Check Weed Control, Watch For Insect Pests

Check Weed Control, Watch For Insect Pests

An update on weed control and crop pest issues you need to be watching for in corn and soybean fields this week.

Iowa State University Extension field agronomist Clarke McGrath, based at Harlan in western Iowa, this week helped some farmers make corn replant decisions. He also looked at a lot of fields where early pre-plant herbicides had been applied and also evaluated fields where preemergence herbicide programs were used. This was mostly corn but a few soybean fields, too, says McGrath, who writes a monthly column for Wallaces Farmer magazine. His column is called "CSI:Iowa" and you'll want to be sure to read what he has to say about the increasing need for using soil-applied herbicides for weed control—when you get your June issue of Wallaces Farmer.

Check Weed Control, Watch For Insect Pests

He evaluated a weedy field this week that had a 75% rate (75% of recommended total rate) of a pre-mix herbicide applied in early April. "This was pretty typical of what we are seeing across the area now—weedy fields," he said on May 16. "The early-preplant applications and the preemegence herbicide applications have been on these fields for a long time this spring, and we expected that they would breakdown sometime soon."

Apply postemergence herbicides soon, before the weeds get too big

Some of the weeds were taking up the herbicide and will likely still be controlled, says McGrath. Agronomists call that type of control action "reach back" by the herbicide. But most of the weeds that haven't been controlled yet by those earlier applied soil herbicide treatments are off and running. You will need to get your postemergence herbicide applied on those weeds in those fields as soon as possible if you want to kill the weeds and keep them from escaping, he says. When you can row your corn, that's the time you should start spraying many of the postemergence herbicides.

"Also, when you are scouting your cornfields, keep an eye out for cutworms," warns McGrath. He has heard of a few fields requiring treatment for this insect pest.

With corn planted essentially finished and soybean planting well underway don't forget to stop and check your corn stands, says Mark Licht, ISU Extension field agronomist in central Iowa. He's been out in fields this week and has noticed a few minor cases where corn seedlings have lost vigor and failed to emerge due to cold temperatures right after planting or because of soil crusting. "Check for those problems now, although they will likely be minor, depending on what happened in each individual field," he says.

Take time to evaluate your stands, compare seeding rate to emerged stands

It is more important to check fields to see how seeding rate and emerged stands compare. This goes for both corn and soybeans, says Licht. Looking closely at emerged populations can also help you determine whether stands were affected by damping off, insect feeding or seed germination issues. If there is missing or double plants you know the issues lies more with your planter. "Knowing what seeding rate you planted and what the count is for plants that actually emerged is useful information to help you fine-tune your planter for next spring. Or it may be enough justification for you to trade in your old planter," he adds.

 "In the coming weeks don't forget to keep an eye out for black cutworm cutting in corn that is less than the 5th leaf corn growth stage. Also pay attention to emerging soybean fields for feeding from bean leaf beetles," advises Licht.

Subsoil moisture is recharged somewhat, but crops will still need timely rain

Paul Kassel, ISU Extension field agronomist in northwest Iowa, reports that corn planting was wrapping up across his area on May 16. Corn is emerging well and is looking good so far, he says. Farmers are encouraged to check their fields for damage from black cutworm. "We don't expect any more or less of a cutworm problem this year than we usually see – but it is always good to check a few fields for this pest," he says.

Farmers had some excitement last on May 11 in areas of northwest Iowa. A storm front came through with some strong winds and heavy rain. The speed and severity of this storm front caught many farmers in the field. "There have been some heavy rains in my area this spring," says Kassel. "Some communities in northern Kossuth County had heavy rain on May 3, May 4 and May 6. It is interesting to note that I have fielded calls from farmers concerned about the lack of subsoil moisture in some areas and from farmers asking about replanting corn due to flooded conditions in other areas."

Subsoil moisture reserves have been recharged somewhat thanks to rains this spring in areas of northwest and north-central Iowa that started out pretty dry. But to get good yields this year timely rains during the growing season will still be needed, he says.

How can you kill glyphosate resistant lambsquarter? Here are some options

The early spring this year has been favorable to the early growth and establishment of lambsquarter. While herbicide resistant waterhemp garnered lots of attention and questions from farmers last winter during the meeting season, lambsquarter has also been known to tolerate glyphosate applications. Those 'tolerant' lambsquarter plants can then be very competitive weeds corn and soybean fields.

Kassel says lambsquarter that have survived tillage in corn has been easily handled with combinations of dicamba or atrazine with preemergence herbicide applications.

However, lambsquarter that survives tillage in soybeans is more difficult to control.

Farmers may want to consider adding glyphosate to preemergence herbicide applications, says Kassel. Also, you need to consider that products that contain Authority, Valor or Sharpen will respond to the addition of crop oil concentrate or methylated seed oil additives to provide added burndown of emerged weeds like lambsquarter. Also consider using products like Envive which contains a low rate of Harmony SG – which is effective on lambsquarters.

TAGS: USDA Extension
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