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 16

Northwest IA Crop Update, July 8, 2002
(Word Document)

Introduction
The moisture situation for western Iowa is becoming critical as most areas missed the rainfall that much of central Iowa received over the weekend.  Corn has begun to tassel in the earliest planted corn fields, soybeans have also moved into the reproductive phase of their development.   I would expect that soybean leaf cupping may become evident once again this year due (not to herbicide drift) but to environmental stress on the plant.  Look for drift patterns if leaf cupping becomes evident.  Western Bean Cutworm moths have been caught near Correctionville, Arthur and Castana; begin scouting corn that is near tassel now.

Weather update
Moisture is the big topic for western Iowa as the corn and soybean crops move into their reproductive phases.  Temperatures are forecasted to moderate by midweek, but than move back into the 90’s by week’s end.  Rain chances remain low for the next 7 days; Wednesday and Thursday look to be the best chance to pick up some precipitation. 

Growing Degree Day Accumulation and Crop Development
Northwest IA picked up 197 Degree-days (base 50) last week and is forecasted to pick up a few less at186 this week (Figure 1).  NW Iowa continues to accumulate greater than normal DD50’s and is well above 11-yr average trend line for this time of the year.

Corn Development has reached VT (tassel) and R1 (pollination) in many of the earliest planted fields.  The VT stage is initiated when the last branch of the tassel is completely visible and the silks have not yet emerged. VT begins approximately 2-3 days before silk emergence, during which time the corn plant will almost attain its full height and pollen shed begins. The time between VT and R1 can fluctuate considerably depending on the hybrid and environmental conditions. Under field conditions, pollen shed (also termed pollen drop) usually occurs in the late mornings and early evenings. R1 begins when any silks are visible outside the husks. Pollination occurs when these new moist silks catch the falling pollen grains. A captured pollen grain takes about 24 hours to grow down the silk to the ovule where fertilization occurs and the ovule becomes a kernel.  Generally 2-3 days are required for all silks on a single ear to be exposed and pollinated.  The shank and husks attain full size between the R1 and R2 stages.

Nutrient uptake through R1 of corn is N = 68%, P = 55%, K = 90% of the year’s total.   Nutrient demand and uptake over the last week has been great in all categories.  With the dry conditions and loss of nutrient availability in the upper soil profile, nutrient deficiencies may be common in area corn fields.  At R1, the corn plant has accumulated 50% of its total dry matter yield

Soybean Development has moved into reproductive stages across most the area (referred to as R1, beginning flower).  You should notice the first flowers on the 3^rd to 6^th node of the main stem.  Flowering will progress up and down the main stem from there. 

Cupped soybean leaves may be common again this year with the environmental stress the crop is experiencing.  Last year many area soybean fields experienced “cupped” leaves similar to symptomology observed from 2,4-D/Dicamba herbicide drift.  The majority of soybean fields with cupped leaves that I looked at last year was not attributed to a growth regulator herbicide, but rather was due to environmental (moisture/heat) stress and/or a soybean herbicide application during stress conditions.  When faced with a field with the “cupped” leaf symptomology, first look for patterns in the field.  Herbicide drift patterns should be evident from the source (corn field) while environmentally induced leaf cupping should be consistent field-wide or more prevalent in areas with more environmental stress (sandy, compacted, pest problems, double-planted areas). 

Paul Kassel (ISU Crops specialist for NC IA) has some helpful insights into deciphering the source of cupped leaves in soybean fields:

Herbicide drift:  symptoms occur a week or so after application and usually confined to a few rows next to a cornfield.  Herbicide Vapor drift:  Symptoms may take two to three weeks to show up.  Vapor is likely coming from corn herbicides that contain dicamba.  Rainfall after dicamba applications will cause vapor production to end.  Dry conditions may enhance vapor production.  Soybean fields affected are usually north of suspect cornfields.  Environmental/surfactant effects:  Surfactants/crop oils may produce leaf symptoms that mimic dicamba drift.  Yield effects on soybeans: Yield reduction is highly correlated to height reduction of soybeans.  Soybeans with cupped leaves and little height reduction generally will have minimal yield reduction.

For more information on “cupped” leaves in soybeans, refer to the following web sites:

Dicamba effects on soybean yields: http://www.ipm.iastate.edu/ipm/icm/2000/7-17-2000/dicamba.html
Malformed soybean leaves appearing: http://www.ipm.iastate.edu/ipm/icm/1999/7-19-1999/malsoy.html
It's Back Again in 2001: http://www.ag.uiuc.edu/cespubs/pest/articles/200115i.html

Pest Management
Western Bean Cutworm moths have been caught in traps in Monona, Sac and Woodbury/Ida counties.  Field scouting should be initiated when western bean cutworm moths are first noticed (NebGuide G98-1359-A).  You can monitor current degree-day accumulations and trap catch numbers at the following web page:  http://extension.iastate.edu/carroll/crops/wbc-2002.htm

For more detailed information on the WBC, refer to the Nebraska publication G98-1359-A. http://www.ianr.unl.edu/pubs/insects/g1359.htm

Bean Leaf Beetle degree-day accumulation has surpassed the 1212 DD mark for soybeans emerging on or before May 20.  Peak 1^st generation emergence will occur around 1212 GDD’s following soybean emergence. Begin sampling fields 1 week after peak emergence.  If the sample is below threshold, continue scouting for two weeks.  If 1st generation beetles are above threshold, scout again mid August to confirm beetles are present, then spray.  Refer to the following web page for population thresholds and updated degree-days http://extension.iastate.edu/carroll/crops/blb-2002-1st-gen.htm

(Word Document)