Despite the fact that corn has struggled with this spring’s cold weather, it eventually will start growing once we get some heat. As corn and weeds start to grow more rapidly, it’s important to keep track of the right timing to apply herbicides if using a postemergence herbicide. Of course, it is important to apply postemergence herbicides to weeds while they are still small enough to be controlled. It is equally important to apply postemergence herbicides during labeled corn growth stages so the corn does not have an increased risk of injury. A classic example of postemergence corn injury is when an ALS-inhibiting herbicide is applied late and causes pinched ears (Fig. 1). However, injury has occasionally been observed even with extremely safe herbicides such as applications of glyphosate made well after the labeled stage of growth. Regardless of the perceived crop safety of a herbicide, it must be applied according to the label’s directions. It’s a wise choice in addition to a legal (i.e. label) requirement.

 

Figure 1. Pinched corn ears caused by an application of an ALS-inhibiting herbicide after the labeled growth stage. 

 Common corn herbicides that can be applied postemergence and their labeled growth stages.

Herbicide	Corn stage
Accent	0-20" or V6; to 36" or V10 with drop nozzles
Aim	0-V14
Atrazine	0-12"
Basagran	no restriction
Basis	0-V2
Beacon	4-20" or V6; to tasseling with drop nozzles
Buctril	4 leaf to before tasseling
Cadet	V2 to 48”
Callisto	0-30" or V8
Celebrity Plus	4-24" or V6
Dicamba (Clarity, etc.)	0-8" or 5 leaf; to 36" with drop nozzles
Glyphosate (Roundup, etc.)	0-30" or V8; to 48” with drop nozzles on RR2 hybrids
Halex	0-30” or V8
Hornet WDG	0-20” or V6; 20-36" with drop nozzles if harvested for grain
Impact	emergence to 45 days before harvest
Laudis	0-V8
Liberty	0-24" or V7; 24-36" with drop nozzles
Marksman	0-8" or 5 leaf
NorthStar	4-20" or V2 -V6; to 36" with drop nozzles
Option	V1-V6; to V7 with drop nozzles
Permit	0-36"
Priority	0-V8
Require Q	V2-V6 or 20”
Resolve	0-12” or V5
Resolve Q	0-V6 or 20”
Shotgun	0-8"; to 12" with drop nozzles
Status	4-36” or V2 to V10
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
Herbicides listed below can be applied postemergence for soil residual herbicide activity. They do not provide postemergence control of grass weeds.
Bicep Lite II Magnum	0-5"
Camix	0-30" or V8
Dual II Magnum	0-40"
G-Max Lite	0-12"
Harness, Surpass, etc.	0-11"
Harness Xtra, Keystone LA	0-11”
Lumax	0-12"
Outlook	0-12"
Prowl	0-30" or V8
SureStart	0-11”

 

In the chart, “V” refers to “V-stage”, which describes the number of leaf collars on a corn plant. The leaf collar is region where the leaf blade joins the leaf sheath, which wraps around the stalk. Fig. 2 shows a V4 corn plant with 4 collars.   

 

Figure 2. Corn in the V4 growth stage.