pre-emergence and post-emergence effects on weeds. Formulated as a granule, such as in Regal's O-O, it can be applied to a range of woody ornamentals. By contrast, Dow's Goal, a sprayable form of oxyfluorfen, is labeled for use on a much more limited number of ornamentals due to the potential for leaf injury.
A disadvantage to the granular form of oxyfluorfen is that it lacks most of the post-emergence activity that the sprayable form possesses.
Another impact of manufacturing a herbicide as a granule can be greater cost. The granular type may be more expensive than sprayable products because it consists primarily of inert ingredients, often as much as 98 percent or more inactive components (such as clay). Therefore, the amount of formulated product required to deliver the same amount of active ingredient may be much greater with granules, resulting in higher shipping and, perhaps, packaging costs.
One advantage of granules is that there seems to be greater public acceptance of this form. Though not necessarily accurate, sprays evoke “toxic” images in many people's minds.
- Ease of application
Another benefit is that granules are fast and easy to apply, especially over large turf areas. Cleanup and package disposal is typically easier as well.
- Application uniformity
A potential advantage of sprayable forms over granular types is more-uniform application. Granules can be difficult to apply uniformly, especially those that contain a relatively high concentration of active ingredient. As the percent active ingredient of a granule increases, fewer and fewer granules per square foot are applied. Thus, for example, you'll find it easier to achieve uniform coverage with a 1-percent granule than with a 5-percent granule.
One issue that relates particularly to dinitroaniline (DNA) herbicides is the potential for staining of sidewalks and other structures. The active ingredients in this class of chemistry have a yellowish or yellowish-orange color. Granular formulations often do not stain badly, whereas a liquid formulation may cause more serious staining. Plus, granules are easy to sweep or blow from concrete surfaces, whereas overspray should be washed off before it dries.
Differences exist among sprayable formulations of DNA herbicides. In my studies, an emulsifiable-concentrate formulation of pendimethalin had a more intense yellow color than a water-dispersible granule.
Variables in liquids
Sprayable formulations can differ in their ease of use. Dry flowable (DF) or water-dispersible granule (WDG) formulations produce much less dust during mixing and loading than wettable powders. Another important advantage of DF and WDG formulations is their ability to quickly form a uniform suspension in the spray tank. However, dry flowables are more likely to plug spray nozzles and tips than emulsifiable concentrates.
An advantage of emulsifiable concentrates is rapid mixing in the spray tank with minimal agitation.
One final note about emulsifiable concentrates: because they contain an oil solvent, you would expect them to provide greater post-emergence weed control than a dry sprayable.
Formulations affect herbicide performance in many ways, from cost, to ease of loading, mixing and applying, to safety to effectiveness. It's impossible to generalize and conclude that one formulation is inherently better than another. Each type involves tradeoffs. The best choice depends on your specific needs.
Dr. Jeffrey F. Derr is professor of weed science at Virginia Polytechnic Institute and State University (Virginia Beach, Va.).
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