Managing problem weeds in Roundup Ready corn

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A postemergence application of glyphosate alone in Roundup Ready corn often can do a good job of controlling many broadleaf and grassy weeds. But producers should not rely strictly on glyphosate alone for several reasons:

  • Relying just on glyphosate for weed control increases the risk of yield loss from early-season weed competition due to intentional or weather-delayed late applications.
  • Control of certain broadleaf weeds, such as kochia, Palmer amaranth, waterhemp, velvetleaf, morningglory, or marestail, often is not adequate with glyphosate alone.
  • Glyphosate-resistant weeds will not be controlled with glyphosate alone and are occurring more frequently in fields across the entire state.
  • Using glyphosate alone will speed up the selection process for glyphosate-resistant weeds, creating control problems for the near future.

The following is a list of some of the most common broadleaf weed problems in corn, both in eastern and western Kansas, and some of the most effective herbicides that can be applied preplant or preemergence, or tank-mixed with glyphosate in Roundup Ready corn, to help control each of these problem weeds.

Waterhemp, Palmer amaranth, and other pigweeds

These are vigorous weeds, with multiple growing points on a plant. With contact herbicides, early application and thorough spray coverage are required for adequate control. These small-seeded pigweeds emerge throughout the summer, making them difficult to control without the use of preemergence herbicides or postemergence herbicides with residual activity. Populations of waterhemp and Palmer amaranth are frequently resistant to glyphosate. Palmer amaranth is now found on dryland and irrigated fields throughout the state.

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

Lumax EZ or Lexar EZ (premixes of Callisto plus S-metolachlor and atrazine), Zemax (a premix of Callisto and S-metolachlor), and Corvus or Balance Flexx (which contain isoxaflutole) are products that contain HPPD-inhibiting herbicides. These products effectively control pigweed species when applied preemergence. Corvus contains Balance Flexx and thiencarbazone-methyl (a grass herbicide), so Corvus will also provide good grass control. Corvus or Balance Flexx performance is always improved if tank-mixed with atrazine. If Corvus or Balance Flexx are applied postemergence to corn from emergence through the 2-leaf stage, only atrazine (no other herbicides or adjuvants) can be tank-mixed. These herbicides will provide varying degrees of residual control for later-emerging waterhemp and Palmer amaranth.

All chloroacetamide herbicides (active ingredients may include acetochlor, metolachlor, S-metolachor, or dimethenamid) -- including the new products Zidua, Anthem, Anthem ATZ, and Fierce (all of which include the active ingredient pyroxasulfone) -- have excellent activity on pigweeds. As rates of these products increase, the length of residual control of pigweeds will increase. Sharpen or Verdict (a premix of Outlook and Sharpen) have excellent activity on pigweeds. However, their use rates are too low to provide extended preemergence control of pigweeds and should be tank-mixed with an additional chloracetamide herbicide and atrazine. There are several other herbicides containing some of the active ingredients listed above which can provide excellent control of pigweeds.

Postemergence products Callisto, Realm Q, Solstice, Impact, Armezon, Laudis, and Capreno contain HPPD-inhibiting herbicides that can be tank-mixed with glyphosate to help control waterhemp and Palmer amaranth. Status, which is Distinct with an added crop safener tank-mixed with glyphosate, will also help control glyphosate-resistant waterhemp and Palmer amaranth. Status will provide a little residual activity compared to glyphosate alone. However, the residual activity is generally very short when applied during days with warm temperatures. Bayer has a new product called DiFlexx which is a premix of Clarity and a safener (which is active soil or POST applied, and is different from the safener used in Status). Tank-mixing any of these products with glyphosate will enhance pigweed control. Halex GT is a premix that includes a high rate of glyphosate along with Callisto and S-metolachlor. This product provides good postemergence pigweed control with residual activity from the S-metolachlor. Adding products such as Warrant, Outlook, Zidua, Anthem, or Dual II Magnum (and generics – see labels) to a POST program can extend residual pigweed control, much like Halex GT will provide.  


Velvetleaf is sometimes not controlled with glyphosate 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 condition of the plants. Frequently, velvetleaf plants in the sprayer wheel tracks will not be effectively controlled. Velvetleaf control often is less with early morning or late evening applications. Velvetleaf tends to have a high concentration of calcium cations on the leaf surface; thus, adequate AMS must be in the spray solution to give good control.

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


Morningglory is another broadleaf weed that is not always controlled with glyphosate. Adding Status (Distinct plus a crop safener) to glyphosate is one of the best ways to improve morningglory control in Roundup Ready corn. Callisto, Realm Q, Solstice, Armezon, Impact, and Laudis may not be the best choice if morningglory is a severe problem, although if a pound of atrazine is added to the tankmix, these herbicides can be very effective. Actually, 2,4-D is very good on morningglory as well. Having a preemergence program in place with the herbicides discussed for velvetleaf in conjunction with a postemerge program, morningglory can be controlled. 


Kochia, like the pigweeds, is a small-seeded broadleaf weed. However, it starts emerging in early spring and often has reached 90% emergence by late April. Successful kochia management in corn is dependent on a February or early March application of effective residual products. Kochia continues to emerge at a low frequency all through the summer. This weed often will escape control if glyphosate alone is used in corn since glyphosate-resistant populations of kochia have spread through western Kansas. Always use full rates of glyphosate (0.75 lb ae/a) and use a good source of ammonium sulfate. To reiterate, we do not recommend that glyphosate be applied alone.

Producers can tank-mix glyphosate with Status, DiFlexx, or other dicamba products to enhance kochia control. Another option to enhance kochia control would be to tank-mix glyphosate with Callisto, Solstice, Realm Q, Armezon, Impact, Capreno, or Laudis; or use Halex GT. If Corvus or Balance Flexx plus atrazine, Lumax EZ, Lexar EZ, or the new pyroxasulfone products with atrazine are applied preemergence, they effectively control germinating kochia and greatly benefit a kochia management program in corn. When controlling kochia with postemergence herbicides, it is important to spray when kochia is small -- 2 to 4 inches in height. Larger kochia likely will not be controlled.

Marestail or horseweed

Marestail can often be a significant problem when corn follows soybeans, especially when marestail was left uncontrolled during the soybean production year. When that happens, it usually means the marestail population is glyphosate-resistant. Fortunately, several herbicides that can be used ahead of corn planting have excellent activity. The best option is a fall application of 2,4-D or dicamba with atrazine and/or glyphosate, which all can provide excellent marestail control. Some sulfonylurea herbicides can be effective; however, if marestail populations are ALS-resistant, marestail will not be controlled with sulfonylurea herbicides unless they are tank-mixed with a growth regulator herbicide.

If no fall applications are made, it is very important that early spring applications (March) be made. The addition of a dicamba-based product to a tankmix is important for early spring marestail control. Dicamba is weak on winter annual mustards, thus having other effective herbicides, such as 2,4-D or glyphosate, in the tank is important. Atrazine continues to have good activity on small rosette-stage marestail. However, as the plant gets larger and bolts, the level of control is reduced. An application of 2,4-D at the rate of 1 quart per acre of 4 lb/gal product can be effective in the spring on small marestail; however, a pint of dicamba has been more consistent. Distinct contains dicamba and can control marestail effectively. 

Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist