Principles of Postemergence Herbicides

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 herbicide selection may 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 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 often is considered the primary barrier limiting 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 influences 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. A main function 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 influencing absorption of postemergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend applications be made when target weeds are small, cautioning of reduced effectiveness if applications are made to larger plants. More and more postemergence herbicide labels also are cautioning users to delay applications if weeds are under "adverse environmental conditions." Examples of such adverse environmental conditions may include low air temperatures or prolonged periods without significant precipitation (dry soil). By contrast, 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 extensive as is seen with translocated herbicides. Thorough spray coverage of the plant foliage is very important with contact herbicides and somewhat less important with translocated herbicides.

Summary of Herbicide Adjuvants

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 spray on the target site. Labeled adjuvants forpostemergence corn and soybean herbicides are shown in Table 2 and Table 3, respectively.

Table 2. Spray adjuvants for postemergence corn herbicides

¹COC = crop-oil concentrate, either of petroleum or vegetable-oil base
POC = petroleum-oil concentrate only
MSO = methylated seed oil
NIS = nonionic surfactant
NH₄ = ammonium fertilizer (UAN, APP, AMS)

Nonionic surfactants (NIS) lower the surface tension of spray droplets, thus increasing spray coverage, so they are often referred to as spreaders or wetting agents. Generally, surfactants are polyoxyethylated aliphatic alcohols, but that may contain fatty acid esters to improve herbicide penetration. Herbicide labels may specify that the NIS should contain a minimum of 75 to 80 percent 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 percent on a volume basis. Table 4 is a listing of some nonionic surfactants. Organo-silicone surfactants have terrific spreading ability but poor penetration. Organo-silicone surfactants are listed in Table 5.

Table 4. Nonionic surfactants (NIS) used with herbicides

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 velvet-leaf. Urea ammonium nitrate (UAN) solution (28-0-0) is the most common fertilizer adjuvant, although ammonium polyphosphate (10-34-0) or ammonium sulfate (AMS) also may be allowed. UAN is used at 2 to 4 quarts per acre, or 2 to 4 percent by volume. Contact herbicide labels may specify that fertilizer adjuvants replace NIS or COC, while translocated herbicides often specify UAN in addition to NIS or COC. Mixtures of ammonium salts plus surfactant are available where a combination is desired (Table 6).

Table 6. Surfactant and N-fertilizer premixes for use with herbicides

Crop oils are phytobland (low aromatic content) paraffinic oils of 70 to 110 second viscosity (water = 1, 30W oil = 300). These oils generally contain 1 to 2 percent emulsifier (surfactant) so they can mix with water. Crop oil is a misnomer, as these oils actually are derived from petroleum but are used on crops. They are also called "mineral oils" 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 per acre for COC, versus 1 to 2 gallons 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 percent oil and 15 to 17 percent emulsifier, while VOCs contain 85 to 93 percent refined vegetable oil and 7 to 15 percentemulsifier. Most herbicide labels allow POC or VOC; but some, such as Assure II and Classic, specify POC only. COCs are used at 1 to 2 pints per acre or 0.5 to 1 percent by volume. Some POCs are listed in Table 7.

Table 7. Petroleum-oil concentrates (POC) used with herbicides

Vegetable oils originated with "Bioveg" in the early 1970s. The linseed (flax) oil market was hurt by growing use of 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 berefined to remove waxes, which raised the price. Bladex allowed VOC but not POC under drouthy conditions, indicating relative activity (safer to corn, that is, possessing less foliar activity), while Assure II and Classic specified use of petroleum oil rather than vegetable oils due to more activity. VOCs (regular and methylated) are listed in Table 8.

Table 8. Vegetable-oil concentrates (VOC) used with herbicides

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. Table 9 is a listing of some commercial compatibility agents.

Table 9. Compatibility agents used with herbicides

Drift-reduction agents are added to the spray tank to reduce small droplet formation and thus minimize drift problems. These agents are poly-acrylamide 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 percent active ingredient). Some drift-reduction agents are listed in Table 10.

Table 10. Drift-reduction agents used with herbicides

Buffer/surfactants or buffer-compatibility agents contain organic phosphatic acids that provide an acidifying effect on spray mixes for which a pesticide is affected by alkaline water. Most herbicides do not need a buffering agent, and some sulfonyl-ureas 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. Table 11 is a listing of some available commercial buffer-surfactants and buffer-compatibility agents.

Table 11. Buffer/surfactants and buffer-compatibility agents to acidify spray mixes

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 a defoamer added to minimize foaming problems. Some commercial defoamers are listed in Table 12.

Table 12. Defoamers used in spray mixes

Foaming agents are often used in marking systems to indicate spray boom width. These agents usually are modified alkylsulfate alcohols that provide a semi-stable foam. Some commercial foaming agents are listed in Table 13.

Table 13. Foaming agents used in spray-swath markers

Table 14. Spray-tank cleaners for rinsing pesticide residues from sprayers

Aaron Hager (hagera@idea.ag.uiuc.edu) and Marshal McGlamery (mmcglame@ uiuc.edu), Department of Crop Sciences, (217)333-4424