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7/2/2012 - 7/8/2012

Corn Yield Potential Estimates Amidst a 'Perfect Storm'

By Roger W. Elmore, Department of Agronomy

As of Sunday, July 1, 16 percent of Iowa’s corn crop had silked,  two weeks ahead of normal (USDA July 1). A good share of the crop will silk in the next few days to two weeks. This is the most critical time for corn in terms of effects on yield potential (see Elmore ICM News), a time when water use maximizes (see Al-Kaisi et al., ICM News). Unfortunately, this timing coincides with dry soils across most of Iowa and a forecast for unseasonably high temperatures at least through the end of this week. Many wonder about the impact of this ‘perfect storm’ on corn yield potential. How will this rare combination of circumstances -  corn at the most critical developmental stage, dry soils and hot dry weather persisting for several more days – impact yield potential? The corn simulation model, Hybrid-Maize, helps us address these questions.

  1. Has 2012 weather from planting through July 2 affected corn yield potential?

    No, at least not at the three sites modeled in the historical as well as the 2012 weather data recorded to date at those sites. Obviously, some Iowa fields have experienced afternoon leaf rolling for a week or more already. At least some of these are related to rootworm feeding (see  Erin Hodgson’s July 2 ICM News article). Other fields like this perhaps experienced soil compaction, excessive tillage or other issues that affected root growth and soil moisture (see Elmore’s  ICM News on emergence issues). Yields are already affected in those fields. However, fields that have not had significant leaf rolling as of July 2 still had high yield potential.

  2. What effect will this week’s extremely hot and dry weather have on yield potential?

    That depends on the location and the kind of year we have for the rest of the season.

    Best-case: If the best year in the weather database occurred at each of the sites, estimated yields in Gilbert/ Ames could be reduced by about 11 percent compared to yield estimates at the same site on July 2 (Figure 1). Crawfordsville would experience a 4 percent reduction, and Sutherland would actually see a 3 percent yield increase over that of July 2. In the best year at Sutherland, significant rain events occurred before mid-July. I need to say here that these are best year – best year comparisons. That is the estimated yields from July 8 are as if the best possible year started on July 9 and the same is true for the July 2 estimates. That also means that the best possible year could and does differ between July 2 and July 8; at Gilbert/Ames for example, the best year following July 2 was 2009, and for July 8 was 2004 (see Table 2 for the specific years at each site).

    Worst-case:  The encouraging part of the analysis is that forecast conditions between July 2-8 are actually better than the worst year weather for all three sites. This corresponds with measurements of stress-days discussed by Elwynn Taylor in the ICM News and as shown on the July 5 daily feature on Mesonet.

Figure 1. Yield advantage for July 2 conditions over those of July 8 within yield groups and locations.


The Modeling Approach Used to Address These Questions

  1. Has 2012 weather from planting through July 2 affected corn yield potential?

    The  Hybrid-Maize uses historic weather data from automated weather stations. In this case, I used weather data from three of Iowa State University's Research and Demonstration Farms - NW Iowa near Sutherland, SE Iowa near Crawfordsville, and central Iowa using a combination of data from Ames and Gilbert.

    The model allows users to compare yield potentials given the weather actually recorded up through the simulation date. In this case it included 2012 weather data through last Monday, July 2. The model generates real-time yield predictions for the current season. What that means is that actual weather conditions up to the date of the simulation are in a sense considered the base from which to start. That is what we have to work with; unfortunately, we can’t change what has happened so far in 2012. Then the model ‘asks’ a series of ‘what if’ questions.  For example: What is yield potential if, from this day forward, we have weather conditions like those we had in the best possible year in the weather database for that location? What if the worst historical weather occurred? The weather record begins in either 1986 or 1988 for the three research and demonstration farms we’re working with here (Table 1).

    Common inputs for all three sites modeled are provided in Table 1. Factors that varied across locations such as soil textures are shown in Table 2.  Residue levels at planting, corn suitability ratings and other field-specific information are not factored into the analysis.  However, some of the variability, especially in the early-season factors, is removed by using emergence date rather than planting date in the model.

  2. What effect will this week’s extremely hot and dry weather have on yield potential?

    The five to seven day NOAA forecast on July 3 was for continued hot-dry weather. Using those forecast high and low temperatures, I generated a new – some might say fictitious - weather file for each location for the crop model (see high/low temperatures used in Table 2). The model then provided real-time yield predictions using actual weather data through July 2, 2012, and the forecast average temperatures through July 8. All other weather variables – i.e. solar radiation, precipitation (which was nil at all three sites) and relative humidity - in these model runs were the same as those of July 2; only high/low temperatures were changed. Because crop canopies were already completely developed or nearly so on July 2,  I assume in this analysis  that light interception, crop water use, etc., are stable during this time frame. 

    As mentioned earlier, the real-time yield predictions are based on weather data from previous years at that location and are summarized into five yield-level groups ranging from best yield to worst yield years. For example, what if the best previous weather occurred and what if the worst previous weather occurred from July 9 through the end of the season. Data from these model runs are summarized in Figure 1.  Bars for the three locations are grouped together according to the five yield-level categories from best to worst.




As of July 2, corn may not have yet experienced yield reductions. Although yield estimates vary with locations based on July 8 model runs, if 2012 turns out as ‘median’ year, we may see yield reductions of 7 to 9 percent due to the weather forecast for the week of July 2-8.  The best news is that yields at this time can still better the worst year experienced at each of the sites.

We must remember that not all of Iowa’s corn emerged on May 1 and will enter critical phases of crop development during better conditions. That, of course, remains to be seen.

We all know that many things can happen between now and harvest. If soil moisture conditions do not improve, yields will be reduced much more than these estimates. Meanwhile, let’s hope for cooler temperatures and rain!


Roger W. Elmore is a professor and extension corn agronomist in the Department of Agronomy. He can be reached at 515-294-6655 or e-mail

Water Stress in Corn: Estimating from Stress Degree Days (SDD)

By Elwynn Taylor, Department of Agronomy

“Water Stress Begins at 86F” according to 10- year-old research on corn in the Midwest. This is better stated: “At temperatures of 86F (30C) corn plants reach incipient water stress more than 50 percent of the time.” Naturally, this does not apply to irrigated crops and specifically it applies to non-irrigated corn in the Midwest. In a year with ideal moisture, a perfectly healthy corn plant in prime soil does not begin to expeience water stress until temperatures exceed 92F and perhaps not then if humidity is high. Still, 86F is the average and because the government tends to work with the average. the average was set as the upper bound for the U.S. Corn Growing Degree Day and as the lower bound for water stress. The number is rarely the exact switch-over point, but it is “good enough for government work.” Because we have the government’s critical point numbers, we can make use of them (while remaining keenly aware of the limitations).

Looking at averaged Midwest corn yields over the past 30 years, I note that when the SDD (Stress Degree Day based on temperature exceeding 86F) total exceeds 140 it is tough to find corn yields above trend.

How does 2012 stand as of July 2nd? Just a sample of the Corn Belt gives us a hint: Most states have nine crop reporting districts.  District 5 is the most central of the nine in most states. District 5 is not necessarily the best district to represent the conditions for an entire state; I simply chose it as an example. As of the reports of July 2nd, 2012, the SDD totals since May 1st are:

  • ND 13, SD(District 7) 65, MN 28, WI 62, MI 33
  • NE 170, IA 87, IL 103, IN 118, OH 82
  • KS 360, MO Missing, KY(district 2) 160

With the exception of the Northern Corn Belt (ND, SD,MN, WI, MI), it is likely, according to current National Weather Service outlooks, that the remaining eight Corn Belt states will accumulate SDD well in excess of 140. Only under extraordinary conditions could we expect all of them to exceed their trend yields for corn in 2012. The U.S. corn trend is about 161 BPA for 2012 according to NASS.USDA.GOV (figure 1).  Should the eight other major producing states of the Corn Belt fall below their trend yields it becomes likely that the U.S. yield will also be below the trend.

Figure 1. The 2012 US Corn trend yield estimated from the USDA yield records back to 1981 is about 160BPA. The trend is the “best” straight line through a 30-year yield record extrapolated to the subsequent year.  Graph source:


The accumulated SDD through June 12, 2012, for Washington, Iowa was near 30 (Figure 2). This early accumulation was a hint of things to come in that it was significant and began accumulating rapidly at about the same time the SDD accumulation began during the disastrous crop year of 1988. In both cases, the accumulation by June 12th was not an indication of crop loss as of that date, but in 1988 it did show the trend of things to come. 

Figure 2. Stress Degree Days (SDD) at Washington, Iowa, for May 1st through June 12th for the years of 1988 (severe drought year), 2004 (record high crop yield year), and 2012 (current year). The graphic indicates that the SDD buildup in 2012 began about the same relative date as in 1988, but temperatures in 2012 were to date less severe than in 1988. The total SDD at this location in 1988 reached 650 by mid-August. Graph source: Elwynn Taylor  from


Farmers concerned about SDD at their farm may find the Iowa Mesonet of benefit to them. To use the Mesonet position your computer browser to:

Farmers using this service throughout the Corn Belt can locate a site that is representative of their area.


Elwynn Taylor is a professor of ag meterology and an extension climatologist. He can be reached at 515-294-7839 or e-mail

New Safety Guidelines Developed for Two-tank Anhydrous Ammonia Wagons

By Mark Hanna, Department of Agricultural and Biosystems Engineering

Size and capacity of farm equipment keeps getting larger and fertilizer equipment is no exception. Each year in Iowa approximately a billion pounds of nitrogen is applied as anhydrous ammonia. As equipment has gotten larger, many dealers are now using multi-tank systems with two anhydrous ammonia tanks mounted on to a single wagon running gear. 

With anhydrous ammonia under pressure, safety is always a concern when plumbing and working around application equipment. “Plumbing for a multi-tank system presents unique challenges,” according to Mark Hanna, Iowa State University Extension Ag Engineer.  “Valve location and selection are important decisions impacting safe use.  As an example, valves meant to provide excess-flow protection should not be oversized.”

A new set of voluntary guidelines for plumbing multi-tank anhydrous ammonia systems has been recently developed by a coalition of state government, academia and industry representatives. “As the use of multi-tank systems increases, tank owners should benefit by having these voluntary guidelines. These suggestions from government, academia and industry experts should help applicators review their practices,” said Max Smith, Smith Fertilizer and Grain, Knoxville, Iowa.

“Safety is an important component in maintaining the anhydrous ammonia application industry for Iowa corn production,” according to Kevin Klommhaus, Feed and Fertilizer Bureau Chief at IDALS. 

The guidelines including sketches of suggested plumbing configurations and can be viewed under the news section at The guidelines are voluntary, but dealers and applicators with multiple-tank systems are encouraged to consider the recommendations as equipment is readied for the fall application season. 


Mark Hanna is an agricultural engineer in the Department of Agricultural and Biosystems Engineering. He can be reached at 515-294-0468 or e-mail

Do You Need to Treat for Adult Corn Rootworm This Year?

By Erin Hodgson and Aaron Gassmann, Department of Entomology

As reported in an earlier article, corn rootworm egg hatch in 2012 was slightly ahead of a normal year. Adults were first detected in Illinois last week and around Iowa this week. We’ve been getting many questions about when or if it is appropriate to treat the adults. Three factors must be taken into account before a foliar insecticide application is warranted.

First, consider the growth stage of the plants. Adult corn rootworm will feed on leaves and cause some scarring; however, this will have little if any effect on yield. Adults can cause yield loss if they are present in the field when corn is silking, which is a critical time period to protect plants. Adults are strongly attracted to silks and will mass on plants to feed and mate (Photo 1). Adults that trim back silks to the husk during pollen shed will interfere with pollination.

Photo 1. Corn rootworm are strongly attracted to corn during pollination and can trim back silks to the husk. Photo by Marlin E. Rice.


Next, consider current growing conditions. Weather plays a huge role on how plants respond to silk feeding. Plants do not tolerate as much feeding during pollination in hot and dry weather. For example, under ideal moisture conditions, plants could tolerate 15 beetles per plant, but that number is reduced to just five per plant under drought stress (Photo 2).

Photo 2. Rolling corn leaves are an indication of moisture stress. If plants are droughty during silking, fewer beetles are needed to cause economic loss. Photo by Virgil Schmitt, ISU Extension and Outreach.


Finally, consider plant maturity. Late-planted fields or late-flowering hybrids compared to neighboring fields are generally attractive to adult corn rootworm. Green silks will still be developing in these fields when older fields have brown or drying silks. Adults may migrate and aggregate in these later-maturing fields.


Management recommendations

It is always a smart idea to scout for insects in corn fields during pollination. But with adult corn rootworm becoming active before silking this year, we highly recommend keeping an eye on fields to protect yield. A foliar insecticide may be warranted if there are five or more beetles per plant, silks have been clipped to less than ½ inch of the ear tip, and pollination is not complete. Also take into consideration other insects that may be feeding on the silks at the same time (e.g., Japanese beetle).


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact her at or phone 515-294-2847. Aaron Gassmann is an assistant professor of entomology with research and teaching responsibilities in insect pest management.

Stress, Anthesis - Silk Interval and Corn Yield Potential

By Roger W. Elmore, Department of Agronomy

Forecast daily high temperatures every day this week hover just below 100 degrees. Across Iowa corn ranges from the 10th leaf stage (V10) to tasseling and silking (VT and R1). As of July 1st, the USDA reports that 16 percent of Iowa’s corn was silking; that's nearly 2 weeks ahead of normal. Unfortunately, dry surface and subsoil moisture conditions prevail (see June 26, 2012, Drought Monitor). In the USDA report linked above, 73 percent of Iowa's subsoil moisture was reported as either short or very short compared to 2 percent last year. Crop water use ranged between 0.2 and 0.4 inches per day in central Iowa the last few days (see Soil Moisture Conditions and Crop Water Use, ICM News, for more information on this). With the majority of Iowa’s corn pollinating in the next week to 10 days, this combination of events does not lead to optimism among Iowa’s corn agronomists.

The impact of stress on corn depends on the timing of two critical events: pollen shed and silking. In May, I discussed the multiple 2012 planting dates in a Crop Minute, as well as some of the pluses and minuses associated with them. The USDA forecast of above trend-line yields released after planting earlier this year reflected optimism often associated with early planting and higher yields. On average, we often say that earlier planting results in greater yields and later planting results in lower yields (see the April 27th ICM News). Of course, as with any average, these generalities are not always true.  Sometimes later planting dates result in the best yields.


Pollen shed and silking

Due to the wide planting window experienced this year, expect a wide window for both pollen shed and silking. No doubt earlier silking in normal years is in part associated with earlier planting, and, in addition, early silking is loosely correlated with higher yields. However, many exceptions to this occur. For example, although we had relatively early silking dates from 2005 to 2007 and again in 2010, silking nearly a week later in 2009 resulted in the highest corn yield in Iowa’s history. Our second best yield occurred in 2004, which silked about the same time as those of from 2005 to 2007, and 2010. Early silking this year, if associated with the anticipated stress, will not be associated with maximum yields.

Stress during the pollination and silking period often reduces yield potential.  Water stress is the worst stress factor although high temperatures, defoliation – from hail, insects, etc. - and extremely high plant populations, among others, reduce yield during this critical time especially when coupled with drought stress.   During flowering, plants use more water (0.35 to 0.40 inches per day) than at any other time.  This is in part because silks have the highest water content among all parts of the corn plant.


Anthesis – silk interval: An indicator of stress

One of the best indicators of how plants respond to stress during flowering is the Anthesis – Silk Interval, ASI.  This measures the time in days between pollen shed and silk emergence. We also are concerned about ‘nick,’ referring to the overlap of these two critical developmental stages. The ASI for older hybrids in good condition might have been 2 to 3 days with a range up to a week or more. Corn breeders over the last 5 to 6 decades worked toward developing hybrids that shed pollen and silk at nearly the same time. They have succeeded. With some modern hybrids, silks actually emerge before pollen shed even begins.

These reductions in ASI over the decades helped stabilize modern corn yields in stressed environments. In situations where water is limited, silk emergence and elongation slows. Pollen shed remains constant or accelerates. In older hybrids, water stress often resulted in a loss of nick; thus when silks emerged, there was no pollen source. Barren plants or ears with fewer kernels per ear resulted. By condensing the window of time between tassel and silk emergence, we are more assured of having good pollination with modern hybrids. This is true even if the silks are delayed a couple days or more due to water stress.

Pollen shed occurs over a 5 to 8 day period and silks are viable and receptive to pollen up to 7 to 10 days. Smaller ASI values means a greater chance of successful seed set, increased kernel numbers and increased yield.

The wide planting window this year offers challenges as well as opportunities. Taking note of ASI in your fields will indicate how the crop is faring at flowering. Did pollen shed and silking coincide in near-perfect synchrony, or did stress delay silking and not pollen shed.

In the long-run, the impact of stress conditions during this time will determine yield. On the positive side:

  • No major storms are expected this week, so thunderstorms and the associated strong winds that lodge and break plants prior to tasseling, i.e. greensnap, won’t be a problem.
  • The wide range of 2012 planting dates means that a portion of the crop will not tassel and silk this week and thus may avoid the threats of an abundance of stress for corn… and everyone associated with it.


Roger Elmore is a corn agronomist in the Department of Agronomy. He can be reached at 515-294-6655 or e-mail

This article was published originally on 7/9/2012 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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