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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 20
Northwest IA Crop Update, August 08, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Degree-day accumulation continues above normal trend
bullet Aphids numbers constant to increasing
bullet Continue aphid scouting
bullet Aphid control options
bullet Crop Update Live
bullet Crop growth statistics

Crop development is progressing rapidly as corn progresses into the dough stage and soybeans continue to fill pods.  Crop condition looks good across most of west-central IA.  Soybean aphids continue to be the key pest at the moment; fortunately population levels remain below treatable thresholds in most fields.  Continued scouting will be needed through at least the middle of the month.  If you haven’t yet had the opportunity to participate in a “Crop Update Live” web meeting, be sure to join in the next session on Friday morning (Aug 12) at 7:30 am.

Weather information
Growing Degree Day  Last week’s 154 degree-day accumulation was less than the forecasted (180) but still above the average for the week (140).  A warmer than average week is again forecasted with 177 degree-days through this week, again above average (139).   The area remains well ahead of normal in degree-day accumulations at all planting dates, currently at 106% of normal and forecasted to be 107% of normal by next week.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through August 14.  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
Aug 1 - 8 154 136 142 140
Forcasted Aug 8-14 177 77 144 139


  Table 2.  2005 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% 106% 104% 104% 106% 107% 106% 106%
this week 108% 108% 106% 106% 107% 109% 107% 107%

Crop Management
Corn Development
is advancing through the milk and early dough stages in the most advanced fields.  R3 Stage (Milk) commonly occurs 18-22 days after silking.  R4 Stage (Dough) commonly occurs 24-28 days after silking.  Continued starch accumulation in the endosperm causes the milky inner fluid to thicken to a pasty consistency.  The reduced fluid and increased solids within the kernel at this time produce a doughy consistency. Just prior to R5, kernels along the length of the ear begin to dent or dry on top.


Soybean Development has moved into the seed fill stage in most fields (R5).  This stage is characterized by rapid seed filling and redistribution of dry weight and nutrients within the plant.  At initial R5 (the R5 stage is reached when one pod in the uppermost four nodes contains a seed 1/8 inch wide) reproductive development can range from flowers just open to pods containing seeds 8 mm long.  Root growth and expansion slows; therefore it's important that water be available where the developed roots are.  At R5.5 the plant attains its maximum height, node number and leaf area.  The nitrogen fixation rate peaks and begins to drop.  The seeds at this point will begin a rapid period of dry weight and nutrient accumulation.  As development approaches R6, nutrients accumulated in the leaves and petioles begin to be redistributed to the seed.

Pest Management
Soybean Aphid  Aphid populations continue to remain steady (5 – 40 per plant) to slightly increasing in most area soybean fields.  I continue to find and hear of small “hot spots” above the 250 aphid per plant threshold.  These hotspots generally are very small, either on field borders or areas less than 20 ft in diameter.  Therefore it is important to scout your entire fields aggressively. 

Minnesota’s pest survey (August 5) also reported a steady aphid population from the previous week.  This is good news in that the current MN aphid population is staying lower than in 2003 and if steady, we hopefully will not see large migrations of aphids to the south.  A couple of cold fronts are forecasted over the next 10 days, be scouting for winged aphids following the passage of these fronts. The most recent SAGE model aphid population growth estimates for west-central IA can be found at the web address listed below. 

Estimated aphid population growth rate.  The SAGE model predictions for this week are showing a slight increase in the aphid population growth potential, with the forecasted temps allowing aphid populations to double in 1.8 to 1.9 days.  Be diligent in scouting fields with known infestations.  Refer to my soybean aphid web page for population growth estimates based on different current populations:  









Asana XL

5.8 - 9.6 oz




2.0 – 2.8 oz



Lorsban 4E

0.75 – 1.5 pt



Mustang Max

3.2 – 4.0 oz




1.9 – 3.2 oz



*Always read the product label before use

How long to scout?   Treatment data form 2003 suggests we need to protect soybean plants at least through August 15 (see chart below) or R5.5

Western Bean Cutworm  View updated graphed trap catch data and degree-day counts for N.W Iowa at my WBC web page:  For trap catch data around the state and in Missouri and Illinois, go to this web URL:

Join me in the 4th “Crop Update Live” web meeting to be held on Friday morning at 7:30 am.  I will review and update crop and pest management information presented in this week’s newsletter.  More information on this web meeting will follow in another e-mail.  To view recordings of previous meetings, select the week listed on my Crops homepage:

Crop Growth Model Statistics:  Following are two tables showing the daily growth and yield statistics of a computer modeled crop for N.W. Iowa (showing the next 7 days).  Note the accumulation of yield in both crops (corn 5-6 and soybean 1 bu/acre per day) and estimated water stress (on a scale of 0 to 1).  This data was generated from a model run on August 1.  The data was generated and averaged across model runs using weather from four locations in N.W. Iowa (Ames, Castana, Kanawha and Sutherland). 

 Simulated corn crop planted on May 1, 2005

Simulated soybean Crop planted on May 15, 2005






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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Reference to commercial products is made with the understanding that no
discrimination is intended and no endorsement by Iowa State University with
any specific product(s) used in this is implied

This page last updated on 08/08/05

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