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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 21
Northwest IA Crop Update, August 15, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Corn beginning to Dent
bullet Estimate corn grain yield
bullet Staging soybean development
bullet Two-spotted spider mites
bullet Stay the course with aphid monitoring
bullet Castana Field Day
bullet Needs assessment survey

Area crops are capitalizing on recent precipitation and the milder temperatures as they fill ears and pods.  Corn development is moving into the dough and early dent stages while soybeans are approaching R6.  Now is a good time to venture into corn fields and get a first good estimate on grain yields.  For most pest situations, soybeans need to be protected through the R5.5 to R6 stage, use the included guides to stage your crop.  Spider mites can be found in area soybean fields, be sure to monitor their populations, particularly if an aphid insecticide was applied to the field.  Soybean aphids remain present, but in most fields, remain below threshold. 

Weather information
Growing Degree Day  Last week’s 147 degree-day accumulation was near the average for the week (139).  Another near normal week is forecasted with an estimated 134 degree-days compared to the average of 127.  Compare these numbers to last year, with only 77 and 90 degree days accumulated during the same period.   Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through August 21.  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 8-14 147 77 144 139
Forcasted Aug 15-21 134 90 189 127

Crop Management
Corn Development is nearing partial or full dent in many fields. At R5 (Dent, 35-42 days after silking) all or nearly all kernels are dented or denting. The kernels are drying down now beginning at the top where a small hard white layer of starch is forming. This starch layer appears shortly after denting.


Estimate corn grain yield
{(Ear #) x (# kernel rows) x (# kernels per row)}/90 = Estimated Yield bu/acre

Soybean Development Criteria for determining soybean developmental stages:
  • R5 = Seed is at least 1/8 inch long in the pod at 1 of the 4 uppermost nodes on the main stem. About 15 days to R6.

  • R6 = Pod with green seed that fills the cavity at 1 of the 4 uppermost nodes on the main stem. About 18 days to R7.

Growth rate of the seeds and whole plant is still very rapid at early R6.  This rapid rate of dry weight and nutrient accumulation begins to slow in the whole plant shortly after R6 and in the seeds shortly after R6.5.

Pest Management
Two-spotted Spider Mites  This is another pest that has been showing up in NW Iowa soybean fields.  Some of the aphid insecticides should help keep these in check, but some do not. Know if mites are there in significant numbers before spraying for aphids, and check the insecticide label for mite control if they are a problem. The good news is that cooler weather slows them down.  Be sure to check for mite activity and not just the foliar symptoms.  I looked at a field late last week that was crawling with mites, I rechecked the field today before advising to spray the field, and the mites were gone.  See this 2002 ISU ICM Newsletter article if you want more info on this pest:

Soybean Aphid  Aphid populations continue to remain steady to slightly increasing in most area soybean fields.  Small “hot spots” with 500 to 1000 aphids per plant continue to be reported.  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 12) reports several fields in southern MN have been sprayed, yet in other fields, aphid populations appear to be declining. 

Estimated aphid population growth rate.  The SAGE model predictions for this week are showing an aphid population growth potential of doubling every 2 days.  But I think with increasing beneficial populations and increased moisture in the canopy, most fields will not see population growth any were near this potential.  Be diligent in scouting fields with known infestations.  Refer to my soybean aphid web page for population growth estimates based on different current populations:  

Crop Update Live
Join me in the “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.  Click on this link” Friday morning to join the meting.  To view a recording of the meet, browse to my Crops homepage:

Fall Livestock and Crops Field Day
The Iowa State University Western Research and Demonstration Farm near Castana, IA will be hosting its annual fall livestock and crops field day on Thursday, August 25, from 1 – 3:30 pm.  Topics covered at the field day include:  Swine Welfare, Animal ID, Producers Role in Meat Quality, New Value Added Meat Cuts, What has happened with aphids and rust,  Solid Waste Settling March ’06 Compliance and Results of feeding Condensed Corn Distiller Soluables.  For more information contact the Monona County extension office at 712-423-2175

ISU Extension Needs Assessment Survey
All Iowans are encouraged to participate in a statewide needs assessment for Iowa State University Extension.  ISU Extension is in the process of doing some long range planning for the years 2006-20011, and your help in identifying what we need to work on is requested. To do that, a web-based survey designed to obtain input from a broad base of Iowans is now available at all Iowa county extension web sites. Results from the survey will be used by local county ISU Extension councils throughout the state to confirm needs, set priorities and identify regional issues.  We would like your help! Go to your local county extension web page (go here:, point and click on your county), and do the survey found in the top right-hand corner of the web page of your county Extension Office.


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 08/15/05

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