Postemergence Herbicide Options for Grass Control in Corn

May 26, 2000
Recently, we received several calls regarding options for postemergence grass control in corn. These calls are generally related to instances where a soil-applied grass herbicide failed due to lack of precipitation after application. Currently, there are several herbicide options available for post- emergence grass control in corn. Some of these options require the use of "special" corn hybrids, such as Roundup Ready (RR), Liberty Link (LL), Clearfield ([CL] Imidazolinone Resistant/Tolerant), or Poast Protected (PP). Other options such as Accent, Accent Gold, Basis, Basis Gold, Celebrity Plus, and atrazine do not require "special" hybrids. When deciding on a postemergence grass-control option, consider several factors.

The first question to ask is, "What type of corn hybrid did I plant?" If a herbicide resistant/tolerant hybrid was not planted, the available options would be limited to those herbicides that do not require the use of a herbicide resistant/tolerant hybrid. These options generally consist of products that contain the active ingredients nicosulfuron (Accent) or primisul-furon (Beacon). Both of these active ingredients are ALS-inhibiting herbicides and will not control grass species that are resistant to this class of herbicides. As mentioned in issue no. 3 of the Bulletin, there are some isolated incidences of ALS-resistant shattercane and giant foxtail in certain areas of Illinois.

Another important consideration is the grass spectrum in a particular field and the size these species are. Atrazine plus crop oil concentrate can provide control of several grass weed species as long as they are less than 1.5 inches in height. Basis provides control of foxtails, barnyardgrass, and fall panicum 2 inches in height or less. Other postemergence grass herbicides can provide control of certain grasses up to 20 inches in height, depending on the herbicide and target grass species. However, grasses should be controlled much earlier than this to prevent corn yield loss due to weed competition. A listing of the postemergence grass herbicides and maximum grass heights for these herbicides can be found in Table 1.

Along with determining the size of the target grass species, it is important to remember there are maximum corn heights or developmental stages listed on the labels of many of these herbicides. If applications are made to corn exceeding these heights or stages, a number of these herbicides can cause significant corn injury that may result in yield reduction. These corn heights should be monitored carefully. For example, Accent Gold and Basis Gold may be applied to corn up to 12 inches tall or at the V6 stage, whichever is more restrictive. Table 1 also lists maximum corn heights for these herbicides. Remember that some of these herbicide labels also have maximum leaf stages listed on their labels that may be more restrictive.

Finally, adjuvant selection is also very important when using these different herbicides for postemergence grass control. The appropriate adjuvant selection increases the activity of the herbicide on a particular weed species, as well as helps reduce injury to the crop. Adjuvant selection options can vary with the herbicide, tank-mix partner, and environmental conditions. Always consult the herbicide label for the appropriate adjuvant selection, and remember that these herbicides should not be applied in a liquid fertilizer carrier, or severe corn injury can occur.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague