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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 07
Northwest IA Crop Update, May 09, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Soil temps and GDD accumulations rebound
bullet When will the corn emerge?
bullet Making replant decisions
bullet PEAQ measurements
bullet Pest management insights

With most of the corn now in the ground, the big question is when it will emerge.  The extended period of cold air and soil temperatures, in conjunction with multiple freezes following corn planting, has greatly delayed emergence.  But much of the corn should be emerging within the next couple of days if it follows the rules according to growing degree-day accumulations (120 for emergence).  If emergence doesn’t occur or is erratic, serious evaluations of seedling/plant health and stand counts will need to occur.  Use Table 2 and ISU Extension’s Corn Planting Guide when making replant decisions.  Alfalfa PEAQ measurements have been taken across N.W. Iowa showing estimated RFV ranging from 217 – 252.  Follow the provided URL links to get updated on multiple pest management topics.

Weather information
Soil Temperature Area soil temperatures have finally rebounded following an eleven day period (April 23 – May 3) of below 50 degrees (4-inch daily average found in Figure 1).   This eleven day period would indicate little to no growth or development potential in corn or soybean seedlings that had not yet emerged.  Continue to monitor soil temperature at this web address:


Growing Degree Day accumulation is shown in Table 1 and Figure 2 and is forecasted through May 15.  As seen in Figure 2, degree day accumulations flat-lined for 13 days (April 21 – May 3).  If this was a heart monitor, I think the patient would have been dead and buried by now.  Fortunately, corn is a bit more forgiving, and given the warmer air and soil temperatures, the corn growth should pick up where it left of and will hopefully finish the emergence process within the next few days.  Given the cool temps observed over the previous 3 weeks, N.W. Iowa still picked up more than the normal amount of degree-days last week (70 vs. 52) thanks to the much warmer than normal temperatures observed late in the week.  Degree-day accumulation for this week may fall a bit short compared to the average (66 vs. 72), but is still much better than what we’ve seen in the past.  Degree-day accumulation is still above the 14-year average for the April 10 planting, below normal for the April 20 planting, and near normal for the April 30 and May 10 planting periods (Figure 2).  Corn emergence typically occurs with an accumulation of 110 to 140 degree-days (base 50). 

Table 1.  Degree-Day Weekly Accumulation
  2005 2004 2003 14-Yr Ave
May 2 - 8 70 70 36 52
Forcasted May 9 - 15 66 79 44 72


Crop Management
Corn planting and Emergence  Corn planting should be nearly complete across much of the region.  Corn emergence is definitely a concern for much of the area’s corn crop and from looking at university extension newsletters from across the Midwest; corn emergence is a concern everywhere else also.  Emergence predications can be made from Figure 2, corn planted before April 15 should be emerged and will be approaching the 2 leaf stage (which may be hard to identify if the leaves were froze off, remember this when making POST herbicide applications).    Corn planted from April 20 to May 4th should be emerging at about the same time, which would be right now, between May 10 and May 12. 

Since much of this corn has been in cool and crusted soils for an extended period of time, disease, insect and potentially frost injury could prevent emergence.  Be sure to monitor the growing point on corn that was emerged and frozen off.  Bacterial soft rots can move down the corn whorl and kill the growing point.  Also be observant in fields that had dry surface soils when the freezing temperatures occurred.  Dr. Elwynn Taylor (ISU Extension Climatologist) commented that when the temperatures dip into the mid to lower 20’s, freezing temps may penetrate dry soil deep enough to injure the growing point of emerged and/or non-emerged corn.   Use information from the ISU Corn Planting Guide when making stand evaluations. 

Table 2.  Corn Yield at Various planting dates and stands, expressed as a percent of the optimum stand and date.


Planting Dates


4/20 to 5/05

5/13 to 5/19

5/26 to 6/01

6/10 to 6/16

6/24 to 6/28


Relative Yield Potential (%)

28 – 32,000






























For the data set in table 1 the optimum stand level (established stand) is set at 100. From this base, relative yields for lower stands at different planting dates are suggested. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. To use the table, determine the present corn population and find what the yield potential is compared to the desired stand of 30,000 at the original planting date. Compare this yield potential to the yield potential of the crop if it was replanted today (assuming a good stand was obtained with the replant). The yield potential of the replant may actually be more than the present stand, but it most likely will not be enough of an increase to cover the cost of the replant plus the risk of not obtaining a good stand with the second try

Replant Decisions:  Research at Iowa State University comparing final plant stands at different planting dates (Table 2) should be used to help make the decision on whether to leave a corn stand or replant it.  Use the description at the bottom of the table to interpret the data.  Replant decisions should not be based solely on plant stands, economics also should be considered.  An excellent worksheet to use with making economic comparisons of leaving the original stand vs. replanting can be printed from this website

Soybean planting is well underway and may be finished in some areas.  A concern this season has been how the cool soil temperatures may impact seed health and emergence.  Dr. Pella Pederson wrote a good article in the May 5 ISU IPM news letter found at this web URL:

Forage Management We’re approaching the time of year when alfalfa growers and livestock raisers are keenly watching their alfalfa fields to determine readiness for cutting.  And with recent frosts, determining this timing may be more difficult.  Cutting at the right stage is important to achieve the desired hay quality that will support growth, weight gain, or lactation.   Following last year’s initial efforts into PEAQ (predictive equation for alfalfa quality) measurements, ISU Extension staff in NW Iowa have once again been monitoring fields and reporting growth according to the PEAQ method (for information on PEAQ, visit this web site:  Results from the May 9th sample locations are listed in table 3. 

Table 3.  Relative Feed Value


Height (in)





Late Veg








Lave Veg




Late Veg




Late Veg












Early Bud


*Relative Feed Value

Alfalfa fields in N.W. Iowa are mostly in the vegetative stage with one field being reported in the bud stage. The alfalfa height ranges from 14 inches to 19 inches.  It’s important to remember variability occurs amongst neighboring fields and can show differences up to 30 RFV points. So it's important for producers to measure their own crop.  This is the third measurement taken this spring.  

Frost Injury and PEAQ  In numerous fields, alfalfa tops were frozen back several inches and in a few instances, the entire top was damaged.  Frost damage will cause PEAQ to misinterpret the quality of the alfalfa.  The PEAQ method will likely underestimate the actual RFV in fields that have areas with variable damage or where only a small percent of stems are damaged. On the other hand, PEAQ used on frost damaged alfalfa will likely over estimate the RFV. The bottom line is to discontinue the use of the PEAQ method in fields with significant damage and to understand that PEAQ used on undamaged plants in fields where only some plants or areas are damaged will most likely underestimate the RFV value.  (Comments from Dr. Stephen Barnhart, ISU Extension Forage Specialist)

Pest Management
Several good pest management articles were written in the ISU IPM newsletter last week, they are as follows:

Bean leaf beetle and bean pod mottle virus management: An integrated approach

Three new products receive Section 18 labels for Asian soybean rust management

Progress in soybean rust sentinel plots;

Herbicides and cold weather;


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 05/10/05

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