Consider Corn Growth Stage When Applying Postemergence Herbicides

Corn field with severe weed population — Figure 1. A corn field in south central Nebraska without preemergence herbicide application. Applying a well-timed postemergence herbicide will be the only control option. (Photos by Amit Jhala).

June 14, 2017

Amit Jhala - Professor and Associate Department Head, Department of Agronomy and Horticulture

Heavy rains and cooler soils in May in many areas of the state delayed planting or the application of preemergence herbicides. Early season weed control is important to avoid crop-weed competition and yield loss. Heavy rains also created a situation where preemergence herbicides washed off and the duration of their weed control efficacy was reduced. With plenty of soil moisture and warm temperatures the last two weeks promoting weed seed emergence and growth, some corn fields now have heavy infestations of weeds (Figures 1, 2).

Application of postemergence herbicide is an important weed management tool in corn, specifically this year in Nebraska. There are several points to be considered when applying postemergence herbicide, including corn growth stage, weed species present, and crop and weed heights.

Taller corn is generally more sensitive to potential herbicide injury, particularly when nozzles are applying herbicide directly into the whorl of the plant. Herbicide product labels provide a maximum corn growth stage after which broadcast applications should not be made (Table 1).

Corn field with heavy weed infestation — Figure 2. Plenty of moisture in the soil and warm temperature in last couple weeks promoted weed seed emergence in a corn field.

How to determine corn growth stage?

Restriction of postemergence herbicide application in corn is based on growth stage listed on herbicide labels.
They are primarily indicated as a particular corn leaf stage or plant height, and sometimes both are listed. The growth stage that is most restrictive should be followed when both are listed.
When using a herbicide tank-mixture, follow the recommendations for the most restrictive label language of the products being used in the tank mix.
Corn height is determined by measuring from the soil surface to the arch of the uppermost leaf that is more than 50% emerged (Figure 2).
Corn leaf stage is determined by counting collars when leaves emerge from the whorl and a band is formed around the stem. Leaves that are just emerging from the whorl will not have a visible collar and are not counted. Leaf stages are designated using a "V" (vegetative) to represent each leaf during vegetative development. The first true leaf on corn is the short, rounded leaf near the soil surface which is counted as V1. Each successive, visible leaf collar is counted as V2, V3, and all the way to V18 which is prior to tasseling.

The Bottom Line

It is important to pay attention to the corn growth stage noted on the label when applying herbicide. Herbicide applied beyond the growth stage may result in crop injury.
Environmental conditions influence the absorption of postemergence herbicides and potential crop injury. Therefore, it is important to apply herbicides when crop and weeds are NOT under stress.
Herbicide product labels should be followed regarding spray additives that are recommended for improved weed control.
Do NOT apply postemergence herbicides with foliar fertilizers.
It is important to clean the spray tank to avoid any potential crop injury due to spray tank contamination.
Drop nozzles can be used in taller corn for better weed coverage and to avoid spraying directly into the whorl.
If postemergence herbicides are applied later than mentioned on the label, corn ear development can be affected.

For more information on efficacy of the herbicides in Table 1, refer to 2017 Guide for Weed, Disease, and Insect Management in Nebraska.

Table 1. Application restriction of postemergence herbicides commonly applied in corn.
Postemergence Corn Herbicide	Corn Growth Stage or Plant height

2,4-D	8” broadcast; pre-tassel directed
Accent Q	20” broadcast; 36” directed
Acuron	12”
Aim	8”
Armezon	V8
Armezon PRO	V8 to 30”
Atrazine	12”
Banvel	1 pt 8”; 0.5 pint 36”
Basis Blend	2 collar
Beacon	20” broadcast; 36” directed
Buctril	Pre-tassel
Cadet	48” or 70 days pre-harvest
Callisto	30” or 8 leaf
Callisto GT	30” or 8 leaf
Callisto Xtra	12”
Capreno	V1 to V6 broadcast; V6 to V8 directed
Clarity	8”
DiFlexx	Broadcast V6 or 36”; V7 to V10 or 36” directed; 15 days before tassel
DiFlexx DUO	V7 or 36”
Glyphosate	Read the label; broadcast to 24”; directed to 30”
Halex GT	30” or V8
Hornet	20”; 85 days pre-harvest
Impact	45 days pre-harvest
Laudis	V7
Liberty (Only for Liberty Link Soybean)	24”; V7; or 70 days pre-harvest
Lumax EZ/Lexar EZ	12”
Realm Q	20” or 6-leaf collar
Resicore	10”
Resolve	12”
Resolve Q	20” or V7
Resource	V10
Revulin Q	V8 or 30”
Sencor	60 days pre-harvest
Solstice	48” or 70 days pre-harvest
Status	36”
Steadfast Q	20” or V6
Stinger	24”
Yukon	36”