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Issue No. 8, Article 5/May 13, 2005

Considerations for Weed Control in Corn

Weed control considerations in the Illinois corn crop range all over the board, from postemergence applications being made for control of grass and broadleaf weed species to replanting fields that were excessively damaged by cold temperatures. We offer the following comments for your consideration.

If a cornfield will be replanted, hybrid selection for the replanting operation should be taken into account, especially if the field has been previously treated with certain soil-applied or post-emergence herbicides that require a particular herbicide resistant/tolerant corn hybrid. For example, if a Clearfield corn hybrid was initially planted and received a preemergence application of Pursuit Plus or already has been treated postemergence with Lightning, the replanted corn must also be a Clearfield hybrid. If a nonresistant/tolerant hybrid is replanted instead, the potential exists for the herbicide to cause a great deal of crop injury. If you initially planted a glyphosate-resistant corn hybrid and have areas of a field that need to be replanted, either replant these areas with a glyphosate-resistant hybrid or take special precautions during the post-emergence glyphosate application if you replant with a conventional hybrid.

Is there an interval between when a herbicide was applied and when corn replanting can occur? For soil-applied and most postemergence corn herbicides, replanting can proceed whenever field conditions are feasible. However, for a small number of postemergence corn herbicides, there are intervals between application and replanting. For example, if a cornfield previously treated with Spirit, NorthStar, Permit, or Yukon is lost and must be replanted, there is a 4-week, 14-day, 1-month, or 1-month interval, respectively, that must elapse between the herbicide application and corn replanting.

While most soil-applied herbicides allow more than one application per season, a few can be applied only once per season. For example, the Epic label indicates that only one application per year can be made; Prowl can be used as a soil-applied or postemergence treatment, but if a previously treated corn crop must be replanted, do not make another application or till the ground prior to replanting. In instances where small areas of a field will be replanted, some may elect to simply replant without applying any additional herbicide. If, however, you elect to make a second application of a particular corn herbicide, keep in mind that many product labels indicate a maximum per-acre rate that can be applied during one growing season.

Questions have been posed about how to control emerged corn from the first planting before the field is replanted. Tillage is an effective option for control of the first planting, but some farmers may not want to disturb the soil prior to replanting and so seek a herbicide alternative. Glyphosate is very effective for controlling emerged corn, and there is no waiting interval between application and planting speci-fied on the label. The postemergence grass-control herbicides used in soybean can effectively control emerged corn, but each of these product labels carries rotational intervals between application and planting corn. Poast Plus is labeled for preplant applications, but applications must be made at least 30 days before corn planting. The labels of Select, Fusion, Fusilade DX, and Assure II indicate rotational intervals of 30, 60, 60, and 120 days, respectively, between application and corn planting.

Obviously, glyphosate will not control emerged glyphosate-resistant corn. With the aforementioned limitations regarding the use of the postemergence grass herbicides, few herbicide options exist to control the first planting of glyphosate-resistant corn before replanting. Potential options to control a first planting of glyphosate-resistant corn include (1) products containing isoxaflutole can cause significant damage if applied to emerged corn, (2) replanting with a glufosinate-resistant hybrid would allow for the postemergence use of glufosinate, and (3) replanting with a Clearfield hybrid would allow for the postemergence use of products such as Lightning.

Labels of postemergence corn herbicides often indicate a maximum crop stage beyond which broadcast applications cannot be made. Frequently, these label restrictions refer to plant height, crop growth stage, or both. For products whose label indicates crop height and growth stage, it is important to follow the more restrictive of the two for each particular field. This is very important given the adverse (i.e., cold) weather that the corn crop recently experienced. When air temperatures are cool, corn usually remains relatively small in regard to plant height; however, it continues to advance developmentally. Loss of leaves and leaf tissue from the recent frosts will make determining corn development stage more difficult with respect to herbicide applications. Application restrictions based on corn developmental stage are usually stated with respect to the number of leaf collars present on the plants. Many agree that the leaf collar method is a more accurate method to determine physiological plant age than is plant height.

So, if a corn plant had two leaf collars before losing those leaves to frost, what "number" should the next leaf collar be considered with respect to a postemergence herbicide application? The safest recommendation is to consider it as the third collar. What if you can't really determine how many collars may have been lost because of frost? You might get some idea based on when the field was planted and how many growing degree-days have accumulated (assuming 80 to 85 growing degree-days are needed for each leaf collar). An additional safety factor would be to count the existing leaf collars and add one. In issue no. 7 of the Bulletin (May 6, 2005), Table 4 summarized a large amount of information for postemergence corn herbicides. The column titled "Apply over the top of corn" indicates the maximum corn growth stage at which broadcast applications can be made.

While corn height appears self-explanatory, some ambiguity often exists with respect to where on the corn plant you measure to. In general, many people determine corn plant height by measuring from the soil surface to the top of the whorl or to the arch of the uppermost leaf that is more than 50% emerged. This should be done on a number of plants and then averaged to account for variability among corn plants in the field.--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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