Source: Curtis Thompson, Kansas State University, Weed Management Specialist

A post-emergence application of glyphosate alone in Roundup Ready corn usually does a good job of controlling most broadleaf and grassy weeds. But there are times when control of certain broadleaf weeds with glyphosate is not adequate.

The following is a list of some of the most common broadleaf weed problems in Roundup Ready 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, to help control each of these problem weeds.

* Waterhemp and Palmer amaranth. Waterhemp and Palmer amaranth are vigorous weeds, with multiple buds on a plant. With contact herbicides, this requires thorough spray coverage for adequate control. These small-seeded pigweeds emerge throughout the summer, unless there is a thick crop canopy to shade the ground. Some populations of waterhemp have become resistant to glyphosate. However, there are several products that can help control waterhemp and Palmer amaranth in corn.

Lumax or Lexar (which contain Callisto plus S-metolachlor and atrazine) and Corvus or Balance Flexx (which contain isoxaflutole) are HPPD-inhibiting herbicides that effectively control pigweed when applied preemergence. Corvus contains Balance Flexx and thiencarbazone-methyl (a grass herbicide), which also will provide good grass control. Apply Corvus or Balance Flexx with atrazine only when applied postemergence (up through 2-leaf stage) to corn. These herbicides will provide varying degrees of residual control for late-emerging waterhemp and Palmer amaranth. Callisto, Impact, Laudis and Capreno contain HPPD-inhibiting herbicides that can be tank-mixed with glyphosate to help control waterhemp and Palmer amaranth postemergence. Status, which is Distinct with an added crop safener, will also help control waterhemp and Palmer amaranth. It can be tank mixed with glyphosate. Status will provide a little residual activity compared to applying glyphosate alone. Halex GT, is a premix that includes a high rate of glyphosate along with Callisto and S-metolachlor, providing good postemergence pigweed control with residual activity.

* Velvetleaf. Velvetleaf is sometimes not controlled with glyphosate alone. This may be due to the time of day glyphosate is applied, poor choice of AMS replacement product in the glyphosate, the condition of the plants, or other factors. As with the pigweeds, adding Callisto, Impact, Laudis, Lumax, Lexar, or Capreno to the glyphosate 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 might reduce the ability of glyphosate to translocate to the growing points, especially in less-susceptible species like kochia or pigweeds.

* Morningglory. This is another broadleaf weed that is not always controlled well 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. Aim EW and Priority can also help with morningglory control. Callisto, Impact, and Laudis may not be the best choice if morningglory is a severe problem, although if a pound of atrazine is added, these herbicides can be very effective. Actually, 2,4-D is very good on morningglory as well.

* Kochia. Kochia is like Palmer amaranth in some ways. It is a small-seeded broadleaf weed that can emerge all through the summer; however, greatest emergence is early spring. This weed can escape control with glyphosate alone unless it is actively growing and is thoroughly covered by the spray. Always use full rates of glyphosate (0.75 lb ae/a) and a good source of ammonium sulfate. Glyphosate resistance in kochia has been documented in several locations in western Kansas and appears to be increasing. Thus we do not recommend that glyphosate be applied alone if kochia is a problem. To improve control and gain a little residual control, producers can tank-mix glyphosate with Status, which is Distinct with a crop safener added. In addition to activity on kochia, Status will generally give excellent control of pigweeds and help control velvetleaf as well. Another option to enhance kochia control would be to tank-mix glyphosate with Callisto, Impact, Capreno, Laudis, Lumax, or Lexar — or use Halex GT. If Corvus and Balance Flexx plus atrazine are applied preemergence or early post, they can be very effective in controlling kochia.