Introduction
The time period for scouting and controlling the soybean aphid is fast nearing an end.  Many soybean fields have set the majority of its potential yield and pre-harvest intervals of insecticides will effectively shut down treatments.  Monitoring soybean developmental stage and aphid population growth is essential in making treatment decisions.  Cool temperatures continued to dominate last week but temperatures should warm this week.  Area corn fields are beginning to dent and the time for silage harvest is just around the corner.  Use table 2 to help determine the harvest moisture needed for the intended storage facility.

Row-Crop and Forage Information
Growing Degree-day Another cold front last week left the area very short on degree-days, accumulating only 101 (DD), compared to the 13-year average of 136.  Cold evening temperatures in the mid to lower 40’s really slowed crop development.  A warmer week is forecasted, with 141 degree-days expected over the next seven days.  This should be very close to the long-term average (140 DD) and the most accumulated in a seven day period since the 3^rd week of July.    Current degree day accumulations for April and early May planting dates remain at yearly lows when compared to the 13-year average (88 to 91% 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 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

Corn Silage:   Predicting when to begin harvest can be a challenging task.  Corn silage that is too wet will yield less, will have silo seepage and will produce sour tasting silage resulting in lower intake by livestock.  If corn silage is too dry then yield is often reduced, heat damage and mold more easily develops in the silo because fermentation is inadequate, and the silage has lower protein and digestibility.  Harvest moisture also depends on the storage structure.

Table 2 lists the recommended harvest moisture for different storage facilities.  Detailed information on harvesting corn for silage can be accessed at the following web site: http://www.uwex.edu/ces/crops/uwforage/Silage.htm

Corn Silage Harvest typically occurs between ½ to ¼ milk line, which under normal conditions will occur from 42 to 47 days post pollination.  According to projections based on the “Projected Pheonology Events” table, plan silage harvest to begin from August 26 to September 05 based on silking dates of July 15 – 25.

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  Although soybean aphids can be found in most area soybean fields, they still remain below the 250 aphid/plant threshold in most situations.  Areas in N.W. IA, and N.E. Nebraska that have received little rain since early July are where soybean aphids have gone over threshold and are problematic.  Thresholds for R5-R6 (beginning seed to full seed) have yet to be determined, but are likely in the 500-1000 aphids per plant range. Treatment after R6 has not been found to increase yields.  Refer to last year’s treatment response curve to see the effectiveness of aphid treatments in late August:  http://www.extension.iastate.edu/carroll/crops/Aphid%20form%20results%20graphed.pdf

How long the aphids will persist in soybean fields is anyone’s guess, they could be here until frost or at least soybean maturity.  Last year they tended to disappear the last week of August.  Generally aphids leave soybean fields in search of buckthorn plants in late August.  Some key criteria to evaluate when making late season treatment decisions include: 1) how many aphids are on the plant (500 – 1000 is the expected threshold), 2) is the population increasing, decreasing or stabilized, and 3) how much yield does the plant have yet to set (i.e. how much yield needs protecting)?  Some area soybean fields are still in early R5, which would indicate a need to protect yield for another 7 to 10 days.  Continue to use the University of MN SAGE model to predict aphid population growth potential.  The SAGE model can be accessed at this URL:  http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm    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  

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 08/23/04