When designing a weed control program based on herbicide use, consider soil type, tillage practices, crops (current and following), weed problems, and overall farming operations. No one herbicide will control all species of weeds, so it is important to select herbicides based on the weeds present in a field. Herbicides are often combined to control more weed species, reduce carryover, or reduce crop injury. Some weeds are not controlled by any of the currently available selective herbicides, and require specialized application of nonselective herbicides. Good herbicide performance depends upon the weather, soil conditions, and accurate application.
Soil-applied herbicides have traditionally been the mainstay of herbicide programs. Preplant and preemergence herbicides have the advantage of eliminating early competition between crops and weeds. However, research has shown that weeds will not reduce crop yields if controlled within 4 to 6 weeks after emergence. Postemergence herbicides are comparable to soil-applied herbicides in effectiveness and economics if applied within this time period. Some weeds are better controlled by soil-applied herbicides, while others are more susceptible to postemergence herbicides. Consider combining soil-applied and postemergence herbicides for maximum control of some populations of weed species.
Herbicide Nomenclature and Formulations
There is often more than one formulation of a particular herbicide. This can make selection and application of various products somewhat confusing. Each herbicide has a trade name (sometimes more than one), a common name, and a chemical name. For example, Lasso and Arena are registered trade names, alachlor is the common name, and 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide is the chemical name for a herbicide used in corn and soybeans.
Prepackaged mixtures typically go by a single trade name yet may contain two or more different herbicides. For example, Bicep (trade name) is actually a mixture of metolachlor and atrazine. For this reason it is important to know common names of herbicides so that one knows exactly what a product contains.
Herbicides are formulated and sold in various liquid or solid formulations, depending upon (1) the solubility of the active ingredient in water, and (2) the manner in which the product is applied (i.e. dispersed in water or applied in the dry form). Formulation type is listed on the herbicide label and may be designated by a letter or letters following the trade name. A herbicide label must also give a list of all ingredients and the amount of active ingredient contained in the product. For liquid formulations, amount of active ingredient contained is expressed both as a percentage of the total ingredients and as the number of pounds in a gallon of product. Active ingredients contained in dry formulations are expressed only as a percentage by weight. Several formulations and abbreviations are listed below.
Emulsifiable concentrate (E or EC) - a liquid formulation containing various emulsifiers that aid in dispersion of the active ingredient in water.
Water Soluble (S, AS, or WS) - usually a liquid formulation containing the active ingredient, water, and sometimes a surfactant and antifreeze agent.
Oil Soluble (OS) - a liquid formulation containing the active ingredient dissolved in oil or some other organic solvent. These herbicides must be applied in an oil-based carrier such as diesel fuel or kerosene.
Liquid Flowable (F or LF) - a thick liquid with a slurry-like consistency containing the active ingredient, water, and stabilizers to help active ingredient stay in suspension. Spray tank agitation is necessary to keep flowable formulations from settling out of suspension.
Wettable Powder (W or WP) - a dry powder containing the active ingredient, a diluent, usually bentonite or attapulgite clay, and surfactants. Spray tank agitation is necessary to avoid settling.
Dry Flowable (DF) - a dry formulation consisting of herbicide-impregnated granules that easily disperse in water. Dry flowables are easier to handle and measure than wettable powders. Spray tank agitation is necessary to avoid settling.
Dispersible Granules (DG) or Water-Dispersible Granules (WDG) - a dry formulation very similar to dry flowable formulations. The ingredients are in granules which easily disperse in water. Spray tank agitation is necessary to avoid settling.
Granules (G) - a dry formulation consisting of the active ingredient coating or adhered to some type of inert granule such as clay, vermiculite, or sand. These formulations are applied just as they are purchased with no mixing. Special granular application equipment is required.
Pellets (P) - a dry formulation of active ingredient coating some type of inert pelleted material - similar to granules only much larger. Pellets are applied directly to the target area by hand or with special spreaders.
Herbicide rates are sometimes expressed on an active ingredient basis in technical publications. To convert pounds of active material to pounds of a commercial dry formulation:
| Pounds of commercial product/A = | Pounds of active ingredient/A | x 100 |
| Percent Active Ingredient in Product |
To convert pounds of active material to gallons of a commercial liquid formulations:
| Gallons of commercial product/A = | Pounds of Active Ingredient/A |
| Pounds of Active Ingredient in Gallon |
Soil-Applied Herbicides
Soil-applied herbicides can be applied preplant (or early preplant), preplant incorporated, or preemergence to the crop. Activity of these herbicides is affected by soil texture, organic matter content, pH, moisture, and tillage. Most soil-applied herbicides are more available for plant uptake in coarse-textured, low organic matter soils than in fine-textured, high organic matter soils. Many herbicide labels specify application rates based on soil texture and organic matter content. Herbicides are more likely to injure crops in sandy soils low in organic matter, and careful herbicide rate selection is required to avoid injury. Some herbicides are not labeled for use in sandy soils low in organic matter.
Soil pH can influence the activity of some herbicides. Triazine herbicides are more available for plant uptake in soils with high pH, resulting in better weed control and increased risk of crop injury. The low pH conditions that can result from continuous no-tillage may reduce the availability of some herbicides, resulting in less effective weed control. In general, herbicides are most effective when soil pH is in the range recommended for optimum crop growth.
Soil-applied herbicides require adequate soil moisture for activity. Uptake of herbicide into germinating weeds occurs from the soil solution only. In the presence of adequate soil moisture, less herbicide is adsorbed to the soil and more available in the soil solution for weed control. Under dry conditions, herbicide is tightly adsorbed by soil colloids, and insufficient herbicide may be available in the soil solution to provide acceptable weed control. Preemergence herbicide applications require rainfall, usually at least 1/2 inch, to move herbicide from the soil surface into the zone of weed seed germination and emergence.
Early preplant herbicides are applied several weeks to a month or more prior to planting without incorporation. Herbicides with a fairly long period of residual soil activity can be applied early preplant. Herbicides with a shorter soil residual may not provide late season control when applied early. Early preplant herbicide programs frequently do not provide adequate season-long annual grass control in fields with high grass populations.
Advantages
Disadvantages
Preplant incorporated herbicides are incorporated into the soil prior to planting. Incorporation of some herbicides is necessary to prevent surface-loss from volatility or photodecomposition. Other herbicides are incorporated to reduce the dependence upon rainfall required to more herbicide into the zone of weed seed germination. Incorporation also provides the herbicide placement required for control of some weeds, especially perennials.
Advantages
Disadvantages
The depth and thoroughness of incorporation depend upon the type of equipment used, the depth and speed of operation, soil texture, and the amount of soil moisture. Incorporation should place the herbicide uniformly throughout the upper 1 to 2 inches of soil for best control of small-seeded annual weeds that germinate at shallow depths. Slightly deeper placement may improve control of certain weeds from deep-germinating seeds,especially under dry conditions. The field cultivator and tandem disk place most of the herbicide at about one-half the depth of operation. The suggested depth of operation for these implements is thus 3 to 4 inches.
The most thorough and uniform incorporation, especially with a tandem disk or field cultivator, requires two passes at an angle to each other. However, some of the newer combination tools can provide more uniform single-pass incorporation. The effectiveness of single-pass incorporation can depend upon the soil conditions as much as the implement used to incorporate. One-pass incorporation is not a good approach with less than optimum soil tilth. Incorporation in soils that are too wet can result in streaked weed control; this may be increased with one-pass as compared to two pass incorporation. High crop residue levels make one-pass incorporation difficult. Two-pass incorporation is advised if residue is sufficient to clog the incorporation tool.
Field cultivators are frequently used for herbicide incorporation. Two passes are recommended for uniform weed control, but field cultivators can give acceptable one-pass incorporation with proper set-up and operation. They should have 3 or more shanks with an effective shank spacing of no more than 8 to 9 inches. These shanks can be equipped with points or sweeps. Sweeps usually provide better incorporation, especially when soil conditions are too wet or dry for optimum soil flow and mixing. Sweeps for "C" shank cultivators should be at least as wide as the effective shank spacing. For one-pass incorporation, wider sweeps or narrower spacing with a 3- to 5- bar harrow or rolling baskets pulled behind will provide more uniform incorporation and weed control.
The recommended operating depth for the field cultivator is 3 to 4 inches. The ground speed should be at least 6 miles per hour. The field cultivator must be operated in a level position. If the back shanks are lower than the front, untreated soil will be brought to the surface and the result will be streaked weed control.
Tandem disk harrows invert the soil and usually place the herbicide deeper in the soil than most other incorporation tools. Tandem disks used for herbicide incorporation should have disk blade diameters of 20 inches or less and blade spacings of 7 to 9 inches. Spherical disk blades provide better herbicide mixing than conical disk blades.
The suggested operating depth for incorporation of most herbicides is from 3 to 4 inches. Two passes are recommended to obtain uniform incorporation with a double disk. A leveling device (harrow or rolling baskets) should be used behind the disk to obtain proper mixing. Recommended ground speeds are usually between 4 and 6 miles per hour. Speed should be sufficient to move the soil the full width of the blade spacing.
Combination tools are tillage and incorporation tools that combine disk gangs, field cultivator shanks, and leveling devices. Many of these tools can handle large amounts of surface residue without clogging, and still leave considerable residue on the surface for erosion control. Combination tools may provide more uniform one-pass incorporation than a disk or field cultivator. Good soil tilth is still a prerequisite for effective one-pass incorporation. One-pass incorporation with these tools is generally no better than two passes with a disk or field cultivator.
Preemergence herbicides are applied to the soil surface after the crop is planted but before crop seedlings and weeds appear above the ground. For maximum preemergence activity, 1/2 to 1 inch of rainfall should occur within one week following application. Where this rain does not occur, a rotary hoe is recommended for control of weed escapes as they are emerging.
Advantages
Disadvantages
Postemergence Herbicides
Postemergence herbicides are applied after the crop and weeds have emerged. Most postemergence herbicides have foliar activity only, while a few do provide foliar and soil activity.
Advantages
Disadvantages
In order to achieve effective postemergence control, it is critical to follow label recommendations on rate and timing of application, weed species controlled, and the use of spray additives. The rate and timing of application are based on weed size and climatic conditions. Weeds can usually be controlled with a lower application rate when they are small and tender. Larger weeds often require a higher rate or an additional spray additive, especially if the weeds have developed under droughty conditions. Avoid applying postemergence herbicides during abnormally cool or dry weather, since weeds may not be actively growing under these conditions. Delaying application until weeds resume active growth will ensure better control. Herbicide penetration and action are usually greater when the temperature and relative humidity are high, resulting in better weed control and greater crop injury.
Many of the herbicides applied postemergence cause some crop injury, and crop size limitations may be specific on the label to minimize injury. Weed control can be reduced if rainfall occurs too soon after application. Postemergence herbicides labels specify an interval of 1/2 to 8 hours between application and rainfall, depending upon the herbicide.
The use of an adjuvant such as surfactant, crop-oil concentrate, or fertilizer solution is often recommended to improve spray coverage and herbicide uptake. Weed control may be increased with the use of additives, but crop injury may be increased also. For this reason, follow label directions regarding the use of additives.
Other considerations for postemergence applications are spray volume, pressure, and nozzle selection. Translocated herbicides (move throughout plant) can be effective with partial foliar coverage, while contact herbicides (active only where in contact with plant) require more complete spray coverage. Foliar coverage increases as spray volume and pressure are increased. For contact herbicides, 20 to 40 gallons per acre are often recommended for ground application. Translocated herbicides can often be applied in a minimum volume of 5 to 10 gallons per acre. Minimum spray pressures of 30 to 40 psi are recommended; this pressure range produces smaller droplets and improves penetration of dense canopies. Flat-fan nozzles are generally preferred for postemergence applications, although some herbicides can be applied with flood-type nozzles. However, most labels do not recommend the use of low-pressure flooding nozzles for postemergence application.
Directed Postemergence Herbicides
Directed postemergence applications minimize crop injury because herbicide is placed on the weeds rather than on the crop. Precise application and a height differential between the crop and the weeds are required for directed applications. If the weeds are smaller than the crop, spray can be directed at the base of the corp so that little herbicide reaches the upper parts of the crop plant. Spray pressure should be set fairly low for this type of application, so that fine spray particles or mist are not produced. Safety of applications directed at the base of the crop will be increased through the use of shielded nozzles.
"Wipers" (sponge or rope wick applicators) operate above the crop canopy to control weeds growing taller than the crop. This type of application works best for soybeans, since weeds must generally be at least a foot taller than the crop. Control of johnsongrass, hemp dogbane, and volunteer corn is often achieved using concentrated solutions of Roundup Ultra, Touchdown, or postemergence grass herbicides applied in this manner.
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