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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 18
Northwest IA Crop Update, July 25, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Cooler temperatures forecasted
Predict corn maturity date
bullet Soybean near full pod development
bullet Late summer alfalfa seeding
bullet WBC moth flight past peak
bullet Soybean aphids – scout mid canopy
bullet Perfect temps for aphid population increase
bullet Soybean rust update

The week’s mild temperatures will be welcome following last week’s heat and very high humidity.  The high humidity made it very uncomfortable for us, but probably helped the crop by reducing the transpiration demand.  This was particularly beneficial in areas that missed last week’s rain showers.  With the corn crop running a bit ahead of normal in development, now may be a good time to start thinking about crop maturity and harvest.  The soybean crop is moving into R4 and will begin filling pods very soon.  More moisture in some areas will be welcome as the bean crop enters this critical yield development phase.   Areas that have received precipitation may want to consider late summer alfalfa seeding as a way to replace spring seeding failures.  Western bean cutworm may be on the decrease, but soybean aphids may be on the rise.  Be sure to continue scouting for both pests.

Weather information
Growing Degree Day  Degree-day accumulations last week did not quite reach the forecasted 200, but 189 was still plenty for one week.  The warm temps over the last several weeks have pushed total accumulations to 107% of normal.  This week looks to be a bit milder and near normal with a forecasted accumulation of 154, just slightly more than the average of 149.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through July 31.  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 11-17 189 153 131 158
Forcasted July 18-24 154 116 167 149


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

Crop Management
Corn harvest may not be on your mind, but you can start making predictions on when the crop will mature.  Joe Lauer, Corn Agronomist for the University of Wisconsin, describes how to use silking date to estimate harvest date.  “Silking date is the first indicator for predicting date of harvest for grain or silage uses. The dent (R5) stage occurs 35-42 days after silking and black layer formation (R6) occurs 55-60 days after silking. Usually silage harvest begins shortly after half-milkline on the kernels. The halfmilkline stage occurs about 13 days prior to black-layer (Wiersma et al., 1993). Thus, the predicted start date of silage harvest (half-milkline) occurs 42-47 days after silking. These predicted start dates for silage harvest can be relayed to custom choppers if silking date is known, so they can begin to schedule their workloads for the coming fall.”   Of course this will vary depending upon climate, planting date and hybrid relative maturity.


Soybean Development  has reached R3 and R4 in fields across the region.  R3 is beginning pod growth.  R4 is full pod elongation.  This stage marks the beginning of the most crucial period of plant development in terms of seed yield determination.  Much of the area has received some rain in the last week; this will help the yield potential of the soybean crop as it begins to fill seed over the next several weeks.

Late Summer Alfalfa Seeding  Establishing an alfalfa stand is typically done in the spring, but it can also be accomplished in late summer in NW Iowa. This procedure can be advantageous with crop rotations involving small grains or when a spring seeded alfalfa stand did not establish very well. Late summer seeded alfalfa should only be attempted when conditions allow for it.  Refer to the following URL for more information:

Pest Management
Western Bean Cutworm  It appears the moth flight peaked last week, which is in line with accumulated degree days.  N.W. Iowa should be past 75% emerged moths by July 25.  Scouting should not stop though, pay close attention to later planted fields or those with uneven development.  The late emerging moths will prefer the less mature corn fields/plants when laying eggs.  Also, the cooler temps may favor larva survival.  View 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:

Soybean Aphid  While scouting last week I was having a hard time finding any soybean aphids until I started looking at the entire plant.  Typically, new aphid infestations will start at the top (newest trifoliate leaves) of the plant and can be easily found by looking in this specific area.  But what I found last week were aphid populations in the mid-canopy of the plant, typically on the newest trifoliates of branches.  This should not be surprising considering the high temperatures the area has had.  Temperature in the mid-canopy is cooler allowing aphid populations to develop more quickly.  So when you are out scouting for aphids, be sure to look in the mid to low canopy areas.  Soybean aphid presence in MN is still increasing with incidence (% plants infested) at over 50% across most the state, but severity (aphids per plant) remains relatively low at less than 100 per plant in 197 out of 201 fields (July 22 MN Pest Report).    So far, aphid severity in MN is running lower than in 2003.

Estimated aphid population growth rate increases.  The SAGE model predictions put aphid population growth for this week in high gear.  Cooler temps this week may allow aphid populations to double in 2 days, down from 3.5 days (Carroll area) last week.  Be diligent in scouting fields with known infestations.  Refer to my soybean aphid web page for population growth estimates based on different current populations:  

Aphid economic threshold:  Remember, we are using an economic threshold of 250 aphids per plant with the aphid population increasing and few predators present.  Treatment is recommended after aphid populations pass the 250 per plant level.   Economic losses typically do not occur until aphid numbers reach 1000 per plant.    

Soybean Rust:  Dr. X.B. Yang’s lab released another assessment on soybean rust in the U.S.  The last two sentences in the article read as follows:  …For Iowa, considering the presented scenario, our assessment is that it is unlikely we will find soybean rust in soybean fields before mid-August. If the rust is found around mid-August in Iowa, the disease is unlikely to cause any significant economic damage.  Read the full article at this URL:


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 07/25/05

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