Corn Is Emerging In Iowa, So Are Weeds

Corn Is Emerging In Iowa, So Are Weeds

Protect your yield by scouting fields for weeds, insects and other common problems that can show up in young corn.

"The corn crop is pretty well planted in my area," reports Paul Kassel, Iowa State University Extension field agronomist at Spencer in northwest Iowa. "We have had some nice weather for fieldwork this spring. Also, there are neighborhoods where about half the soybean crop is planted. Overall soybean planting is still probably around 30% completed."

CHECK SEEDLING CORN: Weeds are starting to emerge along with corn in Iowa fields this week. You can see wild buckwheat, foxtail, giant ragweed and smartweed beginning to emerge. Check fields for herbicide performance, cutworms and other possible issues.

Corn in some fields has already emerged, and in other fields corn is beginning to emerge, he says. Some of the corn that was planted on and around April 19 has emerged. Also, corn planted around April 22 and some planted May 3 is perhaps a couple days away from emerging. Now is the time to scout your fields at least once a week and check for potential weed, insects and other possible problems.

Corn requires about 120 Growing Degree Days for emergence, Kassel explains. The table accompanying this article shows some GDD accumulations for various planting dates this season.


GDDs as of May 12 at Spencer











"The relationship between corn emergence and GDD accumulation is not perfect since corn emergence is just beginning and we have exceeded the 120 GDDs with the early-planted dates," notes Kassel. "Corn emergence from all planting dates may be similar since about half of the GDDs we have received for the last three planting dates have been received in the last week."

Soil temperatures have warmed considerably since mid-April
The weeds are starting to emerge also, he says. You can see wild buckwheat, foxtail, giant ragweed and smartweed starting to come up. Farmers are encouraged to check their fields for herbicide performance and for insects or other possible problems.


Soil temperatures are available on the Iowa Mesonet at depths of 4, 24 and 50 inches (on the webpage, go to "soil temperatures," then click on "ISU soil moisture network"). Soil temperatures at depth were a concern earlier in the spring. There was concern that root growth and crop development would be hindered because of cold soil temperatures in the soil profile. "However, soil temperatures at a 50-inch depth have risen recently to around 43 degrees in northern Iowa," he says. "I'm not sure what the normal would be, but the soil has warmed considerably since mid-April. We could find hard frost at a two-foot depth as recent as April 15."

Corn Is Emerging In Iowa, So Are Weeds

TOP 6 TIPS BEFORE V6: Properly identifying problems early in the season helps corn growers make well-informed decisions on insect management, herbicide application and replanting. You want to keep corn plants healthy and save yield potential.

Six things to scout for in seedling corn
Corn seedlings are most vulnerable and susceptible to damage from planting through the V6 growth stage. "It's very important for you to identify early season problems to make important input decisions and save yields," notes Travis Belt, a Mycogen Seeds agronomist. He encourages growers to understand the development stages of young corn and to scout fields early and often. He suggests scouting for these six problem areas from planting to V6 growth stage of corn.

1. Assess emergence and germination. Uniform, poor emergence patterns can be blamed on various problems, including a jammed planter, excess soil moisture or insect damage. Germination failure can result from a seed's cell tissues rupturing from cold temperatures after planting. Understanding the cause of poor stands is the first step toward corrective action.

2. Identify early season stress. "Environmental conditions or insect pressure may cause early season stress to plants," Belt says. Black cutworms, corn nematodes and slugs all are culprits that cause early season corn damage. Watch for wilted plants, stunted growth, yellowing and slime trails that indicate the presence of these pests.


3. Check for weeds.  "Weed pressure, especially early in the plant's life cycle, robs stands of valuable moisture and nutrients critical for maximizing yields," Belt says. Early germinating weeds include ragweed, velvetleaf and lambsquarters. Belt recommends applying a postemergence herbicide for hard-to-control weeds.

4. Study visual appearance. Look for clues to overall plant health in the appearance of young corn. Purpling sometimes occurs during sunny days and cool nights. Yellowing may signal inadequate heat, sunlight or nitrogen (N). Discoloring caused by temperatures will be corrected in adequate weather. N loss should be further investigated.

5. Detect nitrogen loss. Belt says the best way to determine if the yellowing of corn plants is due to N loss is to take in-season soil samples or leaf tissue samples. If tests show a deficiency, plan for a sidedress application.

6. Weigh alternative options. If poor field conditions delayed planting or if plants have a poor stand, re-evaluate the growing schedule and input decisions. Before considering replanting, Belt says to compare the yield potential of the existing stand with the yield potential of replanted corn.

"Properly identifying problems in the early part of the season helps corn growers make well-informed decisions on insect management, herbicide application and replanting to keep plants healthy and save yields at harvest," Belt says. For more information visit for articles discussing these and other agronomic topics during the growing season.

TAGS: USDA Extension
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