Weed identification is the key to an effective corn weed management program. Incorrect identification of problem weeds can mean the difference between profit and loss. Although a weed’s life cycle, including its method(s) of reproduction, is the most important identifying characteristic, it is sometimes necessary to know the exact species before selecting weed management measures.
Corn growers should make a weed inventory to aid in the selection of weed control programs. By tailoring control programs to fit the problems in each field, growers can minimize weed control costs while maximizing yields and profits. An inventory can be made by scouting fields two or three times during the year and recording the types (such as broadleaf annuals or annual grasses) of weeds present in each field.
The first observation should be made by the time corn is 3 or 4 inches tall. These early-season observations reveal how effective preplant or preemergence herbicides, if used, have been and suggest the possible need for cultivation or for postemergence herbicide applications. A second look at the fields in midsummer (before the corn is waist high) can provide information on the overall effectiveness of weed control practices and provide clues on how the program might be adjusted in future years. This also is a good time to record the types and numbers of weeds present and to map the location of special problem areas in the field. Additional notes on weed types and numbers can be taken at harvest to complete the weed inventory.
Weed Management Methods
Although herbicides can provide effective weed management, corn growers should not depend on herbicides alone. Growers should use good cultural practices so the corn is competitive with any weeds and should integrate chemical control programs with cultivation, especially with difficult-to-control weeds or when weather conditions reduce herbicide effectiveness.
The first step in cultural weed control is the selection of a corn hybrid that is adapted to local growing conditions. Timely planting along with proper fitting in tilled situations or proper adjustment of no-tillage planters ensures rapid germination and a competitive advantage for the corn. Another cultural practice that favors rapid establishment of corn is proper band application of fertilizer at planting.
All primary (plowing) and secondary (fitting) tillage operations help provide a weed-free seedbed. Cultivation of row crops is an effective way to control annual weeds between corn rows. Band application of herbicides over the row at planting, combined with one or two cultivations, provides good control of annual weeds such as common lambsquarters and foxtails. Although rotary hoes effectively destroy weed seedlings in small corn, a row cultivator adjusted to minimize pruning of corn roots should be used after corn is 5 or 6 inches tall. Creeping perennials such as common milkweed and quackgrass are not adequately controlled by one or two cultivations. These weeds regrow from rhizomes (underground stems) following cultivation and are controlled with tillage only if the operations are repeated over long periods. Biennial (wild carrot, etc.) and simple perennial (dandelion, etc.) weeds do not persist in fields that are plowed but can be a problem in reduced and zone/no-tillage fields.
A variety of herbicides are available for preplant, preemergence, and/or postemergence weed control in corn. These herbicides vary in their effectiveness in controlling different weeds (Table 3.7.1) and in the length of time they remain active in the soil. Some corn herbicides, such as *atrazine and Princep, can carry over to affect triazine-sensitive rotational crops such as small-seeded forage legumes, small grains, and soybeans. Knowledge of the weeds present, herbicide effectiveness, and rotational plans should be considered when selecting herbicides.
Cost of chemical weed control dictates that herbicides be applied when they will provide maximum return. Label guidelines for the timing of herbicide applications are based on research and are geared for maximum weed control and minimum crop injury. The labeled application timings for corn herbicides are shown as shaded cells in Table 3.7.2.
Factors Affecting Soil-Applied Herbicides
To be effective, soil-applied herbicides must be available for uptake by the roots and/or shoots of germinating weed seedlings. This means that they must be dissolved or suspended in the soil solution. Soil properties that affect the availability and activity of soil-applied herbicides include soil texture, organic matter level, and pH. All should be considered when determining herbicide rates.
Soil texture is determined by the relative percentages of sand, silt, and clay in a soil. Names describing texture such as loamy sand and clay loam are assigned to soils depending on these percentages. Clay particles are negatively charged and have a large surface area. As a result, soils high in clay content (heavy soils) have the capacity to adsorb or tie up herbicides and generally require higher herbicide rates than coarse-textured or light soils.
Organic matter content also affects adsorptive capacity of soils. Although un-decomposed plant and animal residues can influence herbicide performance, the well-decayed, fine organic matter particles known as humus are of greatest importance. Like clay particles, humus particles are negatively charged but exhibit an even greater capacity to adsorb or tie up herbicides than clay. Consequently, herbicide rates also have to be adjusted to the soil organic matter level.
Rates for soil-applied herbicides in the chemical weed control tables (Tables 3.7.3, 4.11.1 and 6.7.2) are for medium-textured (loam) soils with organic matter levels of 3 to 4 percent. Fine-tuning the rates for other soils can be done by consulting the herbicide label for different soil textures and for varying organic matter levels.
Soil pH can also affect the availability of some soil-applied herbicides. This is important for the triazine herbicides (*atrazine and Princep). These herbicides are most strongly adsorbed (tied up and unavailable for uptake by weeds) on clay and organic matter particles at low pH levels. Although the amount of triazine adsorption increases at all pH levels below 7.0, adsorption is most dramatic at pH levels of 6.0 and below. This is an important consideration for contin-uous zone/no-tillage fields where the surface inch of the soil profile may have a lower pH than is reflected in the results of a normal soil sample analysis. It may be helpful to check the pH in the top inch of the soil profile with a soil pH kit in fields that have been in zone/no-tillage for extended periods.
Soil pH also affects availability of †Python WDG (flumetsulam) and soil applications of *†Hornet WDG (clopyralid + flumetsulam). These herbicides should not be applied to areas where soil pH is greater than 7.8 as this may result in unacceptable crop injury. In addition, soil applications of †Python WDG or *†Hornet WDG should not be made to soils with more than 5% organic matter if soil pH is less than 5.9 as reduced weed control will result.
Herbicide Resistance Management
Triazine-resistant biotypes or strains of common lambsquarters, smooth pigweed, common ragweed, and common groundsel have been confirmed in New York State. Populations of these weeds were originally controlled with one or more of the triazine herbicides (*atrazine and Princep) at normal use rates; however, shifts to weed populations dominated by the triazine-resistant strains have occurred in many locales. These triazine-resistant strains are not controlled with extremely high triazine use rates. In addition, there is cross-resistance among the different triazine herbicides used in corn, soybeans (Sencor), and alfalfa (Sencor and Velpar).
This situation has prompted refinements in the control guidelines for annual broadleaf weeds in field corn. Triazine herbicides have played, and will continue to play, an important role in field corn weed control programs; however, effective control programs for these triazine-resistant strains will involve the use of crop rotation and cultivation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will also delay development of weed populations that are resistant to triazine and other herbicide groups.
Rotating herbicides with different sites of action and the use of tank mixes or sequential applications that involve herbicides with different sites of action are key elements in herbicide resistance management plans. To do this most effectively, everyone involved in decisions about weed management must have site of action classification for herbicides readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619). In this system, a group number is given to all herbicides with the same site of action. To further efforts in management of existing herbicide-resistant weed populations and to delay or avoid development of new herbicide-resistant weed populations, these “GROUP NUMBERS” are included in the “Chemical weed control tables” in each crop section of this guide. Since herbicide resistance management is most effective when practiced across all crops in rotation, “GROUP NUMBERS” for all herbicides in the Cornell Guide For Integrated Field Crop Management can be found in Table 8.2.1. Mode of action/site of action and chemical families for site of action groups can be found in Table 8.2.2.
Weed Control in Zone/No-Tillage Corn
Weed control in zone/ no-tillage corn is more difficult than in reduced or conventionally tilled and planted fields for two reasons. First, some weed control options are eliminated in zone/ no-tillage cropping systems. Second, some weeds are more aggressive and/or more difficult to control in zone/ no-tillage situations. Effective control programs require proper identification of the existing and potential weed problems and knowledge of herbicides available. Chemical weed control in zone/ no-tillage corn may involve the following:
1. Control of existing vegetation, before or at the time of planting, with a burndown herbicide. Herbicides used for this purpose include *Gramoxone Inteon or the various formulations of glyphosate including Roundup products, Durango DMA, Touchdown Total, etc. If burndown applications are made in combination with residual (preemergence) herbicides, they eliminate competition from existing vegetation long enough to get the corn established and to allow for rainfall activation of the residual herbicides. If residual herbicides are not used, burndown treatments provide a window for total postemergence weed control programs to be applied. With herbicide-resistant corn, like glyphosate-resistant (Roundup Ready) or Liberty Link hybrids, burndown and residual herbicides may be applied early postemergence in one-pass.
2. If residual herbicides are not applied with the burndown application, weed control in zone/no- tillage corn may require preemergence or early postemergence herbicide applications to control weeds that emerge after planting. In addition, postemergence herbicides may be required to control escaped weeds such as dandelion.
Zone/no-tillage burndown guidelines are given in Table 3.7.4. Refer to Table 3.7.3 for preemergence or total postemergence weed management guidelines for field corn.
Prepackaged Herbicide Mixtures
There are many prepackaged herbicide mixtures for field corn. Although it is usually known which herbicides are included, questions do arise about the relative amounts of each herbicide in these mixtures. Information about the active ingredients in some of these products is summarized in Table 3.7.5.
Prepackaged herbicide mixtures eliminate the inconvenience of tank mixing herbicides to obtain broad spectrum (grass and broadleaf) weed control needed in most fields. Not only do they make spraying easier, but they can eliminate some of the problems associated with container disposal. Finally, these mixtures often can be purchased for less than the cost of equivalent amounts of the individual products. The disadvantage of these premixes is that the user sacrifices some flexibility in adjusting the rates of each herbicide to what is needed in a given situation.
*Atrazine Application Rates
*Atrazine applications for corn are restricted to a maximum of 2.5 pounds active ingredient per acre (lb. ai/A) per calendar year. In addition, there is a per-application maximum of no more than 1.6 or 2 lb. ai/A depending on soil erodibility (as defined by NRCS) and the amount of crop residue cover.
For preplant and preemergence applications on highly erodible soils with more than 30 percent crop residue cover, use up to 2 lb. ai/A (4 pt./A) per application. For highly erodible soils with less than 30 percent crop residue cover, use 1.6 lb. ai/A (3.2 pt./A). On soils that are not highly erodible, use up to 2 lb. ai/A (4 pt./A). For postemergence applications, a maximum of 2 lb. ai/A (4 pt/A) may be applied if no *atrazine was applied before corn emergence. If a postemergence treatment is required following a preplant or preemergence application, the total *atrazine applied may not exceed 2.5 lb. ai/A (5 pt./A) per calendar year.
Calculating the lb. ai/A of *atrazine is relatively simple when using products that contain *atrazine only. It may be difficult to calculate the total amount of *atrazine being applied when using prepackaged herbicide mixtures. This is especially true if both preplant/preemergence and postemergence premix products are being used. Table 3.7.6 gives common labeled application rates of each premix product along with the lb. ai/A of *atrazine being applied.