Wallaces Farmer

Weed, insect issues confront Iowa farmers

Wet weather this spring forced changes in weed control plans for many farmers.

Rod Swoboda

June 28, 2019

6 Min Read
Cornfield with weeds
WEED ESCAPES: A strong postemergence program that includes a residual herbicide offers the best protection against weeds.

Delayed planting doesn’t always equal delayed weed growth. With widespread rain across the Corn Belt this spring and wet weather continuing in June in some areas, many growers had to wait to get into fields. Fields were finally planted, some without herbicide applied. For others, seed has been in the ground for weeks now, and weed management has become the next concern.

“The unpredictable weather we’ve had over the past weeks and months makes it difficult for growers to get their herbicide applications made in a timely fashion,” notes Daniel Waldstein, technical marketing manager for BASF. “This year especially, flexibility in herbicide application is critical.”

With corn acres across the Midwest planted much later than normal, the crop will potentially grow at a faster rate, developing faster than usual. Because of this, farmers have felt increased pressure in applying their herbicides quickly.

Herbicides for post-application

“Our concern this year is that growers prioritized planting, as they should, but then did not make a timely herbicide application,” Waldstein says. “We’re looking at a scenario this year where we are going after larger weeds postemergence. If you choose the right herbicide, there is an opportunity to do that. For example, Armezon Pro herbicide is effective on weeds that are difficult to control and gives you the opportunity to come back with an application early-post or mid-post, and still do well with the weed control.”

This herbicide can be sprayed on corn up to 30 inches tall. It can be used on all corn for broad-spectrum grass and broadleaf weed knockdown with residual control. “It’s one of the few corn postemergence herbicides that allows the use of MSO, a strong adjuvant often more effective against larger weeds than NIC or COC,” he says.

“For optimal weed control, we recommend glyphosate as a tank-mix partner,” Waldstein says. “However, Armezon Pro can provide excellent weed control in many cases even without glyphosate, due to the combination of chemistries in the product. When choosing a herbicide, read the label and make sure the product is approved for the growth stage your crop is in, especially for soybeans.”

Challenges farmers face

Wet weather and weed management challenges in corn and soybeans, and thistle caterpillars in soybeans, continue to be common issues Iowa State University Extension field agronomists are hearing about and seeing in fields in late June this year. Here’s a sampling of what they’ve seen and heard during the last two weeks of June this year in various regions across the state.

Northwest Iowa. “A stretch of about three weeks of relatively dry weather came to an end this past week with 2 to 3 inches of rain falling in parts of my region,” says ISU’s Paul Kassel. “Farmers and applicators turned their focus to postemergence herbicide applications in soybeans this week. Some postherbicide applications have already been made on soybean acres, specifically acres that didn’t receive the intended preemergence herbicide program.

“Corn development is in a wide range across the area, with late April and early May planted corn at V9 stage and early June planted corn at V3.”

Central Iowa. “My region received anywhere from about 0.5 inch to 3 or more inches of rain last week,” says Meaghan Anderson. “We are seeing a lot of early-season insect feeding in soybeans, with thistle caterpillars being the main culprit. I expect many soybean fields to be flowering soon. This is a good reminder to double-check crop growth stages before making herbicide applications, as some herbicide labels restrict application after R1 or R2 in soybeans.

“I’m hearing about weed control issues in both corn and soybean fields. In addition to weed management questions, I’ve had calls about nitrogen and sidedress applications, and my first dicamba drift on a soybean field.”

Another weed management issue she observed this past week was waterhemp not responding to an application of HPPD herbicide (Group 27).

Southwest and west-central Iowa. “Most early-planted corn has benefited from the sunshine, warmer temperatures and some rainfall,” Aaron Saeugling says. “The early planted corn is V7 to V10. Most postherbicide applications have been applied to corn. Soybeans range from V1 to V4. I’ve seen a few blooms, so some of the early-planted fields are not far from reaching R1. Thistle caterpillars continue to do some feeding in soybean fields. 

“Additionally, soybean gall midge adults have been confirmed in Cass County. Herbicides are being applied to soybean fields as the weather allows. Questions on thistle caterpillars dominated phone calls this past week. I’ve also gotten my first herbicide drift calls of the season.”

East-central and southeast Iowa. “Corn mostly ranges from V2 to V10 or larger growth stage,” Rebecca Vittetoe says. “Most of the early-planted corn is out of the ugly duckling stage and has gained its nice dark-green color. If you look closely, you can still see some unevenness in growth across cornfields. I’ve noticed some pale or yellowish “flag leaves” scattered in cornfields. Soybeans mostly range from V1 to V4.

“I did see flowers starting to appear in early-planted soybean fields last week, so some fields may be at R1 now. Thistle caterpillars have been the main concern in bean fields. Weed control issues in corn and soybeans, weed identification, tissue testing for corn and soybean, and thistle caterpillars dominated calls this past week.”

Eastern Iowa. “Rain this last week ranged from 2 to 4 inches roughly south of Highway 92, 1 to 2 inches between Highway 92 and Highway 30, and a 0.5 inch north of Highway 30,” Virgil Schmitt says. “Most farmers had from zero to three days to do any fieldwork. Corn planting has mostly ended as of June 20. April planted corn is V9 to V10 growth stage, mid-May planted corn is mostly V7 to V8, and June planted corn is mostly V3.”

“On June 21, I noted some pale ‘flag leaves’ in my travels in some fields in eastern Jasper and western Poweshiek counties along I-80. Soybeans are ranging from growth stage V4 down to still in the bag. Soybean planting is about 90% complete as of June 24. Inquiries about herbicides, potential herbicide drift, and cover crops dominated the phone calls I received this past week.”

South-central Iowa. “Rainfall across the region put a halt to field operations during the week ending June 23. Much of southern Iowa received anywhere from 1.5 to 4.5 inches of rain,” Josh Michel says. “As of June 24, corn remains at around 80% to 85% planted, and soybeans are about 60% to 65% planted. Corn growth stage ranges from V8 for the early-planted corn to V1. Earlier-planted soybeans are V4, with many fields possibly reaching R1 by the end of this week.

“Newly planted soybeans continue to have some emergence issues. Bean leaf beetles, Japanese beetles and thistle caterpillars are causing some minor concerns with soybean feeding. Common questions this past week have been focused on nitrogen management, uneven corn stands, late- season weed management, and prevented planting options.”




About the Author(s)

Rod Swoboda

Rod Swoboda is a former editor of Wallaces Farmer and is now retired.

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