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Issue No. 9, Article 7/May 25, 2007

Musings About Postemergence Corn Herbicides

Weed control in the Illinois corn crop has been challenged by both wet and dry soil conditions, depending on location. Much of east-central Illinois is experiencing a stretch of dry soil conditions, while excessive precipitation in other areas has contributed to corn replanting. With most corn planting completed, attention has moved to application of postemergence corn herbicides.

Postemergence herbicides are integral to an integrated weed management program. Applications made after crops and weeds have emerged allow for identification of the weed species present as well as the severity of infestation so that herbicide selection can be tailored to the particular field. Postemergence herbicide applications minimize the interactions of the herbicide with factors associated with soil (such as soil texture and organic matter content), but they often magnify interactions between the herbicide and prevailing environmental conditions. For weed control to be achieved with postemergence herbicides, the herbicide must come in contact with the target, be retained on the leaf surface prior to absorption into the plant, be able to reach the site of action within the plant, and finally induce some phytotoxic response. If for any reason one or more of these steps is restricted or limited, the level of weed control can be expected to decline.

Plant age and size, relative humidity, soil moisture, and temperature are factors that influence absorption of postemergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend that applications be made when target weeds are small and caution about reduced effectiveness if applications are made to larger plants. Also, several postemergence herbicide labels suggest users increase application rates or delay applications if weeds are under "adverse environmental conditions." Examples of such conditions may include prolonged periods without significant precipitation (dry soil) or low air temperatures. On the other hand, high relative humidity, adequate soil moisture, and moderate to warm air temperatures all favor enhanced herbicide absorption. Remember that if conditions favor rapid herbicide absorption into weeds, conditions are also favorable for enhanced absorption into the crop, which may result in crop injury.

Almost all postemergence corn herbicides have application restrictions with respect to maximum corn size (specified as height, leaf number, or sometimes both). Table 1, reproduced from the 2007 Illinois Agricultural Pest Management Handbook, contains summary information for many postemergence corn herbicides, including maximum corn size for broadcast applications.

Farmers are also encouraged to remember that some postemergence corn herbicides have application timing restrictions based on minimum corn size. For example, the Status (diflufenzopyr + dicamba) label indicates that broadcast applications should be made when corn is between 4 (V2) and 36 (V10) inches tall. Table 2 lists some of the postemergence corn herbicides with minimum corn size restrictions on their labels.

As mentioned in previous issues of the Bulletin, across some areas of Illinois preplant and preemergence corn herbicides have been on the ground anywhere from a few days to several weeks without adequate precipitation to move the herbicides into the soil solution. Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed on a dry soil surface with no incorporation (mechanical or by precipitation) for several days following application. Several postemergence herbicides for grass control in corn are described in the following paragraphs. In general, grass weeds should be treated before they exceed 3 to 4 inches in height.

Several of these products have label restrictions regarding applications to corn previously treated with certain soil insecticides. Additionally, not all products can be tankmixed with foliar-applied insecticides, and certain products cannot be applied within a certain number of days before or after a foliar insecticide application. Be sure to consult the label for other restrictions and limitations.

Accent 75WDG (nicosulfuron) can be applied broadcast at 0.33 to 1.33 ounces per acre to corn up to 20 inches tall or through the V6 stage, whichever is more restrictive. Post-directed applications can be made to corn from 20 to 36 inches tall or before the V10 stage. Similar to other ALS-inhibiting corn herbicides, the Accent label cautions about applications to corn previously treated with certain soil insecticides. Do not make more than two applications of Accent per growing season.

Atrazine can be applied as a postemergence treatment before corn exceeds 12 inches in height to control certain annual grasses (not fall panicum) up to 1.5 inches tall. Include a crop oil concentrate (COC) with postemergence atrazine applications.

Basis 75WDG (rimsulfuron + thifensulfuron) can be applied at 1/3 ounce per acre to field corn in the spike through the 4-leaf (2 collars) stage for control of 1- to 2-inch-tall barnyardgrass, foxtails, and fall panicum. Do not apply to corn having 3 fully emerged collars or over 6 inches tall. Applications of Basis must include a COC (petroleum or methylated seed oil [MSO]) or a nonionic surfactant (NIS). An annonium nitrogen fertilizer also must be added with the COC or NIS. The Basis label includes precautionary statements about making applications to corn previously treated with certain soil insecticides.

Beacon 75WDG (primisulfuron) can be applied broadcast at 0.76 ounce per acre to corn between 4 and 20 inches tall. Corn plants less than 4 inches tall may be more susceptible to injury. Applications should include a COC or an NIS; a liquid nitrogen fertilizer also may be included. Beacon is effective on shattercane, johnsongrass, and quackgrass but is weaker than Accent on other annual grass weed species. NorthStar 47.4WDG and Spirit 57WDG are premixes containing the active ingredient of Beacon plus dicamba or prosulfuron, respectively. While primarily used for broadleaf weed control, these herbicides can also control certain annual and perennial grass weed species. Their labels also carry precautionary statements regarding applications to corn previously treated with certain soil insecticides.

Celebrity Plus 70WDG is a premix product containing nicosulfuron + dicamba + diflufenzopyr that controls many of the same grass weed species as Accent. Broadcast applications should be made to corn between 4 and 20 inches tall. An application rate of 4.7 ounces per acre of Celebrity Plus provides the equivalent amount of nicosulfuron contained in 0.67 ounce of Accent 75WDG.

Glyphosate (various formulations) can be applied to glyphosate-resistant corn hybrids for control of annual and perennial grass species. Applications should include a spray grade ammonium sulfate; any other required additives (such as NIS) will be specified on the product label. Calculations of equivalent application rates among various glyphosate formulations should be based on the acid equivalent contained in the respective formulations. Expert 4.88SC and Field Master 4.06S, premixes containing glyphosate, atrazine, and either S-metolachlor or acetochlor, respectively, can be applied postemergence to glyphosate-resistant corn hybrids.

Impact 2.8SC (topramezone) provides postemergence control of certain annual grass weed species. Impact is often used in tankmixes with other herbicides (especially atrazine) to control a broad spectrum of weeds. Applications can be made to corn up to 45 days before harvest but are limited to corn up to 12 inches high if tankmixed with atrazine. Application rates range from 0.5 fluid ounce (north of Interstate 80) to 0.75 fluid ounce per acre. MSO with an ammonium nitrogen fertilizer is usually the preferred spray additive combination, but COC can be substituted for MSO.

Liberty 1.67S (glufosinate) can be applied at 28 to 34 fluid ounces per acre only to corn hybrids resistant to glufosinate. Applications must include a spray-grade ammonium sulfate. Thorough spray coverage of the target vegetation is necessary to achieve consistent grass control, so pay particular attention to application volume, pressure, and nozzle selection.

Option 35WDG (foramsulfuron) can be applied broadcast at 1.5 to 1.75 ounces per acre to corn from V1 through V6 growth stages. Applications of Option must include a methylated (MSO) or ethylated (ESO) seed oil and an ammonium nitrogen fertilizer. The 1.5-ounce rate of Option will provide control of barnyardgrass up to 4 inches tall, green or yellow foxtail and fall panicum up to 3 inches tall, giant foxtail up to 6 inches tall, and shattercane up to 12 inches tall. Several other annual and perennial grass weed species are listed on the label. The Option label includes precautionary statements about making applications to corn previously treated with certain soil insecticides. Equip 32WDG, a premix containing foramsulfuron, controls many of the same grass weed species as Option. Grass sizes on the Equip label are generally smaller than those listed on the Option label. A 1.5-ounce application rate of Equip provides less foramsulfuron than 1.5 ounces of Option.

Resolve 25DF (rimsulfuron) can be applied at 0.5 to 2 ounces per acre to corn up to 12 inches tall but not exceeding 5 leaf collars. Postemergence in-crop applications require the addition of an NIS and an ammonium nitrogen fertilizer. Resolve can provide control of barnyardgrass, foxtails, and fall panicum up to 2 inches tall. The Resolve label includes precautionary statements about making applications to corn previously treated with certain soil insecticides.

Steadfast 75WDG (nicosulfuron + rimsulfuron) can be applied at 3/4 ounce per acre to control foxtails, barnyardgrass, and fall panicum up to 4 inches tall, shattercane up to 6 inches tall, quackgrass up to 8 inches tall, and johnsongrass up to 12 inches tall. Steadfast may be applied to corn up to 20 inches tall or through the V6 stage, whichever is more restrictive. Applications must include a COC or an NIS and an ammonium nitrogen fertilizer. The Steadfast label includes precautionary statements about making applications to corn previously treated with certain soil insecticides.

Steadfast ATZ 89.3WDG (nicosulfuron + rimsulfuron + atrazine) can be applied at 14 ounces per acre to corn up to 12 inches tall or through the V6 stage, whichever is more restrictive. Grass species and sizes listed on the Steadfast ATZ label are nearly identical to those listed on the Steadfast label. Applications must include a COC or an NIS and an ammonium nitrogen fertilizer. The Steadfast ATZ label includes precautionary statements about making applications to corn previously treated with certain soil insecticides.

Stout (nicosulfuron and thifensulfuron) contains the active ingredients of Accent and Harmony GT XP. The product is labeled for postemergence applications in field corn that is up to 16 inches tall and exhibiting up to 5 leaf collars. Applications of 1/2 to 3/4 ounce per acre must include either a COC or an NIS and an ammonium nitrogen fertilizer.

--Aaron Hager

Author:
Aaron Hager

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