University of Illinois

No. 7/May 9, 1997

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 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 herbicide and prevailing environmental conditions.

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

The plant cuticle serves as an outer protective layer that restricts the amount of water lost by the plant through transpiration. It also serves a variety of other functions, and the cuticle can be 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 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. 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 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 spray on the target site.

Nonionic surfactants (NIS) lower the surface tension of spray droplets to allow greater spray coverage and may be referred to as spreaders or wetting agents. Surfactants are generally polyoxyethylated aliphatic alcohol, but may also contain fatty acids to improve herbicide penetration. The organo-silicone surfactants have tremendous spreading ability. 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.

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. Methylated seed oils (MSO) are fatty acid methyl esters that increase herbicide penetration. Most herbicide labels allow POC or VOC, but some specify one or the other. COCs are used at 1 to 2 pints per acre, or 0.5 to 1% by volume.

Ammonium fertilizer adjuvants are added to increase herbicide activity on certain weed species such as velvetleaf. Urea ammonium nitrate (UAN) solution (28-0-0) is the most common fertilizer adjuvant, although ammonium polyphosphate (10-34-0) or ammonium sulfate (AMS) may also be allowed. UAN is used at 2 to 4 quarts per acre, or 2 to 4% by volume. Contact herbicide labels often specify that fertilizer adjuvants replace NIS or COC, while translocated herbicides usually specify UAN in addition to NIS or COC. Mixtures of ammonium salts plus surfactant are available where a combination is desired.

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 diluted forms (1 to 2% active ingredient).

Buffer-surfactants 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 compatibility agent may contain extra free organic phosphatic acid to acidify the spray mix.

Spray-tank cleaners are used to clean pesticide and fertilizer residues from spray tanks to minimize cross-contamination of sprays. These are especially important with postemergence sprays when changing crops. Aqua ammonia or household ammonia may 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 it if ammonium fertilizer residues remain in the tank. Some herbicide labels specify detergent washes if suspendable formulations have been used in the tank.

Aaron Hager, and Marshal McGlamery, Department of Crop Sciences, (217)333-4424

Table 1. Summary of adjuvant functions and rates.

Spray adjuvantFunctions Pints/A% v/v basis
SurfactantsSpreader­ penetrant
Nonionic surfactant (75­80% ai NIS)0.5­1.00.25­0.50
Dash HC (solvents + fatty acid esters)1.00.50
Crop-oil concentratesPenetrant­ spreader­ humectant
Petroleum (POC, 83­85% oil + 15­17% NIS)1.0­2.00.50­1.00
Vegetable (VOC, 85­93% oil + 7­15% NIS)1.0­2.00.50­1.00
Ammonium fertilizer adjuvants Aid velvetleaf control, some buffering
Urea + ammonium nitrate (28-0-0 UAN)4.0­8.02.00­4.00
Ammonium polyphosphate (10-34-0 APP)1.0­2.00.50­1.00
Ammonium sulfate (21-0-0 AMS)2.5 lb/A17 lb/100gal
Drift-reduction agentsMinimize spray drift Depends upon
Depends upon