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5/7/2012 - 5/13/2012

Imbibitional Chilling and Variable Emergence

By Roger Elmore, Department of Agronomy

Sixty-four percent of Iowa’s corn lay in seedbeds as of May 6. This is 6 percent ahead of normal (see USDA report).  Northeast and South Central cropping districts reported 50 percent planted while the Northwest and West Central cropping districts were around 75 percent planted. Indeed many farmers completed planting corn; 7 percent of our soybeans were planted – slightly behind the 11 percent five-year average.

So far we have experienced multiple corn planting ‘windows’ this year. As I write this article, farmers rapidly plant the remaining corn crop.

Walk fields to assess stands – and discover problems

In a May 1 ICM New article I discussed how to think about corn yet unplanted, in terms of yield potential. Planting between May 15 and May 25, on average, results in 87 percent yield potential. But please remember, we have had years where the best planting dates were the last ones of the season!

Also in that article I presented information and links on how to assess planted corn that may be a candidate for replanting. I don’t intend to discuss those items more now. However, let’s talk about some 2012 conditions and field reports.

Reports from emerging fields - 2012
Soil temperatures  in late April backed off to ‘normal’ after an unusually warm March. We all know that cooler soil temperatures slow the germination process and predispose seedlings to fungal infection and other problems – corn is a warm season crop. My colleagues and I  observed, and/or heard reports of seedling growth problems in some parts of Iowa already this year, related to  Imbibitional chilling damage (see Figures 1 and 2) and some variable emergence. Soil crusting was not a problem in these situations.

Figure 1. Imbibitional chilling effect on corn, Story Co. IA. R. Elmore photo, May 10, 2012.

Figure 2. Imbibitional chilling effect on corn planted April 11, 2012, Story Co. IA. R. Elmore photo, May 8, 2012.

Imbibitional chilling refers to the chilling effect seeds may experience when they imbibe, or absorb water especially when soil temperatures are less than 55 F for an extended time. Brittle shoot cell membranes rupture in cold soils. Seedlings may “corkscrew” and may not emerge when exposed to these cool soil temperatures. This may also happen when there are rapid swings in air temperatures. Fortunately, most fields we’ve seen or heard about exhibit less than 5 percent of the plants affected. If these plants do emerge, they will not likely be productive.

In addition, it is not hard to find fields this year with variable plant emergence in both reduced and minimum tillage, as well as maximum tillage – see Figure 3. Variable emergence and growth reduces yield if development stages vary by two leaves or more.

Figure 3. Four plants, three development stages, Story Co. IA. R. Elmore photo May 10, 2012.

What we like to see
We like to see uniform emergence and growth (Figure 4). Perfect plant spacing within a row – a picket-fence stand – is less important than uniformity of emergence and attaining optimum plant populations. Adjust planters, planter speed and seed depth properly.  And of course, plant into good seedbed conditions: ‘mudding in’ corn results in mediocre results. Do what you can to end up with the right number of plants that all look and develop the same (Figure 5).

2012 promises many opportunities to learn more about growing corn.

Figure 4. Uniform emergence and development is more important than uniform plant spacing in the row. Corn following corn, Story Co. IA. R. Elmore photo. May 10, 2012.

Figure 5. Strive for uniformity in emergence and development. R Elmore photo. May 10 2012.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at or (515) 294-6655.


Crop Minute - Soil Erosion

Clarke McGrath, extension field agronomist, says severe soil erosion is a concern across parts Iowa this spring, especially in southwest Iowa. During this week's Crop Minute, he talks about the effect of heavy rain falls and greater than normal spring tillage in that part of the state.

The Crop Minute will be a regular feature of Integrated Crop Management News through the growing season. Each week a member of the Iowa State University Extension and Outreach crops team will cover a current topic of interest.

Emerged Corn, Herbicides and Nitrogen

By Bob Hartzler and John Sawyer, Department of Agronomy

The rapid pace of planting in late April followed by rain has resulted in many fields having emerged corn before preemergence herbicides and nitrogen applications have been completed.  Of particular concern are no-till fields where planting was completed prior to killing emerged weeds.

These fields should be priority for action since weeds that have a head start on the crop can begin to impact crop yields very early in the season.   

  • Researchers evaluating how long weeds can compete with corn before impacting yields (the critical period) often report that corn can tolerate weeds until the V3-V4 stage. However, these findings are based on corn being planted into a clean seedbed. When corn is planted into a weedy seedbed, yields can be impacted much sooner. 
  • Another reason for quick action in these fields is that several of the early-emerging weed species are some of the most difficult to control once they get a little size to them. These tough to control weeds include horseweed/marestail, giant ragweed and lambsquarters.

Most preemergence herbicides also are registered for application after corn has emerged, however, their activity on emerged weeds varies. If weeds are present, determine whether the postemergence activity of the residual herbicides is sufficient to control the weeds present in the field or if an additional herbicide with better postemergence activity is needed.

Another issue is the need to apply nitrogen and the desire to minimize trips across the field.  Urea-ammonium nitrate (UAN) alone can be applied to emerged corn, and the risk of injury to the corn is dependent upon UAN rate, corn stage and weather conditions. Conservative suggestions are to limit postemergence applications of UAN to 90 lb N/acre when corn is at the V3 to V4 stage and to 60 lb N/acre at the V7 stage.  Applications beyond the V7 stage are not recommended, and the risk of injury increases during hot, dry conditions. 

While many preemergence herbicides are applied using UAN as a carrier, this practice is only recommended prior to crop emergence.

  • The combination of herbicides with UAN greatly enhances the foliar activity of these products and poses a real threat of killing all emerged tissue contacted by the spray. 
  • Almost all herbicides prohibit application in nitrogen solutions after the crop has emerged due to the risk of severe crop injury.  
  • Some might try to rationalize this combination if the corn is at the VE-V1 stage since the growing point is still underground. While corn often can recover quickly from loss of the shoot at this growth stage, the herbicide may influence the plants ability to recover and therefore result in yield loss.

Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities. John Sawyer is professor with research and extension responsibilities in soil fertility and nutrient management.

Start Scouting for Stalk Borer

By Erin Hodgson, Department of Entomology, and Adam Sisson, Integrated Pest Management

Iowa has been steadily accumulating degree days ahead of schedule in 2012. Over the weekend, some parts of southern Iowa hit an important degree day benchmark for common stalk borer. About 10 percent of stalk borer larvae can begin moving to corn after accumulating 1,300 to 1,400 degree days. Part of southwestern and southeastern Iowa have reached that threshold and we recommend starting to scout this week in corn (Fig. 1). Central and northern Iowa should start scouting May 12 to May 18, if warm temperatures continue. This is about three weeks earlier than last year. 

Figure 1. Growing degree days accumulated (base 41°F) for stalk borer larval movement in Iowa for 2012. Begin scouting around 1,300 to 1,400 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

Stalk borer eggs are laid on grasses and weeds in the fall, and young larvae will feed in the spring until they outgrow the plant. Corn can become infested when stalk borer larvae move to find bigger host plants, typically adjacent to grassy edges of emerging corn. The most susceptible stages of infestation are at V1-V5, or about 2 to 24 inches in plant height. Larvae can damage corn by defoliating leaves and burrowing into stalks. Stalk borers do not typically cause economic damage when feeding on the leaves, but can clip newly emerging plants and cause death (Fig. 2). More often, larvae kill corn plants by entering the stalk and destroying the growing point (i.e., flagging or dead heart). A dead heart plant will have outer leaves that appear healthy, but the newest whorl leaves die and can cause barren plants.

Figure 2. Stalk borer can shred corn leaves and destroy the growing point.

Stalk borer infestations are more likely in corn surrounded by giant ragweed. Female moths prefer to lay eggs in weedy areas in August and September, so minimizing weeds in and around corn during that time will discourage egg-laying. Using herbicides to kill spring weeds can force stalk borer larvae to infest young corn plants. Long term management requires controlling grassy edges around corn so that mated females will not lay eggs in that area during the fall.

Fields with persistent stalk borer infestations should be monitored every year. Applying insecticides to infested corn is not effective because the larvae are protected once tunneled into the stalk. Instead, target foliar applications to larvae as they migrate from grasses to corn. Look for larvae inside the whorls to determine the number of plants infested. The larvae are not highly mobile and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Young corn is particularly vulnerable to severe damage, but plants are unlikely to be killed once reaching V7 (seven true leaves).

Control. To prevent stand loss, scout and determine the percent of infested plants. The use of an economic threshold (Table 1), first developed by ISU entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.

Table 1. Economic thresholds for stalk borer in corn, based on plant stage, expected yield and market value.

If an insecticide is warranted, some products can be tank-mixed with a fast burndown herbicide, or applied seven days after a slow burndown herbicide. Border treatments should be considered if infestations are localized. Insecticides must be well-timed so that products are reaching exposed larvae before they burrow into the stalk. Make sure to read the label and follow directions, especially if tank-mixing with a herbicide, for optimal stalk borer control.

Life cycle. There is just one generation per year in Iowa, and the egg is the overwintering stage. Like all insects, stalk borers develop based on temperature. Egg hatch typically occurs around April 19 to June 5, or 50 percent egg hatch happens at 494 growing degree days. The number of larval molts is variable depending on food quality, and ranges from seven to nine instars. Fully developed larvae drop to the soil to pupate. Approximately 50 percent of pupation happens at 2,746 growing degree days, with 50 percent adult emergence at 3,537 growing degree days. Peak adult flight occurs during the first two weeks of September.

Description. Stalk borer larvae have three pairs of true legs and four pairs of fleshy prolegs. The body is creamy white and dark purple with brown stripes. Often there is a creamy white stripe running down the back. A distinctive feature is an orange head with two dark lateral stripes (Fig. 3). The adults are dark gray and brown colored moths, with jagged white lines and two to three clusters of white spots.

Figure 3. Stalk borer larva. Photo credit Marlin E. Rice.

For more information on stalk borer biology and management, read a recent Journal of Integrated Pest Management article by Rice and Davis (2010), called “Stalk borer ecology and IPM in corn.”


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847. Adam Sisson is an Integrated Pest Management program assistant. Sisson can be contacted by email at or by calling 515-294-5899

This article was published originally on 5/14/2012 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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