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Central Iowa Crop Update
Updates from an agronomist and weed nerd in central Iowa.
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In this issue:
- Scouting resources
- Delayed PRE applications & other weedy thoughts
- Scouting for insect pests
- Thoughts on early-season hail
- 2022 corn fungicide trial results
- Return program for certain chlorpyrifos products
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Scouting resources
Now that corn and soybean fields are beginning to emerge, it's a great time to enjoy the weather by scouting those crops. The USDA NASS Crop Progress and Condition Report pegged central Iowa at 68% corn planted, 55% soybeans planted, and 7% corn emerged last Monday. This is well ahead of both last year and the five-year average. My informal survey on Wednesday had us at 79% planted overall (corn and soybean), with 23% of those planted acres emerged. My colleague Angie Rieck-Hinz had some nice notes on scouting in her newsletter, so I thought I'd share them here.
Early-season scouting should involved several key items:
- assess stand establishment through stand counts,
- evaluate seedling health (normal vs. not normal, disease, vigorous growth),
- look for insect and weed issues, and,
- evaluate the field for planter problems.
So far, I think the emerged crop looks surprisingly even, with only a small percentage of stand loss from the extended cool period in April. Remember that even seed planted in the best conditions is likely to have some loss and you can check your seed tag to evaluate the % loss that might be expected (% live seed = % viable seed X % purity).
The resources listed below are great summaries of what to look for in fields, as well as the field guides we have available in the Extension Store.
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Delayed PRE applications & other weedy thoughts
Between the wind and the rain, there may be some crop fields that didn't get a preemergence herbicide application at the time of planting. While the crop may not be emerged, numerous weed species are up now (grasses, velvetleaf, morningglories, waterhemp, ragweeds, etc.), so keep in mind several things before applying any delayed PREs... (1) is the crop emerged, (2) is the herbicide mix safe to apply to emerged or nearly emerged crops, (3) are there weed species emerged, and (4) if weed species are emerged, will my current herbicide mix control the present emerged weeds.
For corn, the great majority of PRE herbicides can also be applied POST. For those planning to use UAN as a partial carrier, this article has some good reminders for using UAN on corn that may be emerged.
For soybean, most PRE products cannot be applied after soybeans have emerged due to the likelihood of causing substantial damage and stand loss. Some products, like those containing sulfentrazone and flumioxazin, must be applied within three days of planting, per their labels. Check out this article and check your herbicide product labels before returning to the field.
Aaron Hager, weed scientist with U of Illinois, wrote a really nice article with some reminders about using herbicides to manage weeds like waterhemp. It was yet another wakeup call about how little we know about the best strategies to slow the development of herbicide resistance and how the only way to truly assure no further development of resistance is to eliminate any seed production. Check out his thoughts in "Recommendations to Manage Herbicide-Resistant Weeds: It's Not as Easy as Some Believe."
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Scouting for insect pests
Insect pests have been on the move this spring and as crops emerge, we'll want to check for signs of them.
Black cutworm and true armyworm monitoring continues across the state; catches have been steady, though I caught more true armyworm moths on Monday morning following all the storms Friday PM - Sunday. While catching the moths doesn't necessarily indicate the pest will be an issue (or worth treating), it's important to scout for both of these pests once the corn emerges.
Black cutworm larvae will cut seedling corn plants at or just below the soil surface. Now is the time to begin scouting for these in corn fields. While the damage is usually easy to find (corn whorls laying on the ground next to the row), it can be difficult to find the larvae as they hide under residue or soil during warm days. Read more about predicted cutting dates and black cutworm biology.
Feeding from a young true armyworm larvae on seedling corn in spring 2022. Photo by Meaghan Anderson.
True armyworm larvae will feed on green leaf tissue, often eating entire corn leaves except the leaf midrib. Keep an eye out for true armyworm feeding while scouting and read more about them here.
Alfalfa weevil larvae should be active in central Iowa and it's worth scouting alfalfa fields to check for their presence. You can use a sweep net if alfalfa is tall enough or check for plants at the terminal leaflets if alfalfa is too short to sweep. This article has good information on ID and biology, including common look alikes. Here's a nice video describing how to use a sweep net in field crops.
Bean leaf beetle suffered fairly "average" winter losses in central Iowa this year, so keep an eye on the earliest emerged soybean fields and soybean fields near alfalfa for this pest. This article describes more about scouting and thresholds.
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Thoughts on early-season hail
Some emerged fields of corn, soybean, and alfalfa were affected by hail last weekend. I've got the concerns for each outlined below with links to resources.
Corn affected by hail prior to V6 should have no lasting damage as a result of the injury due to the growing point being below ground. Occasionally, the hailed, dead leaf tissue can tangle the subsequent emerging leaves, but I think all the emerged corn was small enough to keep this from being much of a concern. Regardless, monitor stands for regrowth to make sure new leaf tissue develops normally.
Three soybean plants with varying levels of damage - cut below cotyledons/dead (bottom left), one cotyledon missing (middle), and one that appeared to receive no injury (upper right). Photo taken May 11, 2023 by Meaghan Anderson.
Soybean affected by hail could suffer stand loss due to the small size of emerged plants. The field I observed on 5/11 had varying levels of damage but a number of plants were cut off below the cotyledons, which means certain plant death. Soybean can survive even with the loss of both cotyledons as long as the growing points at the base of those cotyledons are still intact. The best thing to do is to wait to evaluate stands for 7 days or more after the hail event. The field I checked on 5/11 still had some new plants emerging, so waiting until next week will help assure that we're accounting for all plants. This article on soybean stand assessments can help determine if replant is necessary.
Alfalfa damage and management depends on whether the stand is new or established, and how close we are to harvest. Seedling stands should be thickened up if the surviving stand is <25 plants per square foot. Information for established stands is available on this page from the University of Wisconsin.
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2022 corn fungicide trial results
Alison Robertson posted her 2022 summary of foliar fungicide applications in Iowa this week. Take a look at the results and use this information, along with other resources like the Fungicide Efficacy for Control of Corn Diseases document from the Crop Protection Network, to make wise fungicide decisions this growing season.
Additionally - I know concern for tar spot remains high. Don't forget to check out the Tarspotter app to help determine when to scout fields for its presence!
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Return program for certain chlorpyrifos products
For anyone still holding on to chlorpyrifos products, the EPA has announced a return program for certain chlorpyrifos products. Check out this article for more information, including a list of the products approved for return, and reach out to your retailer or distributor to line up return to the manufacturers.
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