Introduction
Very cool temperatures prevailed last week, temperatures will warm this week, but degree-day accumulations will remain below normal for the coming week and much below normal for the year.  Seasonal degree-day accumulations are only slightly less than last year (2003) and the trend mirrors 1992.  Crops are developing slowly but look good for the most part.  Soybean aphids continue to show up in many area soybean fields, but remain at below threshold levels in most areas.  Monitoring aphid populations will be important over the next two weeks.

Row-Crop and Forage Information
Growing Degree-day As predicted, a mass of cold air settled over the region last week bringing with it night-time lows in the 40’s and daytime highs in the 60’s, resulting in a total of 67 degree-days for the week.  This was only 48% of the average of 138 for the same time period.  This was the greatest weekly degree-day deficit the area has seen this year.  The 7-day forecast shows some improvement but still below average with 123 degree-days forecasted compared to the 7-day average accumulation of 136.  According to Figure 1, corn planted on April 10 has accumulated degree-days equivalent to a “normal” May 10^th planting.  Current degree day accumulations for April and early May planting dates have reached yearly lows when compared to the 13-year average (88 to 92% of normal).  Degree-days (average for NW Iowa) can be accessed at this web site:  http://www.extension.iastate.edu/carroll/crops/degree-days-2004.htm

Corn development is slowly advancing through the milk and early dough in the most advanced fields.  R3 Stage (Milk) commonly occurs 18-22 days after silking.  R4 Stage (Dough) commonly occurs 24-28 days after silking. 

Remembering when…So how do degree-day accumulations for this year compare to previous years?  Table 2 lists degree-day accumulations (assuming May 1 planting) by date for Sac City.  As of August 15, degree-day accumulations are only 29 degree-days behind last year (about 2 days), about the same as 1996, and ahead of 1993 and 1992.  So do you remember when your crop matured in 1996, 1993 or 1992?  The degree-day accumulations actually appear to mirror 1992 the most, warm early then cool later in the season. 

Soybean Development
Criteria for determining soybean developmental stages:     

·       R4 = Pod development at least 3/4 inch long at 1 of the 4 uppermost nodes on the main stem. About 7 days to R5.

·       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
Soybean Aphid  Aphid populations can now be found in most area soybean fields, yet they still remain below the 250 aphid/plant threshold in most situations.  Minnesota’s pest survey (August 13)  is also finding aphids in most fields, but still much below last year and below economic thresholds.  Aphid numbers continue to increase and are approaching treatable levels in several fields in far northwestern IA.  Current and forecasted weather conditions could still allow SA reproductive potential to be high. 

Estimating aphid population growth.  Use the University of MN aphid population growth calculator (called the SAGE model) to help predict the growth potential of an existing aphid populations based on current aphid numbers and the 7-day weather forecast.  The SAGE model can be accessed at this URL:  http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm  Click on "Soybean aphid growth estimator v1-2.xls" following the second paragraph.  This is a Microsoft Excel program.  The SAGE Models predicts (for Carroll, IA) aphid populations to double every 2.1 days with the extended 7-day forecast (August 16 – 22).  See my SBA page for more information on identification, scouting and temperature and humidity trends compared to last year.  http://www.extension.iastate.edu/carroll/crops/soybean-aphid.htm  

Use pictures at this web address (http://www.extension.iastate.edu/carroll/crops/soybean_aphid_leaf_counts.htm) to help estimate aphids per leaf.  Print the pictures and take them to the field.

Publication Update:  PM 1168, "Corn Suitability Ratings -- An Index to Soil Productivity" has been updated as of July 2004 and can be found on the Extension Publications Website:  http://www.extension.iastate.edu/Publications/PM1168.pdf

For questions or comments please respond to vagts@iastate.edu

The information given in this publication is for educational purposes only.
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 08/16/04