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ISU Extension Logo

Northwest Iowa Crop Update Newsletter
by Todd Vagts
ISU Extension Crops Specialist
Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.

[Home][Special Topics][Weather Data][Subsoil H20][PDF Info] [ISU Extension][IA State University]

Volume 5, Number 14
Northwest IA Crop Update, June 27, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Sentinel plot field day
bullet Degree-day surge
bullet Determining corn yield potential
bullet Soybean enter reproductive phase
bullet How long can corn and soybeans hold their breath?
bullet Insect monitoring, round 2
bullet A new herbicide injury fact sheet

A unique opportunity will be available on Thursday, June 30th to visit with Dr. Ralph Von Qualen (western IA’s sentinel plot coordinator) on the monitoring of soybean rust sentinel plots in western IA.  We will be meeting at one of the sentinel plots at the ISU western research farm near Castana, please plan to attend and discuss any questions you may have about soybean rust and its management.  Recent hot temperatures have created a surge in degree-day accumulations, pushing accumulations at all corn planting dates to above normal levels.  Rains over the weekend were welcome in many areas, but to much of a good thing in others.  Just how long crops can survive under water depends on many factors discussed in the following pages.  Soybean fields have just entered the reproductive phase and corn will not be far behind.  Environmental conditions from now through August will play a major role in determining yield potential of both crops.  The second major round of insect monitoring will begin next week with the emergence of the 1st generation bean leaf beetle, flight of the western bean cutworm, and continued vigilance on the soybean aphid.

Soybean Rust Sentinel Plot Field Day  An update on soybean rust monitoring in Iowa soybean sentinel plots will be held at the ISU research and demonstration farm near Castana in Monona County on June 30.   Dr. Ralph Von Qualen, western IA’s sentinel plot coordinator, will lead the discussion on soybean rust monitoring in IA.  I will also be on hand to discuss recent developments on the movement of soybean rust in the U.S. and its implications to western IA soybean farmers.   

Participants will be able to observe the sentinel plot located at the research farm and ask questions related to disease and pest management in soybeans.  The program will begin at 10:00 a.m. and should conclude by 11:00.  Contact the Monona County Extension Office at 712-423-2175 for more information.  The farm is located four miles east of Castana on county highway E34.

Weather information
Growing Degree Day  Last week northwest IA didn’t quite make the forecasted 192 degree-days but still brought in an outstanding 180, 31 greater than the weekly average.  We should see a much above normal degree-day week again, with 188 degree-days forecasted through July 3.  With last week’s accumulation, all corn planting dates once again are above normal in degree-day accumulations.  If the forecast holds true, degree-day accumulations and plant phenology will be 4 days ahead of normal by next week.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through July 3.  More detailed degree-day accumulations by planting date can be obtained at this URL:

Table 1.  Degree-Day Weekly Accumulation
  2005 2004 2003 14-Yr Ave
June 13-19 180 80 149 149
Forcasted June 20-26 188 116 138 151


  Table 2.  2004 Degree-Day % of Average (by planting date)
  10-Apr 15-Apr 20-Apr 25-Apr 30-Apr 05-May 10-May Ave
last week 107% 105% 101% 101% 104% 107% 104% 104%
this week 110% 108% 104% 105% 108% 110% 108% 108%

Crop Management
Corn development  A majority of the area corn crop is now at or past the V10 stage (corn planted before May 10) and corn planted between April 20 and May 1should be past the V12 stage and will approach V15 by this weekend.  Nutrient uptake through the 14th leaf stage of corn is N = 38%, P = 25%, K = 40% of the year’s total.  K demand has been large compared to the other primary nutrients, and will be at 80% of the year’s demand in the next 7 to 10 days. 

The earliest planted corn in the area will be approaching tassel in 10 to 14 days.  The number of rows of kernels per ear has already been established, but the determination of the number of kernels per row will not be complete until about one week from silking or about V17.   Area corn fields are entering the most critical period (to avoid moisture stress) in determining corn yield, typically two weeks pre and two weeks post tassel. (For those that received rain over the weekend, the timing was near perfect). Water stress during this period can have major impacts on final yield.  Corn at the V12 stage may take up 0.24 inches of water per day and corn at early tassel may take up to 0.28 inches of water per day.  From now on we need 1 ¾ inches of rain per week to maintain current soil moisture status.  Ideally we should to go into tassel with a full soil profile (for those with irrigation potential, this should be your goal!), then the crop can work with the given rainfall and pull off of soil reserves to fill the grain.


Soybean Development is moving into reproductive stages across much of the area (referred to as R1, beginning flower).  You should notice the first flowers on the 3rd to 6th node of the main stem.  Flowering will progress up and down the main stem from there.   Dr. Emerson Nafziger (University of IL) commented in an article last year that “Soybean plants need to reach the 3-trifoliolate stage before they can flower--and warm nights also speed up the activity of a light-sensitive system in the plant that allows flowering to occur. Flowering may stop during long days and cooler nights and will resume in early to mid-July.”

Nafziger went on to comment that “Some of the flowers present now will form pods, and these will remain ahead in development of the main flush of pods that will form later in July. There are usually not many of these early flowers or pods, though, so their contribution to yield will not be great. You can use them as a general indicator of the state of the plant over the next few weeks, though: If they stay attached and develop seeds, it means that the plant is doing well. But if they fall off the plant, it's probably because the plant is struggling to maintain high rates of photosynthesis and growth.” (Adapted from the Bulletin, No. 14 Article 4,June 25, 2004

Flooded corn and soybean  Rains over the weekend were beneficial to many areas, but were detrimental to areas that received in excess of 3 inches, with some areas of Calhoun, Sac, Pocahontas, B.V and Ida counties receiving over 8 inches.  There is some good and bad news for flooding that occurs at this time of the year.  Because the corn and soybean plants are larger, they are better able to withstand flooding stress, but the warm air and water temperatures can speed up crop injury if the water does not recede in a timely manner.  Following are a few key points on corn and flooding I drew out of an article written last year by Dr. Joe Lauer from the University of Wisconsin:
  • The extent to which flooding injures corn is determined by several factors including: 1) timing of flooding during the life cycle of corn, 2) frequency and duration of flooding, and 3) air and soil temperatures during flooding.

  • Respiration is the plant physiological process most sensitive to flooding.

  • Measurable short term reductions for root and leaf growth rates begin immediately within 1-12 hours, but tend to recover quickly within 2-3 days.

  • Flooding can also result in losses of nitrogen through denitrification and leaching.

  • Flooding causes greater crop yield losses when it occurs early in the season. When corn at a height of 30 inches was flooded for 24 and 96 h, yields were reduced 14 to 30%. With a high level of N in the soil, very little yield reduction occurred even with 96 h of flooding. When flooded near silking, no reduction in yield occurred at a high N level, but yield reductions up to 16% occurred with 96 h of flooding at the low level of N.

  • Mud and sediment caking leaves and stalks could damage plant tissue and allow development of fungal and bacterial diseases not typically seen. A disease problem that may become greater due to flooding and cool temperatures is crazy top, a fungus that depends upon saturated soil conditions to infect corn seedlings.

Dr. Lauer in the same newsletter stated that you can expect soybeans to survive 2 to 4 days under water or in completely saturated soils, again depending on air and water temperature and cloud cover.

Adapted from the Wisconsin Crop Manager, Vol. 11 No. 12.  May 27, 2004.  Read the full article online at: and

Pest Management
Bean Leaf Beetle  Peak emergence of 1st generation bean leaf beetles will begin in early planted soybeans next week.  Therefore, plan to begin scouting for first generation Bean Leaf Beetles in about two weeks in soybean fields that emerged in early May.  Research at ISU now allows us to get an estimate of the population size of the 2nd generation by scouting the 1st generation one week after the peak emergence, which generally occurs after an accumulation of 1212 degree-days following emergence of the soybean field.   More information on when and how to scout and thresholds at

Western Bean Cutworm  Western bean cutworm pheromone traps will be placed throughout Iowa over the next week.  University of Nebraska research indicates that 25, 50 and 75 percent moth flight begins at 1319, 1422, and 1536 degree-days (base 50) respectively (starting from May 1).  As of June 27, N.W. IA degree-day accumulation is 905, which is about 100 degree days or four days ahead of last year.  Typically, an accumulation of 1325 (25% moth flight) occurs by the 3rd week in July.  More information on trap counts, degree-day accumulation and control strategies can be found at this web page:

Soybean Aphids remain scarce around the region yet surrounding states appear to be finding increasing populations.  Be vigilant and scout for aphids whenever you are in a soybean field.  Also look to a new ISU website all about soybean aphids.  The web site can be found at

Other soybean aphid information sources can be found on my own aphid page at this web address:

Herbicide Injury fact sheet:  A new herbicide injury on soybean fact sheet has been developed that covers a wide range of herbicides and "typical" symptoms associated with injury from those herbicides. The front of the fact sheet displays images of soybean injury and is arranged by herbicide site of action, while the back of the fact sheet focuses on symptomology associated with that site of action or a specific herbicide. The fact sheet is currently only available in PDF format; it can be downloaded from the Web at (Please note the large file size of this Adobe PDF, 5 MB before attempting a download).


Print or view this newsletter in PDF or Microsoft Word format.

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366

For questions or comments please respond to

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This page last updated on 06/28/05

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