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5/19/2008 - 5/25/2008

Timeliness Critical to Protect Corn Yields

by Bob Hartzler, Department of Agronomy

Weeds likely are emerging with corn in many fields due to the lack of a preemergence herbicide. Whether this was planned or due to weather constraints, it is critical to control weeds early in order to protect yields.

The term critical period is used to define how long weeds can be allowed to compete with the crop before yields are impacted. To obtain maximum yields, weeds must be controlled before the critical period is reached. The difficulty in relying on total post programs is the variability in the critical period, making it impossible to predict the optimum time for postemergence herbicide application.

The critical period is influenced by many factors, including:  weed density, relative time of emergence of weeds and corn, weed species and cultural and environmental factors.

A multi-state project evaluated the effect of time of weed removal in glyphosate resistant corn. In the 35 experiments, the average yield loss was 2 percent when the initial glyphosate application was made to 2.5 inch weeds (Table 1). 

Table 1.  Effect of early-season competition on corn yields. (Gower et al. 2003. Weed Technol.)
Weed and Corn Competition Table

Delaying the application until weeds were 5 inches tall doubled the yield loss. The variability in the critical period can be seen by looking at the yield response at the sites with the least and most competitive environments.

At sites with low levels of competition, corn yield loss was not affected when application was delayed until weeds were 7.5 inches.

At the other end of the spectrum, corn yields were reduced 13 percent when weeds were only 2.5 inches at locations with high weed competition.

Due to our inability to accurately predict the critical period, it is wise to act conservatively when determining when to apply postemergence herbicides. Weed density is probably the most important factor influencing the critical period, and fields with heavy infestations should be treated as quickly as possible after weed emergence.

 Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities.


Weeds in young corn

Corn Emergence Problems Across Iowa

Roger Elmore, Elwynn Taylor, and Lori Abendroth, Department of Agronomy

Producers recently experienced good planting conditions; these could be the best planting dates for the year. Although we recommend having corn planted by early May if possible, these recommendations are based on research data accumulated from several locations and years where planting date is the only major variable. This kind of information provides insight as to when, on average, is the best time to plant corn in Iowa. Yield variation exists though in relation to planting dates due to the growing conditions which occur during the remainder of the season.

Although much of Iowa’s corn acres were planted after the recommended planting window, it is very possible that the most recent planting dates are the best  for 2008 because of the cold and wet spring conditions. By ‘best’ we mean the ones that maximize yield. We may look back and find these past few days are the ones that maximized yield this year.

Corn emergence issues will likely occupy much attention in the next two weeks. Iowa’s planting season started later and is going longer than in recent history. Delayed emergence and/or poor seedbed conditions in some regions will increase variability in seedling emergence and reduce final plant populations.

As of 18 May 2008, 78 percent of Iowa corn acres were planted compared to an average of 92 percent. Significant progress was achieved this past week relative to the 11 May report (only 46 percent planted then). In 2007, 88 percent of corn acres were planted at this time.

Soils warmed early this spring then cooled. This condition can bring a crop disaster. Many of us remember a couple of years (especially in the 80’s) when crops went in, partially emerged and quit growing when the both the air and soil temperatures cooled. Crop “damping off” problems multiplied. However, current soil temperatures have recovered to near 60 degrees F at the 4-inch depth and will likely continue warming.

This is suitable for corn and soybean planting and growth this time of year. Mid-Iowa soil temperatures seldom fall below 50 degrees F after the first of May. This year soil temperature seems to be about 3 weeks behind normal.  Both corn and soybean grow best at a soil temperature near 80 degrees F.

Iowa producers continue to plant corn and soybean and many have not taken time to assess some of their early planted fields. It is important to assess crop stands as soon as possible, especially in fields where seedbed conditions were poor to marginal at planting. Much of the corn planted before mid-May is coming up well based on reports from different parts of the state, even though many of this corn was planted into marginal conditions. 

Unfortunately, crusting is preventing crop emergence in some fields across the state (Figure 1, Figure 2 and  more photos). In those cases, producers are hoping for a gentle rain to soften the soil surface and allow for better emergence of their early-planted corn. Planting into marginal conditions this year could not be avoided in some regions. 

Corn seedling breaking through soil crust
Figure 1. A corn seedling struggling to emerge through a thick crust.  Story County, Iowa, 22 May 2008.


Corn seedling leafing out underground
Figure 2. A corn seedling leafing out under the soil surface in an unsuccessful attempt to emerge. Some seedlings like this are growing 1 to 2 inches horizontally in an attempt to reach the surface. Story County, Iowa, 22 May 2008.

Be aware that planting into marginal conditions brings about marginal returns. Resources useful in assessing poor stands and other emergence-related maladies are listed below:

1) Information on normal corn root development
2) Early-season corn stress and some things that go wrong
3) Rootless corn syndrome
4) Uneven corn emergence and heights due to variable soil conditions and planter adjustment: with a tool to assess uneven emergence
5) Problem stands and replant decision making:
6) Off-color corn

Roger Elmore is professor of agronomy with research and extension responsibilities in corn production. Elwynn Taylor is a professor with responsibilities for developing and implementing extension education and information programs in agricultural climatology. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production.

La Nina Diminishes

By Elwynn Taylor, Department of Agronomy

The La Nina of the past several months as determined by the 90-day average Southern Oscillation Index (SOI) has ended. The SOI is a measure of the atmospheric pressure deviation from normal and directly influences meteorological conditions in numerous distant locations. The SOI diminished to 0.8 standard deviations on May 19, 2008. 

Although now in “neutral” condition, there is normally a time lag and risk associated with the La Nina though clearly reduced does not fully disappear at least for several weeks (often about 6 weeks). A significant number of (but by no means all) severe droughts in the Midwest are associated with La Nina. 

Planting under less-than-favorable conditions tends to exacerbate the impact of subsequent hot and dry weather. Drought in the southeastern U.S. is often a precursor to development of drought in the Corn Belt. The adverse factors do not make widespread drought likely for 2008 but below trend crop yields are and remain likely.

I will make my next computation of “most likely yield” during early June. At this time it appears that the previously estimated most likely yield of 142 bushels per acre for U.S. corn will be increase somewhat. 

Elwynn Taylor is a professor with responsibilities for developing and implementing extension education and information programs in agricultural climatology.

Where Do I Get Weather Information?

By Elwynn Taylor, Department of Agronomy

There are several sources of weather information available to help farmers make informed crop production decisions.

To see the forecast for your ZIP code go to Click the general location of interest within the U.S. map. Now click your “exact” location (if you miss it a bit you get a chance to put in your ZIP code.)

This gives your 7-day forecast.  Note that you can check the time the wind shifts, the rain starts and other weather events by clicking “Hourly Weather Graph” at the lower right portion of the page.

Information such as the wind speed an low temperature also can be found at Return to the page showing the map of your location. You will see a long column of terms on the left that includes “Climate”  and “Local.”(The link is marked with a brown box at the bottom of the vertical blue bar in the graphic below.) Web page

If you choose “Local,” you will get a screen that looks like the one below. The site automatically takes you to the "Observed Data" tab. The first choice, Daily "Climate Report (CLI)," will give you yesterday’s weather information for the city you select.  You also may choose “Preliminary Climatology Data (CF6)” to get daily records for the past months.

Daily Climate Report Graphic

Data also can be obtained for the ISU Agronomy farms that have weather stations from  Choose the ISU AgClimate network from the list in the lower left corner.  For 20 minute or even minute by minute data of wind direction and speed as well as temperature and other data see the ASOS and AWOS networks listed in a bar across the top of the mesonet page

Elwynn Taylor is a professor with responsibilities for developing and implementing extension education and information programs in agricultural climatology.

Surface Waters: Ammonium is Not Ammonia – Part Three

By John Sawyer, Department of Agronomy and Matt Helmers, Department of Agricultural and Biosystems Engineering

Two previous ICM News articles outlined the difference between ammonium and ammonia, the relationship between the two nitrogen forms, and the implication of a combined (ammonium-N plus ammonia-N) analysis related to water quality criteria for aquatic life and chlorination treatment for drinking water.This article focuses on the potential sourcing of ammonium and ammonia in surface waters.

Ammonium and ammonia in surface water systems can originate from many sources, and are naturally occurring forms of nitrogen. Predominant sources will vary on a watershed or sub-watershed basis. Also, sources and concentrations are greatly influenced by hydrology, including timing and volume of water runoff.

It is difficult, if not impossible, to separate the effects of both source and hydrology. This makes the issue of ammonium and ammonia in surface water systems complex and can complicate implementation of practices to moderate movement to water systems. Of main importance is targeting sources or practices that have the potential to significantly reduce delivery to a surface water system. Equally important is that management practices need to vary depending upon specific situations.

Following is a list of possible sources of ammonium and ammonia in surface water systems. These are in no particular order, and are not ranked in order of importance or greatest possible impact. There are likely other sources not listed.

Land-applied manure and biosolids, septic systems, raw sewage, snow, rainfall, animal feedlot runoff, surface runoff into tile inlets, eroded soil and sediment, airborne ammonia, direct deposit by aquatic organisms, wildlife manure, land-applied fertilizer (ammonium containing) for crop and turf production, fertilizer on sidewalks and driveways, manure storage structures, manure stockpiles, manure spills, fertilizer facilities, fertilizer spills, decay of aquatic organisms and organic materials in water.

The source(s) of greatest importance (in regard to surface water quality) in one watershed may be quite different in another watershed. Sources could also be point (exact source location) or non-point (diffuse source), and can vary depending upon the time of year.

For example, with wintertime application of manure and fertilizer to cropland, the risk of runoff to surface water increases due to greater chance of the manure or fertilizer not interacting with soil, with that risk increasing further when soil is frozen, there is heavy snow cover and especially if applied shortly before a rapid melting event.

Even with other times of application, such as springtime when rainfall can be intense, if there is a runoff event shortly after surface application the potential for runoff increases because the manure or fertilizer has not had adequate time to interact with the soil. However, the time of year may have less impact on something like animal feedlot runoff if there is little interaction with soil downstream of the feedlot either in non-frozen or frozen soil conditions.

Quite interestingly, because snow readily absorbs ammonia, it can have quite high concentrations of ammonium/ammonia. This is not something that most individuals would think of, and a good example of why understanding sources and quantifying sources is important to provide a greater likelihood that any change in management practices will result in an improvement in water quality.

Despite this, it is equally as important for everyone to consider the implications of their actions in regard to potential for ammonium or ammonia to reach surface water systems. This is good for organisms that live in the water and for municipalities that use surface water for their drinking water supply.

John Sawyer is a professor of agronomy, with extension and research responsibilities in soil fertility and nutrient management, and Matt Helmers is an assistant professor of Agricultural and Biosystems Engineering with extension and research responsibilities in hydrology and water quality.

Finally! A Good Work Week

The last week found Iowa farmers able to get to the field, and crop planting progress advanced notably. Although the week followed the seasonal temperature trend, being a bit cooler than average, the rain held off for the most part.


Emerged corn can now be found across Iowa, and soybean planting is well underway.


Over the weekend, ISU Extension Field Agronomist Kyle Jensen reported black cutworm damage in a south-facing field in southwestern Iowa. Refer to the articles posted on Friday, May 16 for predicted cutting dates and a discussion of economic thresholds for black cutworm.

Map of growing degree days in Iowa

For the next 10 days (May 19 through May 28), historical average base-50°F degree-day accumulations are 13 per day for the northern three tiers of Iowa counties, 14 for the central three tiers, and 15 for the southern three tiers.

Rich Pope is an extension program specialist working in the Corn and Soybean Initiative. 

Update on Fungicides for Use on Soybean

By Daren Mueller, Department of Plant Pathology


There have been some changes in the availability of fungicides for soybean. Here is a quick summary.


Alto® (cyproconazole, Syngenta Crop Protection) has been fully registered for use on soybean by EPA. The product will be available for use on soybean in Iowa when the new label is approved by the state. Until then, this product will only be available through its Section 18 label for soybean rust.


ProlineTM (prothioconazole, Bayer CropSciences) has been fully registered for use on soybean. Although registered, Bayer will not be pushing this product for soybean in 2008, but it is available. The only diseases listed on the label for soybean are soybean rust and powdery mildew. This product also is available for wheat. 


CarambaTM (metconazole, BASF) will NOT be available for use on soybean as a Section 3 product. It still has a Section 18 label for this season for soybean rust, but once the Section 18 is expired this product will only be for use on small grains.


MultivaTM (metconazole + pyraclostrobin, BASF) was recently approved by EPA, but like CarambaTM, will not be available for use on soybean. This product will be available for small grains.


Folicur®, OriusTM and UppercutTM (tebuconazole products) are still expecting Section 3 registration during 2008 season. However, as a backup plan, a Section 18 application will be submitted to renew the expired tebuconazole Section 18 label.  Bottom line, the label status for these products is still up in the air for 2008, so stay tuned.


For a complete list of available products for soybean rust, see Using Foliar Fungicides to Manage Soybean Rust  – Appendix B.

This article was published originally on 5/26/2008 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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