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Issue No. 5, Article 3/May 4, 2012

Removing Weeds from Corn: The Smaller, the Better

Proper timing of postemergence herbicide applications provides a corn crop the best opportunity to express its full genetic yield potential. Letting weeds exist too long with the crop ultimately reduces the amount of seed yield the crop will produce. Yield losses can accumulate very rapidly when weeds compete too long with corn, and the cost of lost yield can far exceed the cost of integrated weed management that includes a properly timed application of a postemergence herbicide.

Simply put, size matters, and when it comes to removing weeds from the corn crop with a postemergence herbicide, the smaller the weed, the better. We know that the longer weeds are allowed to remain with the crop, the greater the likelihood of crop yield loss, but we don't know the specific day after planting or emergence when weed interference begins to reduce corn yield. This critical time is influenced by many factors and can vary with the weed spectrum, density of species, available soil moisture, and other factors.

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 crop yield loss through weed interference. A common recommendation is to remove weeds in corn before they exceed about 2 inches tall.

Another reason to apply postemergence herbicides to small weeds is that they are generally easier to control than larger ones. Application rates of post┬Čemergence herbicides are commonly based on weed size, with higher rates often recommended to control larger weeds. To be effective, the herbicide has to be taken into the plant (usually by absorption through the leaves) and then moved to its target site. Herbicide absorption is often more rapid and more complete through younger plant leaves than older ones. Other nonbiological factors, such as temperature, relative humidity, and soil moisture levels, also can influence how rapidly the applied herbicide is taken into the leaves. High relative humidity, adequate soil moisture, and moderate to warm air temperatures all enhance absorption.

The occurrence of herbicide-resistant weed biotypes and populations is likely to escalate across areas of central Illinois during the 2012 growing season. Waterhemp plants with resistance to one or more herbicide sites of action will challenge the effectiveness of many postemergence herbicides. Depending on the mechanism of resistance, herbicide-resistant waterhemp plants may not demonstrate much injury or a reduced growth rate after application of a herbicide to which they are resistant. We anticipate that many herbicide-resistant populations will not be discovered until several days after the initial herbicide application. A follow-up, or "rescue," application to control resistant plants is more likely to be successful if the initial application was made before the waterhemp plants exceeded 3 inches compared with an initial application made to plants 6 inches or larger.

Prior application of certain soil insecticides can impact the selection of foliar-applied corn herbicides. Specifically, using an organophosphate (OP) insecticide at planting or after corn emergence can restrict the use of herbicides that inhibit the ALS or HPPD enzymes. Precautions and restrictions, which typically appear on the herbicide label, are a result of the increased potential for corn injury after use of OP insecticides and ALS- or HPPD-inhibiting herbicides. (See Table 2 for a summary of label information.) As always, consult the most current product label for complete details.

Table 2. Corn herbicide label statements for postemergence applications: Interactions with soil- or foliar-applied insecticidesa.

Corn herbicide

Soil-applied insecticides

Foliar OP insecticides

Counter 15G

Counter 20CR

Thimet

Lorsban

Aztec

Fortress

Force

Days if applied before

Days if applied after

nicosulfuron and rimsulfuron

Accent Q

No

No

UCI

UCI

Yes

Yes

Yes

7

3

Basis

Nob

Nob

Nob

Nob

Yes

Yes

Yes

7

3

Basis Blend

Noc

Noc

UCI

UCI

Yes

Yes

Yes

7

3

Realm Q

SCI

SCI

SCI

SCI

Yes

Yes

Yes

--

--

Require Q

Nob

Nob

UCIb

UCIb

Yes

Yes

Yes

7

3

Resolve Q

Noc

Noc

Noc

Noc

Yes

Yes

Yes

--

--

Steadfast Q

No

Nod

UCI

UCI

Yes

Yes

Yes

7

3

primisulfuron and prosulfuron

Beacon

No

Nod

TCI

TCI

TCI

TCI

Yes

10

7

Northstar

No

Nod

TCI

TCI

TCI

TCI

Yes

10

7

Spirit

No

Nod

TCI

TCI

TCI

TCI

Yes

10

7

flumetsulam

Hornet WDG

No

No

No

TCI

TCI

TCI

Yes

10

10

Python

No

No

No

TCIe

TCIe

TCIe

Yes

--

--

SureStart/TripleFlex

No

No

No

TCI

TCI

TCI

Yes

10

10

mesotrione

Callisto

SCI

SCI

Yes

SCI

Yes

Yes

Yes

7

7

Callisto Xtra

SCI

SCI

Yes

SCI

Yes

Yes

Yes

7

7

Halex GT

SCI

SCI

SCI

SCI

SCI

SCI

Yes

7

7

Lumax

SCI

SCI

TCI

TCI

TCI

TCI

Yes

7

7

Lexar

SCI

SCI

TCI

TCI

TCI

TCI

Yes

7

7

Zemax

SCI

SCI

SCI

SCI

SCI

SCI

Yes

7

7

thiencarbazone

Corvus

No

No

No

No

Yes

No

Yes

7

7

Capreno

No

No

No

No

Yes

Yes

Yes

7

7

thifensulfuron

Harmony SG

No

UCI

UCI

UCI

Yes

Yes

Yes

--

--

isoxaflutole

Balance Flexx

--

--

--

--

--

--

--

7

7

aNo = do not use; Yes = the herbicide may be applied to corn previously treated with this insecticide. UCI = unacceptable crop injury; SCI = severe crop injury; TCI = temporary crop injury; -- = no information on label.
bDo not apply within 60 days of crop emergence where an organophosphate insecticide was applied.
cDo not apply within 45 days of crop emergence where an organophosphate insecticide was applied.
dDo not apply to corn treated with Counter 20CR in-furrow or over the row.
eSoil-applied OP insecticides should be applied in a T-band or a band.

It's important to remember that the labels of most postemergence corn herbicides allow applications at various crop growth stages, but almost all labels indicate a maximum growth stage beyond which broadcast applications should not be made, and a few even specify a minimum growth stage before which applications should not be made. These growth stages are usually indicated as a particular plant height or leaf stage; sometimes both are listed. For product labels that indicate a specific 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 applications are made outside a specified growth stage or range.--Aaron Hager

Author:
Aaron Hager

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