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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 19
Northwest IA Crop Update, August 01, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet String of above normal DD weeks broken
bullet Yield accumulating in corn and soybean
bullet Yield potential remains strong
bullet Late summer forage shortage?\
bullet Preparing for grain harvest
bullet Aphids everywhere, numbers remain low
bullet Investigate yellow areas in soybean fields
bullet Scout for 2nd generation ECB

Last week’s cool front broke a seven week long string of above normal degree-day periods.  Warm temperatures this week will begin another above normal degree-day week.  Both corn and soybean crops are now steadily filling grain.  Rainfall over the last two weeks will greatly benefit both crops during this critical period.  Recent crop model runs suggest very strong yield potentials for both corn and soybean in most of northwest IA.  Pest levels across most of northwest IA remain at relatively low numbers.  Soybean aphids are present now in most fields, but numbers remain low.  Hot spots have been detected, so be sure to thoroughly scout for this pest.  As WBC moth flight decline, ECB moth flight and egg laying will pick up.  Continue scouting corn for these two pests.  Soybean fields continue to remain fairly disease free, yet be sure to check any areas the yellow prematurely.  Be sure to view the 3rd “Crop Update Live” web meeting to be held Wednesday morning (August 3) at 7:30 am. 

Weather information
Growing Degree Day  Last week’s below normal degree-day accumulation broke a seven week string of above normal degree-day accumulations.  The seven day accumulation of 136 was 8% less than the weekly average of 149.  The seven day forecast looks to be warm again (at least for the first half of the week) with a projected accumulation of 180 compared to the weekly average of 140.   The area is still well ahead of normal in degree-day accumulations at all planting dates, currently at 105% 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 7.  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
July 25-31 136 116 167 149
Forcasted Aug 1 - 7 180 136 142 140


  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% 103% 104% 105% 107% 105% 105%
this week 108% 107% 105% 106% 107% 109% 107% 107%

Crop Management

Corn Development Corn Pollination is complete in most area corn fields and is advancing towards early dough in the most advanced fields.  Many fields are in the R3 stage.  R3 Stage (Milk) commonly occurs 18-22 days after silking.  The R3 kernel displays yellow color on the outside, and the inner fluid is now milky white due to accumulating starch.  Ear tip kernel abortion occurs when the youngest kernels (resulting from the most recent pollination) are cut off from nutrient flow because the supply is insufficient to fill all the kernels that have been set. Such kernel abortion is most likely to occur during the first two weeks after pollination (blister stage).


Soybean Development has moved into the seed fill stage in many 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.

Good general comments on grain fill in both corn and soybeans from Dr. Emerson Nafziger (University of IL) at this web address:

Forage: Harvest corn silage to compensate for forage/feed deficit.  If feed supplies are running short, corn can be chopped early (on a daily basis) and fed to meet demand until the crop is ready for harvest and storage.  In general, the digestibility and feeding value is equal from blister to maturity.  Protein tends to be higher for more immature silages.  On the downside, dry matter intake is typically less with more immature silages due to the higher moisture content. 

Grain Harvest Preparations  It is not too soon to begin harvest preparations, particularly around storage areas.  Removal of last year’s crop and sanitation practices should be implemented around unloading and storage areas.  Refer to this University of Missouri document on late summer storage facility preparations:

Crop Growth Model Statistics:  The most recent model runs (August 01) suggest yield potential for corn and soybean is still very good compared to previous years.  The corn and soybean model data indicate greater than 100% yield potential of the previous 5-year (soybean) and 25 year (corn) model run average for 3 of 4 locations.   The model data suggest a great improvement of yield potential for both crops over the last two weeks with the rainfall that has been received in the area.  The four locations include Ames, Castana, Kanawha and Sutherland.  Castana is the only location showing below average yields. 

Pest Management
Soybean Aphid  Aphid populations are now present in most area soybean fields, yet the populations remain very low (1-5 aphids/plant in fields that I’ve been in) and much below the 250 aphid/plant threshold in most fields.  Yet I have heard of some fields with “hot spots” approaching or above the 250 aphid per plant threshold.  Therefore it is important to scout your fields aggressively. 

Minnesota’s pest survey (July 29) reported another large increase in aphid incidence (% plants infested) and severity (aphids per plant). Ten percent of inspected fields in MN had more than 100 aphids per plant.  Why is MN’s aphid population so important?  With increasing aphid numbers, more winged aphids may develop in that area creating opportunities for mass aphid migrations to other areas, such as western Iowa.  This was the situation we observed in 2003 with large aphid migrations from north to south.  Current and forecasted weather conditions (cool and somewhat dry) could allow SA reproductive potential to be high.  Also look for those cool fronts that pass through and bring northerly winds that could bring aphids into our area.  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 just about the same as last week, with the forecasted temps allowing aphid populations to double in 2 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:  

Yellowing soybean fields and Cyst Nematode   I’ve notice in the last week areas of soybean fields are turning yellow.  The question you should ask yourself if this is occurring in your field “is this an area that typically turns yellow”.  If not, further investigation should be done, in particular, look for soybean cyst nematode.  Look to this recent article by Dr. Greg Tylka (ISU Extension Plant Pathologist) for more information.

Soybean Rust:  A few new finds of soybean rust have occurred in the extreme southeastern U.S., but the occurrence of rust in that area is still at very low levels.  With favorable weather conditions for rust development in the southeastern U.S. continuing through last weekend, more infected fields may be found.  Continue to monitor their situation at the USDA website:

2nd generation ECB scouting should take place in late maturing non-B.t., seed-production and popcorn fields.  Fields that have green silks and are shedding pollen are most susceptible to second-generation infestation.  Each egg develops a black spot just before hatching. Best control is realized when application is timed to the first significant egg hatch and when young larvae are still located in the leaf axils.  As the plant approaches blister stage and beyond, potential economic benefits of an insecticide application rapidly decline.

Yield loss from the 2nd generation ECB occurs mainly from physiological yield loss (reduced yield from reduced water and nutrient uptake through the stalk).  Several worksheets (some interactive on the web ( have been developed to help you make the decision on whether to treat the pest.  These worksheets can be access online at or

Western Bean Cutworm  WBC moth catch has declined to a slow trickle south of highway 7, yet traps continue to catch WBC moths in more northern areas of northwest and north central IA.  Scouting for WBC should be concentrated on late tasseled corn that still has green silks.  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 3rd “Crop Update Live” web meeting to be held on Wednesday 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 access the web meeting, log in by hitting this URL: at 7:30 am Wednesday morning.


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

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This page last updated on 08/01/05

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