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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 10
Northwest IA Crop Update, May 31, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Degree-day accumulations near normal
bullet All crops emerged
bullet Corn plant becomes self sufficient
bullet Monitoring soybean development
bullet Insects on the move
bullet Control weeds sooner rather than later

Favorable weather over the weekend and forecasted warm temps this week should really get the crops moving ahead.  Degree-day accumulations remain near normal as does crop development (except for the late April plantings).  The corn plant is becoming self sufficient as the nodal root system takes over.  Be aware that the growing point is moving above the soil surface.  Soybean development is moving ahead at about one new node every five days, do you know how to stage soybeans.  Corn rootworm hatch should occur on time during the second week of June, expect movement of stalk borers within the week.  Weeds are doing well also, be sure to remove them from the field soon so that they do not rob yield from the corn or soybean crop.

Weather information
Soil Temperature Area soil temperatures have stabilized in the mid 60’s (Figure 1), still good for getting the last planted soybeans out of the ground.   Since most of the crops are now out of the ground, this will be the last reporting of soil temperature, yet you can continue to monitor area soil temperatures at this web address:


Growing Degree Day  Last week, N.W. Iowa accumulated near normal degree-days (79 vs. 82).  The seven day forecast looks favorable with 118 degree-days predicted compared to the average of 92.  This could be our first week with greater than 100 degree-days accumulated in a seven day period. All planting dates (except for plantings during April 20 – 30th) are near normal in degree-day accumulations (Figure 2) and may move to above normal by next week.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 2 and is forecasted through June 05.  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
May 23-29 79 75 80 82
Forcasted May 30-05 118 93 84 94


  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 99% 94% 83% 84% 90% 97% 85% 90%
this week 104% 100% 91% 93% 98% 103% 96% 98%

Crop Management
Corn development
   Corn across much of the region ranges from the V2 to V5 leaf stage.  Corn planted during the cool April 20 – 30th time frame should be at or approaching the V4 stage and near V5 by next weekend.  Corn planted in early April should be at the V5 leaf stage, approaching V6.  The growing point typically moves above the soil surface and is more susceptible to injury at the V5 stage.  Corn at V6 is at the beginning of a period of greatly increased stalk elongation and nutrient uptake. 

The nodal root system is becoming the major functioning root system. By the time a plant reaches approximately V4 (four visible leaf collars), three “rings” of nodal roots should be visible at the crown of the plants. Such plants are essentially independent from any further sustenance that the kernel may yet be able to furnish. Total uptake of nutrients has been small through the 6th leaf stage of corn (N = 8lbs, P = 5 lbs, K = 8 lbs).  But rapid accumulation of nutrients and plant dry matter will occur from V6 through V18.  Use the corn development chart (figure 2) to help predict timing of corn V stages.  If you want to figure you own crop development schedule, figure that corn requires about 75 degree-days (base 50) for each new leaf.  


Soybean Development  Soybeans range from VE to V1 (1 nodes) and are growing well in most areas.  New V stages will appear about every 5 days through V5 and every 3 days through R5.  Growth stages of soybeans are defined by the uppermost fully developed leaf node.  A fully developed leaf node is one that has a leaf above it with unrolled or unfolded leaflets (leaflet edges are no longer touching).

Replant Decisions:  Obtain the Corn Planting Guide and the Soybean Replant Guide for help in making replant decisions.  Both can be obtained on the web or from your local ISU Extension office.

Pest Management
Corn Rootworm Hatch  Typically, corn rootworms begin to hatch around 500 degree-days (base 50) from May 1.  According to my calculations, west central IA will have accumulated 475 (south of Hwy 20) and 425 (north of Hwy 7) degree-days by June 6. Expect and accumulation of between 15 and 20 DD per day.  Plan your post-hatch rootworm treatments accordingly. 

Stalk Borer Migration  Stalk borers will soon be leaving areas of tall grass (ditches, waterways, fence lines, etc.) for the edge rows of corn fields. Watch for heads of bromegrass and orchardgrass that turn brown early as these are the source of the insects that will move into the corn field. Scouting should begin when 1,300 - 1,400 Degree Days (Base 41) have accumulated. Based on current weather forecasts, we should reach 1,300 GDD south of Highway 20 by about June 4 and north of highway 20 by June 8.

Bean Leaf Beetle:  The BLB’s in fields now will remain in fields until mid-June.  BLB feed on young, new tissue and can quickly cause noticeable defoliation on seedlings.  Noticeable defoliation and economically significant defoliation are different.  Early-season damage does not gain economic significance until cotyledons are lost and regrowth is suppressed by feeding activity.  If the concern is preventing bean pod mottle virus, then VE-VC stage soybeans should be sprayed with an insecticide as soon as the soybeans emerge and beetles are present.  Remember the goal is to prevent the introduction and spread of the virus by feeding beetles.

Weed Control  Timely application of postemergence products is critical to protect corn and soybean yields, especially in fields relying solely on postemergence herbicides. Weeds can begin to affect yields as early as two weeks after crop emergence. The specific time at which weeds begin to reduce yields varies widely depending upon many factors, but the longer application is delayed, the greater the risk of a yield penalty.



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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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 05/31/05

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