Northwest Iowa Crop Update Newsletter by Todd Vagts ISU Extension Crops Specialist Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.
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Volume 2, Number 13

Northwest IA Crop Update, June 17, 2002
(Word Document)

In this issue 
DD50 accumulation and Corn development
Soybean Development
Hail damage assessment for corn and soybeans
European Corn Borer Scouting and Thresholds

Introduction
Corn and soybean development across west-central Iowa has been progressing nicely.  Precipitation covered much of the region last week and is forecasted to continue through much of the upcoming week.  A few areas were hit hard by wind and hail, hail damage assessment information is provided in this newsletter.  No abnormal insect pest activity has been reported, yet ECB moth flight has been reported to the south.  Scouting in the more mature, non-B.t. corn fields should begin soon.

Weather update
West central Iowa picked up a very general rainfall last week with many counties getting over two inches.  Some wind and hail damage was reported, but for the most part the rainfall was very beneficial.  The next seven day forecast appears to be warm and wet, rain showers are forecasted through Saturday, June 16. 

Growing Degree Day Accumulation and Crop Development
Northwest IA picked up 159 Degree-days (base 50) last week and is forecasted to pick up 170 this week (Figure 1).  NW Iowa has been accumulating greater than normal DD50’s over the last three weeks and has surpassed the 11-yr average for this time of the year (Figure 2).

Corn Development   The majority of corn is at or shortly past the V-8 leaf stage.  Loss of the two lowest leaves may have already occurred. Ear shoots (potential ears) are visible upon dissection of a V-9 plant.  An ear shoot will develop from every above-ground node, except for the last six to eight nodes below the tassel. Only one or two ear shoots develop into a harvestable ear.  By V10, the time between the appearance of new leaf stages will shorten, generally occurring every two to three days. Nutrient uptake through the 8^th leaf stage of corn is N = 10%, P = 8%, K = 18% of the years total. 

Soybean Development  ranges from V1 (1 node) to V3 (3 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.  At the V-3 stage, the soybean plant has accumulated only 4% of the potential end-of-season dry matter yield.  The plant has taken up 5 and 4 percent of it’s total Nitrogen and Potassium requirements for the season.

Hail damage assessment information on Corn and Soybean can be accessed at the following web addresses: 

Corn:  http://extension.iastate.edu/carroll/crops/hail_corn.htm
Soybean:  http://extension.iastate.edu/carroll/crops/hail_soybean.htm

For more detailed information on corn and soybean replant options, refer to the ISU publications PM 1885 (Corn planting Guide) and PM1851 (Soybean replant Decisions)

http://www.extension.iastate.edu/Publications/PM1885.pdf
http://www.extension.iastate.edu/Publications/PM1851.pdf

In Figure 2, the line with squares equates to Monona, Crawford, Carroll and southern Ida, Sac and Calhoun counties.  The line with Diamonds equates to Pocahontas, Buena Vista and northern Ida, Sac, and Calhoun counties.  The dotted line is predicted GDD and crop development in 7 days. 

Pest Management
European Corn Borer ECB moths and egg masses have been found in southern IA indicating that over the next several weeks, non-Bt cornfields should be scouted for the European Corn Borer.  Dr. Marlin Rice (ISU Extension Entomologist) states in the 6-19-2000 ICM newsletter that Scouting should start in the tallest cornfields because adults (moths) are often attracted to these fields in larger numbers. Look for shot-holes in the leaves after corn reaches 17 to 21 inches in extended leaf height. When shot-holes or larvae are found in the taller corn, then scout fields with smaller corn. There is very little reason to scout a field for larvae if no shot-holes are detected in the leaves.

Dr. Rice further states that you should scout the field by walking at least 100 feet in from the field edge. Each field and each variety within a field should be scouted separately. Pull the whorls from five plants at five locations across the field (10 plants at 5 locations would be even better). Whorls should be selected at random. Do not pull whorls only from plants with shot-holed leaves because this approach overestimates the European corn borer population. Unwrap the whorl leaves and count the number of live larvae. Counting larvae inside the whorls is important because you need to estimate the potential yield loss. Do not count shot-holed plants because this approach cannot determine the number of live larvae.

To determine the need for an insecticide application, consult the charts found at the following web address:  http://www.ipm.iastate.edu/ipm/icm/2000/6-19-2000/scoutforecb.html . Treatment is based upon the average number of European corn borers per plant, the expected corn yield, the market value of the corn, and the insecticide and application costs.

For more information on ECB scouting and thresholds, refer to the following web addresses:

Scouting, treatment thresholds and control products:
http://www.ipm.iastate.edu/ipm/icm/2000/6-19-2000/scoutforecb.html

ECB development rate:
http://www.ipm.iastate.edu/ipm/icm/1999/6-28-1999/ecbgrowth.html

(Word Document)