Postemergence Herbicides on Corn

Chris Boerboom, Ext. Weed Scientist

Tuesday, May 16, 2006

In most years, I worry about herbicides being applied to corn that is too tall.  This year’s rainy weather may actually create the opposite situation.  In some fields, preemergence herbicides were not applied and people may be considering early postemergence options.  In most cases, postemergence corn herbicides can be applied to corn anytime after emergence, but there are a few exceptions such as Beacon, NorthStar (which contains Beacon), Buctril, Distinct, and Celebrity Plus (which contains Distinct).  Most of these herbicides need to be delayed until the corn is at least 4 inches tall (see table below).    

Speaking of delayed herbicide applications, if an application of preemergence herbicides in a liquid fertilizer carrier were planned, but cannot be made before corn emergence, do not apply this combination.  Most preemergence herbicide labels prohibit applications using a liquid fertilizer carrier after corn has emerged because of the risk of crop injury. The labels that don’t prohibit it warn against it for the same reason.  Rather than fertilizer, water should be used as the carrier after corn emergence.

Remember that the preemergence grass herbicides (ie. acetochlor, Define, dimethenamid, metolachlor, pendimethalin) do not control emerged grasses even though these herbicides are labeled for postemergence application.  The value of these herbicides when applied postemergence is the residual grass control that they provide. 

One other caution about early postemergence herbicide combinations relates to mixtures of growth regulator herbicides and ALS inhibiting herbicides.  Several ALS grass herbicides carry the warning that tank mixtures with dicamba or other growth regulating herbicides to stressed corn that is V3 or smaller may cause the corn to rattail later in the season.  These ALS grass herbicide include Accent, Accent Gold, Basis, Resolve, Steadfast and Stout.  Where I have seen this happen in the past is when a combination of the ALS herbicide, the growth regulator, and often a preemergence grass herbicide with an emulsifiable concentrate formulation are applied to V1-V2 corn during cold, wet conditions.  Delaying the application until warmer weather (faster herbicide metabolism) should greatly reduce the risk of injury from this type of application.

Despite a few specific precautions noted above, it is still best to target postemergence herbicide applications in corn on the early side to reduce the risk of yield loss from early season weed competition. 

A Chinese proverb says - Don't let slip an opportunity; it may never come again. In the case of emerged weeds in corn, this proverb should be changed from “may never” to “will never”.  

(beta testers, I formated the real "table" quickly. Look below and see how tabbed text comes in.)

Herbicide                    Corn stage                                      

Accent                         0-20" or V6; to 36" or V10

                                     with drop nozzles

Accent Gold WDG    0-12" or V5

Acetochlor                  0-11"

    (Harness, etc.)

Aim                              0-V8

Atrazine                      0-12"

Basagran                    no restriction

Basis                            0-V2

Beacon                       4-20" or V6; to tasseling

                                     with drop nozzles

Bicep Lite II Mag.    0-5"

Buctril                         4 leaf to before tasseling

Callisto                        0-30" or V8

Camix                         0-12"

Celebrity Plus             4-24" or V6

Define                         0-V5

Dicamba                     0-8" or 5 leaf; to 36"  

    (Clarity, etc.)          with drop nozzles

Dimethenamid          0-12"

    (Outlook, etc.)

Distinct                        4-24"; 24-36" with drop nozzles

Glyphosate                 0-30" or V8

    (Roundup, etc.)

G-Max Lite                0-12"

Hornet WDG              0-20" or V6; 20-36" with drop

                                     nozzles if harvested for grain

Impact                        emergence to 45 days before      

                                     harvest

Liberty                        0-24" or V7; 24-36" with drop

                                     nozzles

Liberty ATZ               0-12"

Lightning                    0-20" or V6

Lumax                        0-12"

Marksman                 0-8" or 5 leaf

Micro-Tech                0-5"

Metolachor                0-40"

    (Dual II Magnum, etc)

NorthStar                    4-20" or V2 -V6; to 36" with

                                     drop nozzles

Option                         V1-V6; to V7 with drop nozzles

Pendimethalin           0-30" or V8

    (Prowl, etc.)

Permit                          0-36"

Priority                        0-V8

Ready Master ATZ  0-12"

Resolve                       0-12” or V5

Resource                     V2-V10

Shotgun                      0-8"; to 12" with drop nozzles

Steadfast                    0-20" or V6

Steadfast ATZ           0-12"

Stinger                         0-24"

Stout                            0-16” or V5

Yukon                         0-36"

2,4-D                           0-8"; to before tasseling with

                                     drop nozzles