Weed Management

Alan C. York, Crop Science Department, N. C. State University
A. Stanley Culpepper, Agronomy Department, University of Georgia

Weed management is one of the most important components of a corn production system. Herbicides are normally necessary in profitable production systems. However, herbicides are only one component of a weed management program. Weed management is most successful when an integrated strategy consisting of crop rotation, good agronomic practices that promote crop competition with weeds, cultivation when appropriate, and judicious herbicide use are combined in a planned and coordinated program. Herbicides should be used only as needed and only after careful consideration of the weed problems and the impact those weeds will have on production efficiency of both the current and future crops.

Weed Management Strategies to Use in Conjunction with Herbicides

Crop Rotation

Crop rotation should be an integral component of a weed management program. Crop rotation generally leads to healthier crops that are more competitive with weeds. Moreover, certain weeds are more easily or more economically managed in one crop than in another. In general, most weeds are more easily managed in corn or soybeans than in other agronomic or horticultural crops. Good control in corn can reduce weed problems in rotational crops. Additionally, crop rotation allows use of different herbicide chemistries on the same field in different years. This can prevent weed population shifts (changes in the species composition), avoid evolution of herbicide resistance, and help to keep the overall weed population at lower levels.

Some herbicides may carry over and damage rotational crops. Before using any herbicide, consider your rotational plans and check the rotational restrictions on the label. Rotational restrictions for corn herbicides on a number of common crops are listed in Table 8-5.

Cultivation

Most corn is no longer cultivated as more growers use no-till methods and more effective herbicide programs are available. However, cultivation can supplement chemical control, and used alone it may be sufficient for light weed infestations. Weed control is the only benefit of cultivation except where special soil problems such as severe crusting or poor drainage occur. Cultivation should be shallow to reduce crop root damage and to avoid breaking through any residual herbicide layer and bringing up untreated soil and weed seed. Cultivation is most effective when weeds are small. If postemergence herbicides are planned, do not cultivate for at least a week before or after herbicide application.

Selecting a Herbicide Program

Before selecting a herbicide program, you should know what weeds are present or expected to appear, your soil texture and organic matter content, capabilities and limitations of the various herbicides, and how to best apply the herbicides. Weed Scouting

Fields should be surveyed for weeds during the growing season and in the fall. On a field map or another suitable place, record the species present and the general level of abundance. If the weeds are not uniformly distributed across large fields, it is helpful to designate areas of fields infested with particular weeds as it may be feasible to treat only portions of fields or to treat portions of fields differently. Knowledge of the weed problems during the growing season and those weeds present in the fall which escaped treatment is very helpful in planning for subsequent years. By knowing ahead of time what species to expect, you can better plan your crop rotations and herbicide programs. Considering weed maps over a period of years will improve your accuracy in predicting problems and help you detect shifts in the weed population.

Scouting to determine the need for postemergence herbicides or cultivation should begin about 10 to 14 days after planting. Fields should then be scouted weekly for the next 4 to 5 weeks. It is important to detect and identify weed problems early so an appropriate treatment can be applied at the proper time. Application timing is critical for success with postemergence herbicides. Soil Characteristics

For best weed control and to avoid crop injury, application rates of most soil-applied herbicides must be adjusted according to soil texture and organic matter content. In addition, soil texture and organic matter content may determine whether certain herbicides can be used on a given field. Some herbicides cannot be used on coarse-textured, low organic matter soil while others cannot be used on soils with a high organic matter content.

Reports on soil samples submitted to the NCDA&CS, Agronomic Division, list the percentage of humic matter, not the percentage of organic matter. Herbicide activity is probably more closely related to humic matter content than to organic matter content. However, herbicide labels specify application rates based upon organic matter content.

There is not a reliable conversion between humic matter and organic matter contents. Fortunately for most corn herbicides, it is not necessary to know the exact organic matter content. Suggested rates of most soil-applied corn herbicides are based upon organic matter content being less than or greater than 3 percent. Soil-applied herbicides are generally not used on soils classified as organic on NCDA&CS reports. Soils classified as mineral/organic on the NCDA&CS reports will have greater than 3 percent organic matter. Soils classified as mineral on NCDA&CS reports will usually have 3 percent or less organic matter.

If you need to know precisely the organic matter content, it is advisable to have a determination made by a private laboratory. Except on newly cleared land, one soil test for organic matter every 5 years is sufficient. Herbicide Selection

Before using any herbicide, you should know the important capabilities and limitations of various products labeled for the particular use. You need to know what weeds are controlled by a given product, what rate to apply, when to apply it, how best to apply it, the crop injury potential, rotational restrictions, Worker Protection Standard requirements, and any special precautions related to crop safety or environmental protection. Product labels are the best source for most of this information. Much of the necessary information has been compiled in the tables that begin on page 103 to help you in herbicide selection.

Surface and Groundwater Protection

When properly used, herbicides are environmentally safe. If improperly used, they may enter surface waters and ground water. The practices outlined below are practical ways to avoid contaminating water resources with herbicides or other pesticides.

General Guidelines

Store pesticides properly and away from wells. Store pesticides in their original containers in a secure, dry, well-ventilated facility at least 100 feet from a well. Do not store near combustible materials.

Calibrate sprayers accurately. Calibrate your sprayer at the beginning of the season and recheck the calibration periodically during the season. Recalibrate anytime nozzles are exchanged or pressure or travel speed are changed. Improper calibration can lead to poor weed control if the sprayer applies less than intended or crop injury if it applies more than intended. Either situation can reduce profits. Applying excess herbicide may also increase the potential for environmental contamination.

Proper handling of pesticides. Do not mix or load pesticides, rinse containers, or wash equipment within 50 feet of a well unless the operations are done on a properly constructed pad. Specifications for mixing/loading pads can be found on the labels for atrazine-containing products. In the absence of a pad, it is best to haul water to the field in a nurse tank and do the mixing, loading, and equipment cleaning in the field.

Do not mix or load pesticides or wash equipment within 50 feet of a perennial or intermittent stream or river or within 50 feet of a natural or impounded lake or reservoir.

Do not apply pesticides where surface water is present, to saturated soils, or to intertidal areas below the mean high water mark.

Do not apply pesticides within 50 feet of a perennial or intermittent stream or river or within 50 feet of a natural or impounded lake or reservoir.

As noted later on the next page, labels for products containing atrazine are more restrictive.

Mix the proper amount. Before mixing, calculate the amount of spray solution needed as closely as possible and mix only that amount so there will be little to none to dispose of after the job is completed.

Dispose of excess spray solution properly. Do not dump excess spray solution on the ground. Ideally, excess spray solution is applied to an untreated field planted to a crop for which the pesticide is registered. In most cases, it is also acceptable to dilute small volumes of excess spray solution with additional water and apply to the same field you just finished spraying. A third, and less desirable, alternative is to spray out excess solution on field borders. However, do not spray over or near ditches or other drainage structures. Also, see the stream and drainage system setback requirements for atrazine-containing products.

Properly rinse and dispose of containers. Upon emptying a container, thoroughly rinse it using a pressure-rinse nozzle, allowing the rinsate to go into the tank of spray material being mixed. Alternatively, triple rinse the container and pour the contents into the spray tank. Punch holes in the bottom and sides of the container. Take rinsed and punctured containers to your county pesticide container recycling center, if available, or to your county land fill. Do not store empty containers near a well.

Reduce water runoff. The most common cause of surface water contamination by pesticides is runoff from fields during heavy rains shortly after application. Pesticides dissolved in water or attached to soil particles can exit a treated field in runoff water and enter surface water resources. Good conservation practices that reduce runoff and soil erosion will greatly reduce this problem. Vegetated buffer strips around the field will also greatly reduce the amount of pesticide ultimately reaching streams and other surface waters.

Specific Product Restrictions

Acetochlor. Acetochlor-containing products such as FulTime, Harness, Harness Xtra, Surpass, and TopNotch should not be applied to soils with the following textures if groundwater depth is 30 feet or less: sand with less than 3 percent organic matter; loamy sand with less than 2 percent organic matter; sandy loam with less than 1 percent organic matter. Because of groundwater restrictions, acetochlor-containing products are sold only in the Piedmont and Mountains and selected counties in the Blacklands.

Atrazine. In addition to the general guidelines above, atrazine-containing products such as AAtrex and various generic brands, Basis Gold, Bicep II, Bicep II Magnum, Bullet, FulTime, Guardsman, Harness Xtra, Laddok, Lariat, LeadOff, Liberty ATZ, and Marksman should not be applied within 200 feet around natural or impounded lakes and reservoirs.

Atrazine-containing products should not be applied within 66 feet of points where field surface water runoff enters perennial or intermittent streams and rivers. If applied to highly erodible land, the 66-foot setback from runoff points must be planted to a crop or seeded with grass.

Atrazine-containing products should not be applied to well-drained sand or loamy sand soils where the water table is close to the surface.

If atrazine is applied preemergence on highly erodible soils (as defined by the NRCS) and less than 30 percent of the soil surface is covered by plant residue at planting, the maximum rate is 1.6 pounds of atrazine (active ingredient) per acre. If atrazine is applied preemergence on soils not classified as highly erodible or on highly erodible soils with at least 30 percent of the soil surface covered with residue at planting, the maximum rate is 2 pounds per acre. For postemergence application, the maximum rate is 2 pounds if no atrazine was applied at planting. If atrazine is applied postemergence following a preemergence application, the maximum rate is 2.5 pounds active ingredient per acre per calendar year.

Dicamba. Dicamba-containing products such as Banvel, Celebrity, Celebrity Plus, Clarity, and Marksman should not be applied to soils classified as sands with less than 3 percent organic matter and where groundwater depth is shallow.

Dimethenamid. Dimethenamid-containing products such as Frontier, Guardsman, and LeadOff should not be applied to soils classified as sands with less than 3 percent organic matter and where groundwater depth is shallow.

Simazine. Simazine-containing products such as Princep and various generic brands should not be applied to very permeable soils such as loamy sands where the water table is close to the surface.

Drift Management

If herbicides are allowed to drift off the target site, they can sometimes damage nearby sensitive crops. Drift can occur as either spray drift or vapor drift.

Spray drift is the movement of airborne spray particles beyond the target site. The amount of spray drift is determined by droplet size, height of the boom, and amount of wind. Spray drift can occur with any herbicide.

Many herbicide labels contain specific application instructions to avoid spray drift. Follow those directions carefully. Below are general precautions to avoid spray drift.

  • Do not spray when the wind speed exceeds 10 MPH. Try to limit spraying to times when the wind speed is less than 5 MPH.
  • Do not spray when the wind is blowing toward a sensitive crop.
  • Avoid nozzles and pressures that create fine droplets. Use drift-reducing nozzles such as Spraying Systems Turbo TeeJets or XR-type nozzles. Use the highest spray volume practical; nozzles with higher flow rates produce larger droplets. Use a pressure within the lower recommended pressure range for the particular nozzle.
  • Keep your boom as low as possible while still getting the proper overlap of spray from individual nozzles.

Vapor drift is the movement of vapors, or fumes, from the target site. Spray drift is generally a short-distance problem. Vapor drift, on the other hand, can potentially injure highly susceptible crops a mile or more away.

The potential for vapor drift is directly related to the particular herbicide or formulation of herbicide being used. Most corn herbicides have low volatility and do not pose a threat of vapor drift. The major exceptions are ester formulations of 2,4-D and products containing dicamba. Ester formulations of 2,4-D have been particularly problematic on cotton in North Carolina. Cotton is highly susceptible to 2,4-D, and a number of cotton fields are injured by 2,4-D vapor drift each year. Ester formulations (or acid/ester mixes such as Weedone 638) of 2,4-D should not be applied within a mile of a cotton field. It is best to totally avoid ester formulations of 2,4-D during summer months. Amine formulations of 2,4-D are non-volatile and, hence, do not pose a threat of vapor drift. When using liquid nitrogen as the carrier, premix amines in water and then pour the mixture into the spray tank containing liquid nitrogen while vigorously agitating. Do not add amines directly to the liquid nitrogen.

Follow the drift precautions on labels of all herbicides, including dicamba-containing products.

Spray Adjuvants

Many postemergence herbicide labels specify use of a spray adjuvant. Spray adjuvants are any nonpesticidal material added to the herbicide to improve its effectiveness. Adjuvants increase herbicide absorption into plants. This generally leads to better weed control but also can increase potential for crop injury.

The most commonly used spray adjuvants are nonionic surfactants and crop oil concentrates. Some labels also recommend use of a nitrogen-containing fertilizer in addition to a surfactant or crop oil concentrate. Adjuvant recommendations for postemergence corn herbicides are listed in Table 8-7.

Some postemergence herbicide labels specify use of only nonionic surfactants, some specify use of only crop oil concentrates, and some specify use of either. Labels for a few herbicides specifically direct the applicator to not use an adjuvant. Labels for a few products do not mention adjuvants at all.

There are too many brands and types of adjuvants for researchers to evaluate each. Following are general guidelines on use of adjuvants with corn herbicides:

  • Follow herbicide label directions for adjuvant use carefully. If the label specifically states not to use an adjuvant, do not use one. This statement appears on the label because research has shown no benefit from an adjuvant or, more importantly, unacceptable crop injury may occur when an adjuvant is used.
  • If the label does not mention use of an adjuvant, it is probably because little benefit is likely to be obtained from such use or because use may lead to crop injury. However, there may be specific situations where use of an adjuvant would be of value. Use personal experience or advice from an expert or the herbicide manufacturer’s representative as your guide.
  • Adjuvant recommendations for tank mixes may differ from recommendations for the products applied alone. Read the labels carefully.
  • Whenever possible, use the exact adjuvant specified on the herbicide label. If the label specifies only the type of adjuvant, such as nonionic surfactant, but not a specific brand, buy a reputable brand from a reputable dealer.
  • Be aware that adjuvants vary greatly in quality. Good nonionic surfactants, often referred to as 80-20 surfactants, will contain at least 80 percent surface-active agents. The percentage of surface-active agents should be specified on the surfactant label. Good crop oil concentrates should contain 80 to 85 percent phytobland oil and 15 to 20 percent emulsifiers.
  • Use the rate of adjuvant specified on the herbicide label. Adding more is generally not better, and adding less may give poor results.
  • Be aware that there are a lot of “snake oil” adjuvants on the market. If the adjuvant manufacturer makes claims about an adjuvant that sound too good to be true, they probably are not true. Disregard claims that a particular adjuvant will allow you to greatly reduce herbicide rates. Do not believe that adjuvants improve performance of soil-applied herbicides. There is no evidence to suggest the need in North Carolina for adjuvants that adjust pH of the spray solution (although specific exceptions may exist). When the salesman says his product has been tested by a leading university, ask to see the specific data and then call that researcher for his advice.

Burndown in No-Till Systems

A general recommendation for burndown in no-till corn is Gramoxone Extra at 32 ounces, Roundup Ultra at 24 ounces, or Touchdown 5 at 20 ounces per acre. These burndown herbicides may be mixed with any preemergence herbicide. Specific recommendations vary according to the weed species or cover crop and weed size. See the labels of individual herbicides for specific recommendations. If the problem is predominately annual bluegrass, Carolina foxtail, or little barley, 24 ounces of Gramoxone, 16 ounces of Roundup, or 12 to 16 ounces of Touchdown will be adequate. These rates are also sufficient for many of the common winter annual broadleaf weeds, such as bittercress, buttercup, chickweed, corn speedwell, field pennycress, purslane speedwell, shepherdspurse, and Virginia pepperweed. Roundup at 24 ounces or Touchdown at 20 ounces is preferred for horseweed (marestail) and fleabane. Gramoxone plus an atrazine-containing herbicide is recommended for Carolina geranium or field pansy. See Table 8-7 for adjuvant recommendations.

A wheat cover crop can be killed by Gramoxone at 24 to 32 ounces, Roundup at 16 ounces if 6 inches or less or 24 ounces if larger, or Touchdown at 20 ounces if 6 inches or less or 28 ounces if larger. Rye can be killed by Gramoxone at 20 to 24 ounces, Roundup at 24 ounces if 20 inches or less or 32 ounces if larger, and Touchdown at 28 ounces.

Two burndown herbicide applications are suggested for fields with annual ryegrass or cutleaf eveningprimrose and where corn will be planted into a fescue or alfalfa sod.

For annual ryegrass, apply Roundup at 32 ounces or Touchdown at 24 ounces 2 weeks or more ahead of planting. At planting, apply 32 ounces of Gramoxone plus an atrazine-containing herbicide.

Cutleaf eveningprimrose is not adequately controlled by Gramoxone, Roundup, or Touchdown alone. Apply 2,4-D at 1 pint per acre 2 weeks or more ahead of planting. Roundup or Touchdown can be mixed with the 2,4-D if needed for control of other weeds. Apply Gramoxone at 24 to 32 ounces plus an atrazine-containing herbicide at planting.

If planting into a fescue sod, apply 32 ounces of Gramoxone, Roundup, or Touchdown 2 to 4 weeks ahead of planting. At planting, apply 24 to 32 ounces of Gramoxone plus an atrazine-containing herbicide.

In old alfalfa stands on medium or fine soils, apply 1 pint per acre of Banvel or Clarity at least 14 days ahead of planting. Banvel or Clarity may be mixed with Roundup or Touchdown if desired for control of grassy weeds. Follow with Gramoxone, Roundup, or Touchdown at 32 ounces plus an atrazine-containing herbicide at planting. If on coarse-textured soils, apply 32 ounces of Roundup or Touchdown at or before planting and follow with Banvel, Celebrity, Celebrity Plus, Clarity, or Marksman at normal use rates applied postemergence to corn.

Curly dock is not controlled well by Gramoxone, Roundup, or Touchdown. The best program for curly dock is Harmony Extra at 0.5 ounce applied in early spring. The Harmony Extra label specifies application at least 45 days ahead of planting although injury has not been observed with Harmony Extra 14 days ahead of planting. Follow the Harmony Extra with a normal at-planting burndown treatment.

Weed Management Programs in Conventional Hybrids

A number of options, both preemergence and postemergence, are available for annual grass and broadleaf weed control. On mineral soils, most growers prefer to start with a preemergence treatment and follow with postemergence herbicides as needed. On organic soils, growers prefer postemergence programs because many preemergence herbicides are ineffective on their soils. Acetochlor (Harness, Surpass, TopNotch) and alachlor (Lasso, Micro-Tech, Partner) are the only soil-applied herbicides that perform adequately on soils with more than about 5 to 6 percent organic matter.

Preemergence Herbicides for Annual Weeds

Annual grasses. Preemergence herbicides for annual grasses include acetochlor (Harness, Surpass, TopNotch), alachlor (Lasso, Micro-Tech, Partner), dimethenamid (Frontier), and metolachlor (Dual II, Dual II Magnum). Although there are minor differences in the spectrum of weeds controlled by these herbicides, their effects are much more similar than dissimilar (Table 8-1). These herbicides control broadleaf signalgrass, crabgrass, fall panicum, foxtails, and goosegrass. Heavily infested fields may benefit from a lay-by application of Evik or Lorox. These preemergence herbicides do not adequately control shattercane, Texas panicum, or seedling johnsongrass. See the section on “Control of Specific Troublesome Weeds” on page 96. As noted in the section “Surface and Groundwater Protection” (page 87), there are some restrictions on the use of products containing acetochlor and dimethenamid.

Eradicane and Sutan+ were once used widely in North Carolina but are seldom used today. These herbicides normally control most annual grasses (Table 8-1) for about 4 weeks. Because of the short residual of these herbicides, heavily infested fields normally benefit from a postemergence or lay-by application of a grass-controlling herbicide. Eradicane and Sutan+ must be incorporated immediately after application.

Annual Broadleaf Weeds. Atrazine is the most commonly used preemergence herbicide for broadleaf weed control. Applied preemergence to mineral soils, atrazine controls most annual broadleaf weeds (Table 8-2), and corn tolerance is excellent. Note the restrictions on atrazine use in the section “Surface and Groundwater Protection” (page 87). Premixes containing atrazine plus acetochlor (FulTime, Harness Xtra), atrazine plus alachlor (Bullet, Lariat), atrazine plus dimethenamid (Guardsman, LeadOff), atrazine plus metolachlor (Bicep II), and atrazine plus S-metolachlor (Bicep II Magnum) are available.

Delayed Application of Preemergence Herbicides

Application of preemergence herbicides immediately after planting is recommended. However, some preemergence herbicides can be applied early postemergence to corn if you fail to get them applied at planting.

Bicep II, Bicep II Magnum, Bullet, FulTime, Guardsman, Harness Xtra, and LeadOff can be applied after corn emergence. Bicep II, Bicep II Magnum, or Bullet can be applied before corn exceeds 5 inches tall. Guardsman and LeadOff can be applied to corn up to 8 inches tall. FulTime and Harness Xtra can be applied to corn up to 11 inches tall. All of these herbicides should be applied before the weeds exceed the two-leaf stage. If weeds are small enough, crabgrass, foxtails, goosegrass, and most broadleaf weeds will be controlled. None of these products will control emerged broadleaf signalgrass, fall panicum, or Texas panicum. If one of these species is present or if other weeds exceed the two-leaf stage, a tank mix will be needed. Guardsman or LeadOff can be mixed with Accent, Banvel, Clarity, or Marksman and applied to corn up to 8 inches. Harness Xtra can be mixed with Banvel, Clarity, or Marksman and applied to corn up to 8 inches or mixed with Accent or Permit and applied to corn up to 11 inches. FulTime can be mixed with Banvel, Clarity, or Marksman and applied to corn up to 8 inches or mixed with Accent or Basis Gold and applied to corn up to 11 inches.

Postemergence Herbicides for Annual Weeds

Annual grasses. Accent, applied alone or in combination with a herbicide for broadleaf weeds (Table 8-8), will control broadleaf signalgrass, fall panicum, foxtails, shattercane, Texas panicum, and seedling johnsongrass (Table 8-3). Note that Accent alone will not adequately control crabgrass or goosegrass. However, if applied in combination with atrazine and crop oil, good control of crabgrass and goosegrass can be obtained. An Accent treatment can be followed by Evik or Lorox at lay-by if needed to control grasses emerging after Accent application.

Celebrity is a co-packaged mixture containing the active ingredients found in Accent and Banvel or Clarity. Celebrity will control the annual grasses normally controlled by Accent plus most broadleaf weeds (Tables 8-3 and 8-4). Celebrity Plus is a prepackaged mixture containing the active ingredients found in Accent and Distinct. Weed control by Celebrity and Celebrity Plus will be similar.

Basis Gold will control broadleaf signalgrass, fall panicum, foxtails, shattercane, and seedling johnsongrass and suppress several other annual grasses (Table 8-3). Used in conjunction with cultivation or a lay-by application of Evik or Lorox, adequate control of all common annual grasses should be achieved. Timing of application of Basis Gold is critical. See the label for recommended sizes of weeds to treat.

See Table 8-11 for precautions on using Accent, Basis Gold, Celebrity, or Celebrity Plus on corn treated with organophosphate insecticides. Annual Broadleaf Weeds. A number of options exist for postemergence control of annual broadleaf weeds (Table 8-4). Selection of a herbicide or tank mix (Table 8-8) should be based upon the weeds present, size of the weeds, soil-applied insecticides that have been used (Table 8-11), crop growth stage (Table 8-6), rotational plans (Table 8-5), and potential for drift to susceptible crops (see section “Drift Management” on page 88). In severely infested fields or fields where no preemergence herbicide was used, both an early postemergence and a lay-by treatment may be required for adequate control.

Speciality Use Herbicides

Basagran. Basagran, a commonly used soybean herbicide, has a limited fit in corn. Although Basagran controls several broadleaf weeds (Table 8-4), a number of commonly occurring weeds are not controlled. Basagran could be used to control susceptible broadleaf weeds when presence of soil-applied herbicides or nearby sensitive crops such as cotton or tobacco prohibit use of other postemergence broadleaf herbicides. Also see the discussion on nutsedge in the section “Control of Specific Troublesome Weeds” (page 96).

Basis. This herbicide has a limited fit in North Carolina. It is intended for very early postemergence application (Table 8-6) to control a limited spectrum of weeds. Timing of application is extremely critical. The label claims control of only fall panicum and foxtails 1 to 2 inches tall and pigweed, lambsquarters, and velvetleaf 1 to 3 inches tall. Control of broadleaf signalgrass is not claimed on the label but research in North Carolina has shown control of this grass if treated when 1 inch tall or less. If Basis is used, you can count on needing to cultivate or put out a follow-up treatment. Other herbicides can be mixed with Basis to improve broadleaf weed control (Table 8-8). See Table 8-11 for precautions on using Basis on corn treated with organophosphate insecticides.

Buctril. Buctril can be applied postemergence to corn (Table 8-6). Contact burn on the corn, occasionally severe, will occur. The corn appears to recover and grow normally. Buctril has no activity on grasses or sedges. At typical use rates, Buctril is less effective on some broadleaf weeds than other postemergence corn herbicides (Table 8-4). However, it is very effective on cocklebur, jimsonweed, lambsquarters, nightshade, ragweed, and smartweed. Buctril would be an option for susceptible weeds where a nearby sensitive crop such as tobacco or cotton prohibits application of 2,4-D or dicamba-containing herbicides and the weeds are too large to control with other typical postemergence herbicides.

Resource. Applied postemergence, this herbicide has activity on several broadleaf weeds (Table 8-4). However, its major fit in North Carolina would be to control velvetleaf too large to control with other herbicides. Resource is extremely effective on rather large velvetleaf. It can be tank mixed with certain other herbicides for broader spectrum control (Table 8-8).

Tough. Tough, applied postemergence to corn, has activity on several broadleaf weeds if applied when the weeds are small (Table 8-4). The primary fit for Tough in North Carolina would be to control lambsquarters that are too big to control with other herbicides. Tough is extremely effective on rather large lambsquarters. It can be tank mixed with several other herbicides (Table 8-8).

Herbicide-Insecticide Interactions

Interactions between organophosphate insecticides such as Counter, Dyfonate, Lorsban, or Thimet and various ALS-inhibitor herbicides can lead to corn injury. Precautions concerning use of organophosphate insecticides with these herbicides are listed in Table 8-11. No adverse interactions occur between these herbicides and carbamate (Furadan) or synthetic pyrethroid (Ambush, Asana, Force, Fortress, Karate, Pounce) insecticides.

Do not apply organophosphate insecticides foliarly within 10 days before or 7 days after ALS-inhibitor herbicides. See labels for specific details.

Weed Management Programs in Herbicide-Tolerant Hybrids

Herbicide-tolerant hybrids, developed through either traditional breeding and selection techniques or through genetic engineering, offer additional options for weed management. Extreme care should be taken to identify fields planted to a herbicide-tolerant hybrid. Conventional hybrids will be severely injured or killed if mistakenly sprayed with a herbicide intended for a herbicide-tolerant hybrid. Additionally, when treating herbicide-tolerant hybrids, extreme care should be taken to avoid herbicide drift to non-tolerant corn or other crops.

Information on the agronomic performance of herbicide-tolerant hybrids is limited. Growers are encouraged to not plant large acreages to inadequately tested hybrids.

Any herbicide registered for application to a conventional hybrid can be applied to a herbicide-tolerant hybrid.

Clearfield Hybrids

Clearfield hybrids (previous known as IMI Corn) are tolerant of imidazolinone (IMI) herbicides and are designated by “IT” or “IR” following the hybrid number (for example, Pioneer 32Z18 IT). Lightning is the IMI herbicide suggested for use on Clearfield corn in North Carolina. Lightning controls most broadleaf weeds although control of sicklepod can be erratic (Table 8-4). Lightning’s spectrum of control can be increased by tank mixing with other herbicides such as atrazine, Banvel, Clarity, 2,4-D, or Marksman (Table 8-8). Lightning or a Lightning tank mix will control most annual broadleaf weeds in the absence of soil-applied herbicides. However, a lay-by application of 2,4-D, Banvel, Clarity, Evik, or Lorox may be needed in heavily infested fields.

Lightning will normally control broadleaf signalgrass, foxtails, and shattercane (Table 8-3). In heavily infested fields, a lay-by application of Evik may be beneficial. A preemergence herbicide is suggested if crabgrass, fall panicum, or goosegrass is expected. Lightning will not adequately control Texas panicum. See discussion on Texas panicum in the section “Control of Specific Troublesome Weeds” (page 96).

Lightning will usually adequately control light to moderate infestations of johnsongrass. Adequate control of heavier infestations will often necessitate a follow-up application of Accent or Beacon. This is not only an expensive approach but it also exposes the weeds to two applications of herbicides with the same mechanism of action (Table 8-15). Hence, use of Lightning in fields with heavy johnsongrass pressure is not recommended. Additionally, Lightning is generally not recommended for corn grown in rotation with crops that will be treated with ALS-inhibiting herbicides (Table 8-15).

Liberty Link Hybrids

Liberty Link hybrids are tolerant of the herbicide Liberty. Except on Liberty Link crops, Liberty is a non-selective, limited-translocation herbicide with no residual activity. With a few exceptions, it controls both annual grasses and annual broadleaf weeds (Tables 8-3 and 8-4). However, Liberty is generally more effective on annual broadleaf weeds than on annual grasses. It is normally inadequate on perennial grasses, perennial broadleaf weeds, and sedges.

Research has shown that Liberty is generally not a stand alone treatment. Control is best when Liberty is used as a component of an overall program. Liberty can be tank mixed with most postemergence corn herbicides registered for use on conventional hybrids (Table 8-8). The most effective tank mix appears to be Liberty plus atrazine. Excellent control of both annual grasses and broadleaf weeds has been achieved in a system of Liberty plus atrazine postemergence followed by Evik at lay-by if needed. Excellent control also has been obtained in systems with a preemergence herbicide such as Bicep II followed by Liberty plus atrazine postemergence.

Liberty ATZ is a prepackaged mixture containing the active ingredient in Liberty plus atrazine.

Poast Protected Hybrids

Poast Protected hybrids, designated by “SR” in the hybrid name, are tolerant of the herbicides Poast and Poast Plus. These hybrids are not adequately tolerant of other ACCase inhibitors such as Assure II, Fusilade, Fusion, or Select.

Poast or Poast Plus control all common annual grasses plus bermudagrass and johnsongrass (Table 8-3). These herbicides can be applied postemergence to Poast Protected hybrids to control annual grasses escaping preemergence herbicides, to control johnsongrass or bermudagrass, or in conjunction with postemergence broadleaf herbicides for a total postemergence program. See comments on control of johnsongrass and bermudagrass in the section “Control of Specific Troublesome Weeds” (page 96).

Compared with many other postemergence herbicides for grass control in corn, Poast and Poast Plus do not interact with organophosphate insecticides.

Poast and Poast Plus labels allow for tank mixing with atrazine, Basagran, 2,4-D, and Laddok (Table 8-8). The Poast label also allows for tank mixing with Banvel, Clarity, and Marksman. Tank mixes with Basagran, Laddok, or 2,4-D should generally be avoided as North Carolina research has shown antagonism (reduced grass control) with these combinations. Tank mixes with atrazine, Banvel, Clarity, or Marksman have generally performed well in North Carolina tests. One of these tank mixes followed by Evik at lay-by has controlled annual grass and broadleaf weeds well. However, sequential applications of Poast or Poast Plus and broadleaf herbicides are generally preferred. If Poast or Poast Plus is applied first, broadleaf herbicides can be applied as early as 1 day later. If a broadleaf herbicide is applied first, delay Poast or Poast Plus application 5 to 7 days. If a broadleaf herbicide is used that causes burning on the grasses, delay Poast application until new growth is present on the grass.

In addition to atrazine, Poast (but not Poast Plus) can also be tank mixed with other residual herbicides such as Dual II, Dual II Magnum, Frontier, Guardsman, Harness, and Surpass. This might be an option in fields where no soil-applied herbicides were used.

Roundup Ready Hybrids

Roundup Ultra can be applied postemergence over-the-top or directed to Roundup Ready hybrids from emergence to the eight-leaf stage (Table 8-6). Roundup controls all the common annual grasses and johnsongrass and bermudagrass (Table 8-3). See comments on control of johnsongrass and bermudagrass in the section “Control of Specific Troublesome Weeds” (page 96). Roundup also controls most annual broadleaf weeds (Table 8-4). Note that control of morningglory can be marginal.

In North Carolina research, a single application of Roundup in the absence of preemergence herbicides or a lay-by treatment has generally not provided adequate season-long control of most weeds. Although most weeds are controlled well initially, lack of residual activity from Roundup allows later-emerging weeds to become problems. An atrazine-containing preemergence herbicide, especially one containing atrazine and a grass-control herbicide such as Bicep II, has significantly increased weed control. However, this system still may not be adequate in fields heavily infested with morningglory.

Systems consisting of two postemergence applications of Roundup or one application of Roundup followed by Evik at lay-by, with or without preemergence herbicides, has controlled weeds well. The second application of Roundup will need to be directed in order to obtain good coverage on the weeds.

Atrazine, Clarity, Distinct, Permit, or Resource may be tank mixed with Roundup (Table 8-8). Roundup-Permit tank mixes have not been adequately evaluated in North Carolina. However, with the exception of nutsedge, one would not expect Permit to add much to the spectrum of weeds controlled by Roundup. Adding atrazine to Roundup has improved late-season control of most broadleaf weeds, probably due to atrazine’s residual activity. Adding atrazine, Clarity, Distinct, or Resource to Roundup also has improved morningglory control. However, in heavily infested fields, Roundup plus one of these tank mix partners alone has not been adequate on morningglory.

Roundup may also be tank mixed with Bullet, Harness, Harness Xtra 5.6L, Micro-Tech, or Partner. These tank mix partners will provide residual control of susceptible weeds (Table 8-1).

Control of Specific Troublesome Weeds

Broadleaf Signalgrass

Broadleaf signalgrass has become one of the most common annual grasses in North Carolina. The preemergence herbicides Dual II, Dual II Magnum, Harness, Surpass, and TopNotch, and the premixes Bicep II, Bicep II Magnum, FulTime, and Harness Xtra will provide acceptable early season control (Table 8-1). However, these herbicides usually do not give adequate season-long control in moderately to heavily infested fields. Good broadleaf signalgrass control requires one of the above herbicides at planting plus Evik at lay-by.

Good control of broadleaf signalgrass also can be obtained with postemergence programs (Table 8-3). Accent, Basis Gold, Celebrity, or Celebrity Plus in any hybrid, Liberty or Liberty ATZ in Liberty Link corn, Lightning in Clearfield corn, Poast or Poast Plus in Poast Protected corn, or Roundup Ultra in Roundup Ready corn all control broadleaf signalgrass. With any of these herbicides, a lay-by application of Evik will be beneficial in moderately or heavily infested fields.

Texas Panicum

This annual grass is currently not a wide-spread problem in North Carolina. However, it is present in several Piedmont and Coastal Plain counties and it appears to be slowly spreading.

Soil-applied herbicides are only marginally effective at best on Texas panicum (Table 8-1). However, the chloroacetamide herbicides (Dual II, Dual II Magnum, Frontier, Harness, Lasso, Micro-Tech, Partner, Surpass, and TopNotch) and premixes containing chloroacetamides (Tables 8-15 and 8-16) will usually suppress Texas panicum early in the season so that it is small enough for effective kill with Evik applied at lay-by. Evik should be applied at 2.5 pounds per acre. Eradicane or Sutan+ preplant incorporated tend to be more effective on Texas panicum than the chloroacetamides (Table 8-1).

Texas panicum can be controlled well with Accent, Celebrity, or Celebrity Plus applied postemergence (Table 8-3). Apply these herbicides before Texas panicum exceeds 3 inches. Follow with Evik at lay-by if needed.

Texas panicum can also be controlled very well with Poast or Poast Plus in Poast Protected hybrids (Table 8-3). Apply Poast or Poast Plus before the Texas panicum exceeds 8 inches (4 inches suggested). Follow with Evik at lay-by if needed.

Roundup Ultra in Roundup Ready corn is very effective on Texas panicum (Table 8-3). Apply Roundup to Texas panicum 6 inches tall or less and follow with Evik if needed.

Johnsongrass

Johnsongrass can be controlled with Accent, Celebrity, or Celebrity Plus applied overtop before the weed exceeds 18 inches tall (Table 8-3). One application is normally sufficient. If needed, however, a second application can be made to corn 20 to 36 inches tall using drop nozzles. Add crop oil concentrate or surfactant according to label directions.

Beacon also can be used to control johnsongrass although it is often somewhat less effective than Accent (Table 8-3). Beacon can be applied once at 0.75 ounce per acre to corn 4 to 20 inches tall and before johnsongrass exceeds 16 inches. Split application of Beacon at 0.5 ounce overtop followed by 0.25 ounce applied with drop nozzles may be more effective than a single application of 0.75 ounce. Add crop oil concentrate or surfactant according to label directions.

Poast or Poast Plus can be used to control johnsongrass in Poast Protected hybrids (Table 8-3). Apply Poast at 1.5 pints per acre or Poast Plus at 2.25 pints per acre before johnsongrass exceeds 20 inches tall. If needed, make a second application of Poast at 1 pint or Poast Plus at 1.5 pints when regrowth or new plants are 12 inches or less. The second application should be made with drop nozzles to achieve better coverage. Add crop oil concentrate at 2 pints per acre. Adding either 1 gallon of 30 percent UAN or 2.5 pounds of ammonium sulfate per acre, in addition to the crop oil concentrate, may improve control.

Roundup Ultra applied to Roundup Ready corn is very effective on johnsongrass (Table 8-3). In many cases, one application will be adequate. However, a second application can be made if needed.

Bermudagrass

Bermudagrass cannot be controlled in conventional corn hybrids. However, it can be controlled in crops rotated with corn, such as soybeans, cotton, or peanuts. Bermudagrass can be controlled in soybeans or cotton treated twice with Assure II, Fusilade DX, Fusion, Poast, Poast Plus, or Select. Alternatively, Roundup Ultra can be applied twice to Roundup Ready cotton or soybeans. Poast, Poast Plus, or Select can be applied twice to control bermudagrass in peanuts.

Bermudagrass can be controlled in Poast Protected hybrids. Apply Poast at 1.5 pints or Poast Plus at 2.25 pints per acre before bermudagrass stolons (runners) exceed 6 inches. Make a second application of Poast at 1 pint or Poast Plus at 1.5 pints 2 to 3 weeks later or when regrowth on the bermudagrass is 4 inches or less. The second application should be made with drop nozzles to achieve better coverage. Add crop oil concentrate at 2 pints per acre. Adding either 1 gallon of 30 percent UAN or 2.5 pounds of ammonium sulfate per acre, in addition to the crop oil concentrate, may improve control.

Bermudagrass can also be controlled with Roundup Ultra or Touchdown 5 after harvest of corn silage or early grain harvest. Apply Roundup at 3 quarts per acre or Touchdown at 6.33 pints per acre to infested areas and wait at least 7 days before tillage.

Annual ryegrass

Ryegrass is primarily a problem in no-till systems. It is usually controlled by burndown herbicides applied at or ahead of planting (see section on “Burndown in No-Till Systems” on page 90). However, if ryegrass is a problem after corn is emerged, it can be easily controlled by Accent applied postemergence.

Nutsedge

Nutsedge is generally not very competitive with corn unless present at high populations. However, many growers want to maintain good control in corn grown in rotation with other crops where nutsedge is more problematic.

The most effective treatment for both yellow and purple nutsedge is Permit applied postemergence (Table 8-3). Apply Permit at 1 ounce per acre to nutsedge 4 to 12 inches tall. The spectrum of weeds controlled by Permit is limited (Tables 8-3 and 8-4). However, Permit can be mixed with other herbicides to broaden the spectrum of control (Table 8-8).

Dual II, Dual II Magnum, or Frontier (and the premixes Bicep II, Bicep II Magnum, Guardsman, and LeadOff) will suppress yellow nutsedge (Table 8-1). Dual is generally somewhat more effective on yellow nutsedge than Frontier. Also, Dual is more effective on yellow nutsedge when applied preplant incorporated.

Basagran plus crop oil concentrate applied postemergence will control yellow nutsedge, but two applications are often needed. Basagran does not control purple nutsedge.

In Clearfield hybrids, Lightning applied postemergence will suppress yellow and purple nutsedge (Table 8-3). A lay-by application of Evik will enhance control.

Roundup Ultra applied to Roundup Ready corn will control both yellow and purple nutsedge, but two applications may be required for good control. Good control would also be expected with Roundup applied once postemergence followed by Evik at lay-by. For heavy infestations, a tank mix of Roundup plus Permit should be considered.

Liberty, applied to Liberty Link hybrids, will not control nutsedge (Table 8-3). However, a tank mix of Liberty plus Permit would be very effective.

Sicklepod

Sicklepod is particularly troublesome in soybeans and other crops grown in rotation with corn. Good control in corn will reduce problems for the rotational crop.

There are a number of corn herbicides which control sicklepod (Tables 8-2 and 8-4). On mineral soils, an atrazine-containing product applied preemergence is suggested. Because sicklepod has a long germination period, a program consisting of multiple herbicide applications is normally needed for good control.

Following the preemergence treatment, scout the field and be ready to treat postemergence if needed. Herbicides which can be applied postemergence over-the-top to control sicklepod include atrazine plus crop oil, Banvel, Celebrity, Celebrity Plus, Clarity, 2,4-D, Exceed, and Marksman (Table 8-4). A nonionic surfactant should be included with 2,4-D when treating sicklepod. Banvel, Clarity, Evik, Lorox, or 2,4-D plus surfactant applied at lay-by will control sicklepod.

In organic or mineral/organic soils, use early postemergence overtop herbicides (Table 8-4) followed by a lay-by treatment if needed.

Sicklepod can be controlled in Clearfield hybrids with Lightning or Lightning plus atrazine, Banvel, Clarity, Distinct, Marksman or 2,4-D applied overtop. A tank mix would perform more consistently than Lightning alone. Follow with a lay-by treatment if needed. On mineral soils, a preemergence application of an atrazine-containing product is suggested for heavily infested fields.

In Liberty Link corn, sicklepod can be controlled by Liberty, Liberty plus atrazine, or Liberty ATZ postemergence followed by a lay-by treatment as needed.

In Roundup Ready corn, sicklepod can be controlled by Roundup or Roundup plus atrazine applied postemergence overtop followed by a lay-by treatment as needed.

Burcucumber

A concerted effort and well-planned strategy is required to control burcucumber. Either of the first two programs outlined below should provide good control in corn. Omitting any step may reduce control. If burcucumber is a problem on organic soils, use the third program.

Program 1

A. Preemergence: atrazine at 1.5 pounds active ingredient/acre.

B. Early postemergence: Exceed at 1.0 ounce plus atrazine at 1 pound active plus crop oil concentrate 1 quart per acre before corn exceeds 12 inches or burcucumber exceeds 8 inches. If Accent or Beacon fits better on other weeds, they may be substituted for Exceed. However, Exceed is preferred. Celebrity at 6.67 ounces plus 1 pound active of atrazine or Celebrity Plus at 4.7 ounces plus 1 pound active of atrazine would also be options.

C. Lay-by: Banvel or Clarity at 0.5 pint per acre with drop nozzles when corn is 30 to 36 inches.

Program 2

A. Preemergence: Atrazine at 2 pounds active per acre at planting. Reduce rate to 1.6 pounds if necessary to meet label restrictions on highly erodible soil.

B. Early postemergence: Marksman at 2 pints per acre before corn is taller than 8 inches.

C. Lay-by: Banvel or Clarity at 0.5 pint per acre with drop nozzles when corn is 30 to 36 inches.

Program 3

A. Early postemergence: Exceed at 1 ounce plus Banvel at 0.25 pint per acre plus surfactant before corn is taller than 20 inches or burcucumber is taller than 8 inches. Celebrity at 6.67 ounces or Celebrity Plus at 4.7 ounces would also be options.

B. Lay-by: Banvel or Clarity at 0.5 pint per acre with drop nozzles before corn is taller than 36 inches.

Alligatorweed

Terrestrial alligatorweed can be controlled in corn with Banvel or Clarity at 0.5 pint, Marksman at 2 pints, Celebrity at 6.67 ounces, or Celebrity Plus at 4.7 ounces per acre applied postemergence over-the-top of corn early followed by a lay-by application of Banvel or Clarity at 0.5 pint per acre. Single applications are considerably less effective than two applications. Alligatorweed can be controlled or suppressed in narrow-row Roundup Ready soybeans with Roundup Ultra at 2 to 3 pints per acre or in any variety with Scepter DG at 2.8 ounces plus Blazer at 1.5 pints per acre.

Perennial Broadleaf Weeds

Perennial broadleaf weeds such as common milkweed, horsenettle, trumpetcreeper, hemp dogbane, maypop passionflower, cinnamonvine, bigroot morningglory, curly dock, dewberry, and blackberry can be significant problems, especially in continuous no-till systems.

Perennial broadleaf weeds can be controlled or suppressed in corn with various herbicides applied postemergence. Roundup Ultra or Touchdown 5 would be preferred as the burndown herbicide at planting. However, choice of burndown herbicides often has little impact because most perennial broadleaf weeds are either not emerged or are in a non-active state at the time of corn planting. The major exception is curly dock, which will be growing well at planting time. The most effective option for curly dock is Harmony Extra prior to planting. Although application 14 days ahead of planting has not caused adverse effects on corn, the Harmony Extra label specifies application at least 45 days ahead of planting. A second option would be Banvel or Clarity alone or mixed with Roundup or Touchdown 3 to 4 weeks ahead of planting.

To control or suppress perennial broadleaf weeds in corn, one of the strategies listed in Table 8-12 is suggested. Two applications in corn, early postemergence and lay-by, are usually more effective than one application. If soybeans are rotated with corn, additional suppression can be obtained with Roundup Ultra applied to Roundup Ready soybeans. For erect-growing weeds such as hemp dogbane and common milkweed, two applications of Roundup Ultra are most effective. One application of 2 pints per acre should be made about 2 to 3 weeks after planting to control annual weeds and to suppress perennial weeds. A second application of 2 pints per acre can be made about 2 to 3 weeks later. For low-growing perennials such as trumpetcreeper or horsenettle, a second application may not be beneficial as the crop canopy will likely be closed and coverage on low-growing weeds will be poor. Roundup Ultra at 3 pints per acre is suggested for single applications to perennial broadleaf weeds.

Post-harvest treatment is also an option for perennial broadleaf weeds. Options include Roundup Ultra, Touchdown 5, and Banvel, Clarity, or Distinct plus 2,4-D. Spot spray only infested areas to reduce costs.

Following corn harvest and prior to a killing frost, Roundup Ultra or Touchdown 5 can be applied to suppress perennial broadleaf weeds for the following year. Apply Roundup at 2 quarts per acre for trumpetcreeper, 3 quarts per acre for common milkweed, and 4 quarts per acre for other species. Apply Touchdown at 3.33 pints for trumpetcreeper, 4.67 pints for common milkweed, and 6.33 pints for other species. Adding 1 pint of 2,4-D or 0.5 pint of Banvel or Clarity to Roundup or Touchdown will usually enhance control of these weeds. Do not till or otherwise disturb the weeds for at least 7 to 10 days following treatment.

A combination of Banvel or Clarity at 1 pint plus 2,4-D at 2 pints per acre can be applied following corn harvest. Do not till or otherwise disturb the weeds for at least 14 days after treatment. Delay wheat planting for 20 days after treatment.

Harvest Aid Treatments

There are few situations where a harvest aid treatment is justified or recommended for corn. Exceptions might be fields having a morningglory control failure or very weedy fields where the corn was blown down during a hurricane.

Sodium chlorate, certain brands of 2,4-D, and Roundup Ultra are registered as harvest aids in corn.

By necessity, harvest aids may have to be applied by air. Extreme caution should be exercised to avoid drift to other crops.

Sodium chlorate (various brands) can be used to desiccate grasses. Apply sodium chlorate at 6 pounds active ingredient per acre after corn is in the dent stage. Apply at least 14 days prior to anticipated harvest. Apply on a bright, sunny day when the temperature is at least 75 F. Sodium chlorate will usually desiccate grasses but will have little impact on broadleaf weeds.

Various brands of 2,4-D ester formulations and one amine formulation (Formula 40) are registered for harvest aid in corn. Use of ester formulations is discouraged if cotton or other susceptible crops are located within 1 mile of the corn being treated.

Formula 40 can be applied at 1 to 2 pints per acre after corn is in the dent stage to fields where broadleaf weeds such as morningglory will make harvesting very difficult. Apply at least 7 days prior to anticipated harvest.

Roundup Ultra at 1 to 3 quarts per acre by ground or 1 quart per acre by air can be applied after black layer formation and grain moisture is 35% or less. Apply at least 7 days prior to harvest.

Herbicide Resistance Management

Herbicide resistance is defined as the inherited ability of a biotype of weed to survive a herbicide application to which the original population was susceptible. A biotype is a group of plants within a species (a subset of the original population) that has biological traits, such as resistance to a particular herbicide, not common to the population as a whole.

Herbicide resistance is a relatively new but increasingly significant problem. Worldwide, there are over 200 biotypes with resistance to one or more herbicides in at least 15 different chemical classes. Many of these biotypes have been discovered within the past 5 years. If current trends continue, herbicide resistance may become a significant obstacle to economical weed control.

Herbicide-resistant biotypes have been confirmed on every continent. Examples of species with resistant biotypes in North Carolina include annual ryegrass resistant to Hoelon, goosegrass resistant to Treflan and Prowl, cocklebur resistant to DSMA and MSMA, smooth pigweed resistant to imidazolinones (such as Pursuit), Palmer amaranth resistant to Treflan and Prowl, and cocklebur resistant to sulfonylureas (such as Classic) and imidazolinones (such as Scepter). In other areas of the southern United States, johnsongrass resistant to Poast and pigweed and lambsquarters resistant to atrazine have been discovered.

Until recently, there was limited concern over the evolution of herbicide resistance in the Southeast. Although some resistant biotypes existed, their presence was easily explained by unique cropping systems such as the monoculture of cotton and use of the same herbicides over a long period of time.

The situation, however, has changed in recent years because of development and widespread use of many herbicides having the same mechanism of action (specific process by which the herbicide kills plants). Today, herbicides having the same mechanism of action can be used on nearly every agronomic crop grown. Of particular concern are those herbicides that inhibit the ALS enzyme system (Table 8-15). Several of our most commonly used herbicides are ALS inhibitors.

There are two prerequisites for herbicide evolution. First, a few individual plants possessing genes conferring resistance must be present in the population. Second, selection pressure resulting from extensive use of a herbicide to which these rare individuals are resistant must be exerted on the population. Growers have no way to know if a few plants carrying resistance are present on their farm. Hence, the only way to prevent resistance evolution is to utilize management systems that reduce selection pressure on any resistant individuals that may be present.

Components of a Resistance Management Strategy

Management strategies to avoid resistance contain three basic components. The first and most important component is wise herbicide selection. Avoid continuous use of herbicides in the high, moderate to high, and moderate risk categories (Table 8-15). These herbicides, and especially those in WSSA group 2, should not be applied to two consecutive crops if other alternatives exist. For example, if Classic, Pursuit, Raptor, or Scepter is used on soybeans, try to avoid using Accent, Beacon, Exceed, or Lightning on corn rotated with the soybeans. Another example to avoid would be use of Assure, Fusilade, Fusion, Poast, or Select on soybeans and Poast on Poast Protected corn.

The second component of a resistance management strategy is use of tank mixes or sequential applications of herbicides having different mechanisms of action (more specifically, different WSSA group numbers). This is important when using herbicides in the high, moderate to high, or moderate risk categories. For example, if Accent is used in corn, apply it following a preemergence application of atrazine or tank mix it with atrazine or Banvel.

The third component of a resistance management strategy is to take advantage of non-chemical control options when practical. This would include cultivation, maximization of crop competition, and crop rotations.

Detecting herbicide resistance

Early detection of herbicide resistance is critical in order to maximize effectiveness of resistance management programs. Early detection means management will be easier, and it increases the potential to avoid spread to other fields or farms. Unfortunately, resistance often is not detected until the resistant biotype has become relatively well established.

The vast majority of weed control failures are not due to resistance. Before assuming weeds surviving a herbicide application are resistant, eliminate other possible causes of poor control. Potential causes of poor control include things such as misapplication (inadequate rate, non-uniform application, poor spray coverage, poor incorporation, failure to use an adjuvant, etc.); unfavorable weather conditions for herbicide activity; improper timing of herbicide application (in particular, applying postemergence herbicides after weeds are too large to control); and weeds emerging after application of a short-residual herbicide.

Once other possible causes of poor control have been eliminated, the following are common features that characterize fields where resistant populations occur:

  • The weed in question is initially confined to small patches within the field.
  • All species normally controlled by the herbicide except one are controlled well.
  • Healthy plants of the species in question are interspersed among plants of the same species that were killed.
  • The species not controlled is normally very susceptible to the herbicide in question.
  • Great dependence has been placed on the herbicide (little to no cultivation or other non-chemical control methods).
  • The field has a history of extensive use of the herbicide in question or herbicides with the same site of action.

If resistance is suspected, immediately stop using the herbicide in question and other herbicides within the same WSSA group number (Table 8-15). Contact a weed control expert for advice on how to stop seed production of the resistant weed and planning herbicide programs for future years. Take steps to avoid spreading the weed to other fields and farms.


Tables of Information for this Chapter

TableClick on the links below to see the tables associated with this webpage.
8-1 Grass and sedge response to preplant incorporated and preemergence corn herbicides.
8-2Broadleaf weed response to preplant incorporated and preemergence corn herbicides.
8-3Grass and sedge response to postemergence corn herbicides.
8-4Broadleaf weed response to postemergence corn herbicides.
8-5Rotational restrictions for corn herbicides.1
8-6Corn growth stage for application of postemergence herbicides.1
8-7Adjuvant reccomendations for corn herbicides applied at burndown or postemergence.1
8-8Labelled two-way tank mixes of postemergence corn herbicides.1
8-9Pre-Harvest intervals (PHI) for corn herbicides applied at burndown or postemergence.
8-10Rainfree requirements for corn herbicides applied at burndown or postemergence.
8-11Precautions for ALS-inhibiting herbicides on corn treated with organophosphate.
8-12Suggested treatments for perennial broadleaf weeds in corn.
8-13Formulations, active ingredients, leaching potential, restricted use classification, signal words of caution.
8-14Restricted entry intervals (REI) and personal protective equipment (PPE) required by Worker Protection Standards.
8-15Resistance risk category, herbicide family designation, mechanism of action, and WSSA resistance management grouping for commonly used herbicides.
8-16Active ingredients in packaged herbicide mixtures and co-packs.



This page created by: Alan D. Meijer, Agricultural Research Technician II on 08/29/00 and last revised on 8/30/00.