No. 9 Article 1/June 3, 2011

Postemergence Herbicides in Corn

The Illinois corn crop ranges in development from only recently emerged to surpassing the 7-leaf stage in some earlier plantings. Applications of postemergence herbicides continue to be made across most of Illinois, although recent precipitation has delayed them in some areas. Regardless of delays, adequate soil moisture coupled with warm temperatures will certainly promote rapid growth of emerged weeds.

Proper application timing is critical to the goal of removing weed interference from the corn crop before the weeds reduce yield. Unfortunately, it's not possible to accurately predict the specific day after planting or emergence when weeds begin to reduce yield. Weed scientists generally suggest an interval, based on either weed size (in inches) or days after crop/weed emergence, during which postemergence herbicides should be applied to avoid yield loss through weed interference. The interval is influenced by many factors and can vary with the weed spectrum, the density of certain species, available soil moisture, and so on. In corn the common recommendation is to remove weeds before they exceed about 2 inches tall. The longer weeds remain with the crop, the greater the likelihood of yield loss.

It's important to remember that although the labels of most postemergence corn herbicides allow applications at various crop growth stages, almost all of them indicate a maximum growth stage beyond which broadcast applications should not be made (Table 1), and a few even state a minimum growth stage before which applications should not be made (Table 2). These growth stages are usually indicated as a particular plant height or leaf stage; sometimes both are specified. For products that indicate both corn height and growth stage, be sure to follow the more restrictive of the two. Application restrictions exist for several reasons, but of particular importance is the increased likelihood of crop injury if an application is made outside a specified growth stage or range.

Table 1. Maximum corn growth stage for broadcast applications of postemergence herbicides.

Product

Maximum corn growth stagea

2,4-D amine (3.8 lb ae)

Up to 8 inches

2,4-D ester (3.8 lb ae)

Up to 8 inches

AAtrex 90DF or 4L

12 inches

Accent Q 54.5 WDG

20 inches or V6 (6 leaf collars)

Aim 1.9EW

V8

Basagran 4S

No limit

Basis 75WDG

6 inches or 4 leaves/2 collars

Beacon 75WDG

20 inches

Buctril 2EC

Pretassel

Cadet 0.91EC

48 inches or pretassel

Callisto 4SC

30 inches or 8 leaves

Callisto Xtra 3.7SC

12 inches

Camix 3.67L

12 inches

Capreno 3.45SC

V5

Clarity 4L

8 inches or 5 leaves

Expert 4.88SCb

12 inches

Glyphosate (various)b

30 inches or V8 (8 leaf collars)

Halex GT 4.39CSb

30 inches or 8 leaves

Harmony 50SG

16 inches or V5 (5 leaf collars)

Hornet WDG 68.5WDG

20 inches or V6 (6 leaf collars)

Ignite 2.34SLc

24 inches or V7 (7 leaf collars)

Impact 2.88SC

45 days before harvest

Laudis 3.5SC

V8

Northstar 47.4WDG

20 inches or V6

Permit 75WG

Layby

Peak

20 inches or V6

Rage D-Tech 4.06EC

8 inches or 5 leaves

Realm Q 38.75WDG

20 inches or through V6

Require Q 54.37

20 inches or through V6

Resolve DF 25WDG

12 inches or through 5 leaf collars

Resolve Q 22.4WDG

20 inches or through V6

Resource 0.86EC

10 leaf collars

Sequence 5.25Lb

30 inches or V8

Shotgun 3.25L

8 inches or 4 leaves

Spirit 57WDG

20 inches or V6 (6 leaf collars)

Starane 1.5L

V5 (5 leaf collars)

Status 56WDG

36 inches or V10

Steadfast Q 37.7WDG

20 inches or V6 (6 leaf collars)

Stinger 3S

24 inches

Stout 72.5WDG

16 inches or 5 leaf collars

Yukon 67.5WSG

20 inches

aStages listed are for broadcast applications. Several products can be applied to larger corn using directed applications.
bApply only to glyphosate-resistant corn hybrids.
cApply only to glufosinate-resistant corn hybrids.

 

Table 2. Postemergence corn herbicides with minimum corn size application restrictions.

Herbicide

Minimum corn size

Beacon (primisulfuron)

4 inches

Harmony SG (thifensulfuron)

2 leaf (1 leaf collar)

Northstar (primisulfuron + dicamba)

4 inches (V2)

Peak (prosulfuron)

4 inches

Require Q (rimsulfuron + dicamba)

4 inches (V2)

Resource (flumiclorac)

2 leaf

Spirit (primisulfuron + prosulfuron)

4 inches

Status (diflufenzopyr + dicamba)

4 inches (V2)

Though corn plant height is specified on many herbicide labels, height does not always accurately indicate a plant's true physiological maturity. Determining plant height may seem straightforward, but using different benchmarks for measurement can lead to different results. Generally, height for a corn plant is measured from the soil surface to the arch of the uppermost leaf that is at least 50% emerged from the whorl. Be sure to measure several plants in a given field and average the numbers.

Plant height is influenced by many factors, including genetics and the growing environment. Adverse environmental conditions, such as cool air or soil temperatures and hail, can greatly retard height and result in corn plants that are physiologically older than their height suggests.

Many agronomists agree that leaf number is a more accurate measurement of corn developmental stage than plant height. Counting leaves and counting leaf collars are the two primary techniques used. To count leaves, begin with the short first leaf (the one with a rounded tip) and end with the one that is at least 40% to 50% emerged from the whorl. To count leaf collars, also begin with the short first leaf, but then include only leaves with a visible collar (the light-colored band where the leaf joins the stem). Leaves in the whorl or those without fully developed collars are not counted. Counting leaf collars often stages a corn plant at one leaf less than does counting leaves.--Aaron Hager

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