How late can postemergence herbicide applications be made in corn?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Many areas of the state have been hit with wet weather this spring that has kept them out of the fields during ideal postemergence herbicide application windows. As I have driven around the state, particularly in the southern tiers of counties, I have seen many fields where weeds have taken control and are as tall as the corn or taller. The question then becomes “How late can I spray my corn?” or “How tall can my corn be when I spray?” These are important questions to be considered, and the answer is not always obvious since delayed postemergence applications in corn have largely become a thing of the past.

The majority of herbicides applied postemergence to corn can be found in Table 1J in 2010 Weed Control Guide for Field Crops Extension Bulletin E-434, which gives the height restrictions for many common postemergence herbicides. An abbreviated table is listed below (Table 1).

It is extremely important to note that atrazine and atrazine-containing products can only be applied on corn up to 12 inches in height. Most of the corn in the state is now past this stage, so other herbicide options will have to be utilized. It is also important to remember that glyphosate (Roundup Ready corn) can only be applied to corn up to 30 inches or 8 collars and Ignite (Liberty Link corn) can only be applied to corn up to 24 inches or 7 collars, so plan your herbicide applications accordingly. There are several products that can be used alone or in combination with these non-selective herbicides including the HPPD inhibiting herbicides Callisto (30 inches or 8 collars), Impact (up to 45 days prior to harvest), and Laudis (8 collars) or ALS containing products such as Hornet WDG (20 inches or 6 collars) and Resolve Q (20 inches or 7 collars) to provide both additional postemergence and residual control of weeds. Postemergence applications of Accent Q or Steadfast Q can be made to corn up to 20 inches or showing 6 collars for grass and small seeded broadleaf control. Several products can be applied up to canopy closure, including Permit and Resource for control of certain broadleaf weed species. In rare cases where applications need to be made later still, Buctril can be applied until tassel emergence or Impact can be applied up to 45 days before harvest, leaving a wide window of application.

For all delayed postemergence applications, it is extremely important to consider rotation crops for the 2011 season. The later an application is made in the season, the greater the chance for injury to occur the following season in sensitive crops. Consult Table 12 in the 2010 Weed Control Guide for Field Crops Extension Bulletin E-434.

If residual control of grass weeds is needed, products containing metolachlor (Dual II Magnum, Cinch, Stalwart C, Parallel) can be applied up to 40-inch corn and Prowl H20 can be applied up to 30-inch corn. As always, rainfall is needed for activation of preemergence herbicides. For further information, see 2010 Weed Control Guide for Field Crops Extension Bulletin E-434 or consult herbicide labels.

Table 1. Maximum height for postemergence herbicides in corn.

Herbicide Rate/A Maximum Height
Accent Q 0.67 oz 20” or 6 collars
Aim EW 0.5 oz 8 collars
Atrazine4L 2 qt 12”
Basagran 2 pt None
Beacon 0.76 oz 20”
Buctril, Moxy, others 1.5 pt Before tassel emergence
Callisto 3 fl oz 30” or 8 collars
glyphosate (Glyphosate-Resistant Corn) 0.75 lb a.e. 30” or 8 collars
Hornet WDG 3.0 oz 20” or 6 collars
Impact 0.75 oz 45 days before harvest
Laudis 3 oz 8 collars
Ignite (Liberty Link Corn) 24 fl oz 24” or 7 collars
Northstar 5 oz 20” (V6)
Option 1.5 oz 6 collars or 16”
Permit 0.67 oz Canopy closure
Resolve Q 1.25 oz 20” or 7 collars
Resource 4 fl oz 10 collars or canopy closure
Steadfast Q 0.75 oz 20” or 6 collars
Status 5 oz 36”(V10)
Stinger 0.25 pt 24”
Unity 0.083 oz 16” or 5 collars

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