Principles of Postemergence Herbicides

May 12, 2000
Postemergence herbicides are an integral part of an integrated weed-management program. Applications made after crops and weeds have emerged allow for identification of the weed species present as well as the severity of infestation so that herbicide selection can be tailored to a particular field. Postemergence herbicide applications minimize the interactions of the herbicide with factors associated with soil (such as soil texture and organic matter content), but often magnify interactions between the herbicide and prevailing environmental conditions.

To achieve weed control with postemergence herbicides, the herbicide must come in contact with the target, be retained on the leaf surface prior to absorption into the plant, be able to reach the site of action within the plant, and finally induce some phytotoxic response. If for any reason one or more of these steps is restricted or limited, the level of weed control can be expected to decline.

The plant cuticle serves as an outer protective layer, or "barrier," that restricts the amount of water lost by the plant through transpiration. It also serves a variety of other functions, and the cuticle is often considered the primary barrier that limits herbicide absorption. The cuticle is composed primarily of waxes and cutin, substances that effectively limit water movement out of (transpiration) or into (absorption) the plant. The type and amount of wax that comprises the cuticle influence the degree of wetting that can be achieved, and this composition can change with age and in response to changes in the environment. Older plants and plants under environmental stress generally have more wax or a different structure of the wax comprising their cuticles and are thus more difficult to wet. One of the main functions of certain spray additives is to enhance herbicide penetration through the cuticle.

Plant age and size, relative humidity, soil moisture, and temperature are other factors that influence absorption of postemergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend that applications be made when target weeds are small and caution of reduced effectiveness if applications are made to larger plants. More and more postemergence herbicide labels are also cautioning users to delay applications if weeds are under "adverse environmental conditions." Examples of such adverse environmental conditions may include prolonged periods without significant precipitation (dry soil) or low air temperatures. On the other hand, high relative humidity, adequate soil moisture, and moderate-to-warm air temperatures all favor enhanced herbicide absorption. Remember that if conditions occur for enhanced absorption into weeds, conditions are also favorable for enhanced absorption into the crop, which may result in crop injury.

Postemergence herbicides vary in their mobility with the plant. Some demonstrate very limited movement following absorption and are commonly referred to as "contact" herbicides. Others can move extensively within the vascular elements of the plant and are referred to as "translocated" herbicides. Contact herbicides do show some limited movement following absorption but not nearly as extensively as translocated herbicides. Thorough spray coverage of the plant foliage is very important with contact herbicides and somewhat less important with translocated herbicides.

Adjuvants are added to the spray mix to improve herbicide performance and minimize potential failures under adverse conditions. The most common adjuvants are nonionic surfactants (NIS), crop oil concentrates (COC), and ammonium fertilizer salts. These are used to increase the effect of the herbicide on the target plants. Table 1 is a summary of herbicide adjuvants, their functions, and their application ratios. Labeled adjuvants for postemergence corn and soybean herbicides are shown in Table 2 and Table 3, respectively.

Nonionic surfactants (NIS) lower the surface tension of spray droplets, thus increasing spray coverage, so they are frequently referred to as spreaders or wetting agents. Surfactants are generally polyoxyethylated aliphatic alcohols but may contain fatty acid esters to improve herbicide penetration. Herbicide labels may specify that the NIS should contain a minimum of 75% to 80% active ingredient or otherwise use a higher rate of NIS. NIS is usually applied at 0.5 to 1 pint per acre or 0.125% to 0.5% on a volume basis. Organosilicone surfactants have terrific spreading ability but poor penetration.

Surfactant molecules can be synthesized to achieve specific solubility characteristics, referred to as the hydrophilic-lipophilic balance (HLB). HLBs vary from 1 to 40, with 1 to 20 being most common. Higher HLBs indicate more water (hydro-) solubility, while lower HLBs indicate oil (lipo-) solubility. HLB numbers may be specified on surfactant technical information sheets.

Ammonium fertilizer adjuvants are added to increase herbicide activity on certain weed species such as velvetleaf. The two most common ammonium fertilizers used are ammonium sulfate (AMS) and urea ammonium nitrate (UAN) solution (28-0-0). Another ammonium fertilizer that is sometimes used is ammonium polyphosphate (10-34-0). AMS is used at 2.5­4.0 pounds per acre or 8.5­17 pounds per 100 gallons of spray solution. UAN is used at 2 to 4 quarts per acre or 2% to 4% by volume. Contact herbicide labels may specify that fertilizer adjuvants replace NIS or COC, while translocated herbicides often specify the addition of UAN or AMS to NIS or COC. Mixtures of ammonium salts plus surfactant are available where a combination is desired.

Crop oils are phytobland (low aromatic content), paraffinic oils of 70- to 110-second viscosity (water = 1, 30W oil = 300). These generally contain 1% to 2% emulsifier (surfactant) so they will mix with water. Crop oil is a misnomer, as these are actually derived from petroleum but are used on crops. They are also called "mineral oil" or dormant spray oils. Crop oil concentrate (COC) was developed when the oil crisis raised the price of "mineral oil." COC is another misnomer, as it contains less oil and more emulsifier than "crop oil." The concentration factor was a use rate of 1 to 2 quarts for COC versus 1 to 2 gallons per acre for "crop oil."

Crop oil concentrates (COC) are phytobland oils with emulsifiers added to allow mixing with water. The oil may be of petroleum (POC) or vegetable (VOC) origin. Oils increase spray penetration through the leaf cuticle. POCs contain 83% to 85% oil and 15% to 17% emulsifier, while VOCs contain 85% to 93% refined vegetable oil and 7% to 15% emulsifier. Most herbicide labels allow POC or VOC but some may specify POC only. COCs are used at 1 to 2 pints per acre or 0.5% to 1% by volume.

Vegetable oils originated with "Bioveg" in the early 1970s. The linseed (flax) oil market was hurt by latex paints, so another use was sought for linseed oil. However, palm oil was much cheaper than linseed or sunflower oil. The American Soybean Association also began to promote soybean oil. It was soon found that soybean oil needed to be refined to remove waxes, which raised the price. Bladex allowed VOC but not POC under droughty conditions indicating relative activity (safer to corn, i.e., less foliar activity), while Assure II and Classic specified use of petroleum oil rather than vegetable oils because of more activity.

Methylated seed oil (MSO). It was discovered that seed oils could be fractionated to recover their fatty acids, and these fatty acids could be methylated to form their methyl esters, drastically increasing their penetration. One of the first was Sun-It made from sunflower oil, but Sun-It II or Scoil was made from soybean oil. Now MSOs have similar activity to POCs.

Compatibility agents are spray-tank adjuvants added to improve mixing, especially with a liquid fertilizer spray carrier. Compatibility agents are usually phosphatic esters of alkyl, aryl, polyoxy ethanol, or ethylene glycol plus an alcohol solubilizer. Extra phosphatic acid may be added for buffering (acidifying) effects. Herbicide labels often specify a "jar test" to determine the need for a compatibility agent when mixing herbicides with liquid fertilizer. The rate is usually 1 to 4 pints per 100 gallons of spray mix.

Drift reduction agents are added to the spray tank to reduce small-droplet formation and thus minimize drift problems. They are polyacrylamide or polyvinyl polymers. The use rate per 100 gallons of spray is 2 to 10 fluid ounces of concentrated forms and 2 to 4 quarts of dilute forms (1% to 2% active ingredient). Many drift-reduction agents are now sold in combination with a retention agent or a retention agent and AMS.

Buffer-surfactant or buffer-compatibility agents contain organic phosphatic acids that provide an acidifying effect on spray mixes where a pesticide is affected by alkaline water. Most herbicides do not need a buffering agent, and some sulfonylureas such as Classic and Pinnacle should not be acidified. A buffering effect is provided by ammonium sulfate added to the spray mix to help some herbicides such as Roundup. A compatibility agent may contain extra free organic phosphatic acid to acidify (buffer) the spray mix.

Defoamers are added to the spray tank to minimize foaming and air entrapment, particularly where spray agitation is excessive. Defoamers are usually dimethylpolysiloxane products. Many surfactants already have defoamer added to minimize foaming problems.

Foaming agents are often used in marking systems to indicate spray-boom width. These are usually modified alkylsulfate alcohols that provide a semistable foam.

Spray-tank cleaners are used to clean pesticide and fertilizer residues from spray tanks to minimize cross-contamination of sprays. These are very important with postemergence sprays, especially when changing crops. Aqua ammonia or household ammonia can also be used as a spray-tank cleaner and is recommended when 2,4-D or dicamba has been in the spray tank. Hypochlorite bleach has been used, but do not use if ammonium fertilizer residues remain in the tank.

There are more than 400 commercial adjuvants from over 34 different companies available for use in commercial agriculture. A complete listing of these products is compiled every 2 years by Dr. Bryan Young at Southern Illinois University. This compilation, The Compendium of Herbicide Adjuvants, contains descriptions of each commercial product by adjuvant type and also some standard adjuvant terms and definitions. The compendium may be purchased for $3.00 per copy from Southern Illinois University and can be viewed on the Web at Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague