CurtisThompson K-State Extension Agronomy State Leader and Weed Management Specialist cthompso@ksu.edu

A postemergence application of glyphosate alonein Roundup Ready corn often can do a good job of controlling most broadleaf andgrassy weeds. But producers should not rely strictly on glyphosate alone forseveral reasons:

•        Relyingjust on glyphosate for weed control increases the risk of yield loss fromearly-season weed competition due to intentional or weather-delayed lateapplications.

•        Control ofcertain broadleaf weeds, such as kochia, velvetleaf, morningglory, ormarestail, often is not adequate with glyphosate alone.

•        Glyphosate-resistantweeds will not be controlled with glyphosate alone and are occurring morefrequently in fields across the entire state.

•        Usingglyphosate alone will speed up the selection process for glyphosate-resistantweeds, creating control problems for the future.

The following is a list of some of the mostcommon broadleaf weed problems in corn, both in eastern and western Kansas, and some of themost effective herbicides that can be applied preplant or preemergence, or tank-mixedwith glyphosate in Roundup Ready corn, to help control each of these problemweeds.

Waterhemp, Palmer amaranth, and other pigweeds. Waterhemp, Palmer amaranth, and otherpigweeds are vigorous weeds, with multiple growing points on a plant. Withcontact herbicides, thorough spray coverage and early application is requiredfor adequate control. These small-seeded pigweeds emerge throughout the summer,making them difficult to control without preemergence herbicides orpostemergence herbicides with residual activity. Populations of waterhemp arefrequently resistant to glyphosate and Palmer amaranth populations resistant toglyphosate are increasing. Palmer amaranth has been primarily found on goodbottom ground in eastern Kansas, irrigated anddryland fields in central Kansas, andirrigated fields in western Kansas.In the last few years, Palmer amaranth has spread to dryland acres throughoutthe state.

There are several products that can helpcontrol waterhemp and Palmer amaranth in corn.

Lumax EZ or Lexar EZ (which contain Callistoplus S-metolachlor and atrazine), Zemax (which contains Callisto andS-metolachlor), and Corvus or Balance Flexx (which contain isoxaflutole) areproducts that contain HPPD-inhibiting herbicides that effectively controlpigweed species when applied preemergence. Corvus contains Balance Flexx andthiencarbazone-methyl (a grass herbicide), which also will provide good grasscontrol. Corvus or Balance Flexx performance is always improved if tank-mixedwith atrazine. If Corvus or Balance Flexx are applied postemergence to corn,only atrazine (no other herbicides or adjuvants) can be tank-mixed when appliedto corn from emergence through the 2-leaf stage. These herbicides will provide varyingdegrees of residual control for later-emerging waterhemp and Palmer amaranth.

All chloroacetamide herbicides (activeingredients may include acetochlor, alachlor, metolachlor, S-metolachor, anddimethenamid), including the new products Zidua, Anthem, Anthem ATZ, and Fierce(all of which include the active ingredient pyroxasulfone) have excellentactivity on pigweeds. As rates of these products increase, the length ofresidual control of pigweeds will increase. Verdict (which contains Outlook andSharpen) or Sharpen have excellent activity on pigweeds. However, their userates are too low to provide extended preemergence control of pigweeds. Thereare several other herbicides containing some of the active ingredients listedabove with can provide excellent control of pigweeds.

Postemergence products Callisto, Impact, Laudis,and Capreno contain HPPD-inhibiting herbicides that can be tank-mixed withglyphosate to help control waterhemp and Palmer amaranth. Status, which isDistinct with an added crop safener tank mixed with glyphosate, will also helpcontrol glyphosate-resistant waterhemp and Palmer amaranth. Status will providea little residual activity compared to glyphosate alone, however the residualactivity is generally very short when applied during days with warmtemperatures. Halex GT, is a premix that includes a high rate of glyphosatealong with Callisto and S-metolachlor and provides good postemergence pigweedcontrol with residual activity.  

Velvetleaf. Velvetleaf is sometimes not controlled withglyphosate alone. This may be due to the time of day glyphosate is applied,poor choice of the AMS replacement product with the glyphosate, or the stress conditionof the plants. Frequently, velvetleaf plants in the sprayer wheel tracks willnot be effectively controlled. Velvetleaf control often is less with earlymorning or late evening applications. Velvetleaf tends to have a highconcentration of calcium cations on the leaf surface; thus, adequate AMS mustbe in the spray solution to give good control.

As with the pigweeds, adding Callisto, Impact,Laudis, Lumax EZ, Lexar EZ, or Capreno to the glyphosate, or using Halex GT,can help with velvetleaf control. Corvus or Balance Flexx applied preemergenceup through 2-leaf corn can provide good velvetleaf control. Another option isto tank-mix glyphosate with Cadet, Aim EW, or Priority (a premix of Aim EW andPermit, an ALS herbicide). These herbicides are excellent on velvetleaf. One ofthe concerns about a tankmix of Aim or Cadet and glyphosate, however, is thatthese herbicides may rapidly burn leaf tissue and reduce the ability ofglyphosate to translocate to the growing points. Adding Sharpen or Verdict to achloroacetamide/atrazine tankmix, or using either the new Zidua, Anthem, orAnthem ATZ products as a preemergence or Fierce as an early preplant, willgreatly enhance a velvetleaf control program -- provided the preemergenceherbicides are rainfall activated.

Morningglory. This is another broadleaf weed that is notalways controlled with glyphosate. Adding Status (Distinct plus a crop safener)to glyphosate is one of the best ways to improve morningglory control inRoundup Ready corn. Callisto, Impact, and Laudis may not be the best choice ifmorningglory is a severe problem, although if a pound of atrazine is added tothe tankmix, these herbicides can be very effective. Actually, 2,4-D is verygood on morningglory as well. As discussed with velvetleaf, having apreemergence program in place with the herbicides discussed for velvetleaf inconjunction with a postemerge program, morningglory can be controlled. 

Kochia. Kochia, like the pigweeds, is a small-seeded broadleaf weed. However,it starts emerging in early spring and continues to emerge at a low frequencyall through the summer. This weed often will escape control with glyphosatealone as glyphosate-resistant populations of kochia have spread through westernKansas.Always use full rates of glyphosate (0.75 lb ae/a) and use a good source ofammonium sulfate. We do not recommend that glyphosate be applied alone.Producers can tank-mix glyphosate with Status or other dicamba products toenhance kochia control. Another option to enhance kochia control would be totank-mix glyphosate with Callisto, Impact, Capreno, or Laudis; or use Halex GT.If Corvus or Balance Flexx plus atrazine, Lumax EZ, Lexar EZ, or the newpyroxasulfone products with atrazine are applied preemergence, they can be veryeffective in controlling germinating kochia and greatly benefit a kochiamanagement program in corn. When controlling kochia with postemergenceherbicides, it is important to spray when kochia is small, 2 to 4 inches inheight. Larger kochia likely will not be controlled.

Marestail. Marestail can oftenbe a significant problem when corn follows soybeans, especially when marestailwas left uncontrolled during the soybean production year usually meaning it'sglyphosate resistant. Fortunately, several herbicides that can be used ahead ofcorn planting have excellent activity. The best option is a fall application of2,4-D or dicamba with atrazine and/or glyphosate, which all provide excellentcontrol. Some sulfonylurea herbicides can be effective; however, if marestailpopulations are ALS resistant, marestail will not be controlled withsulfonylurea herbicides unless they are tank mixed with a growth regulatorherbicide. If no fall applications are made, it is very important that earlyspring applications (March) be made. The addition of a dicamba-based product toa tankmix is important for early spring marestail control. Dicamba is weak onwinter annual mustards, thus having other effective herbicides in the tank isimportant. Atrazine continues to have good activity on small rosette-stagemarestail, however, as the plant gets larger and bolts, control is reduced.2,4-D at the rate of 1 quart per acre of 4 lb/gal product can be effective inthe spring on small marestail, however, a pint of dicamba has been moreconsistent. Distinct contains dicamba and can control marestaileffectively. 

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