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 15

Northwest IA Crop Update, July 1, 2002
(Word Document)

Introduction

Hot and dry conditions prevail across western IA.  The corn and soybean crop is holding its own, but with continued stressful conditions, yield potential in the corn may be losing ground.  Corn has reached a very critical period for yield determination, typically two weeks pre and two weeks post tassel.  Water stress during this period can greatly reduce yield potential.  Soybeans have begun to flower; water stress in the soybean crop will not become critical until beginning pod fill.  First generation bean leaf beetles have begun emergence and will peak at 1212 degree-days following emergence of the soybean field.  Scout the first generation to help determine control strategies for the more damaging 2^nd generation in August and September.  Several pheromone traps have been place to help monitor Western Bean Cutworm moth emergence in NW Iowa.  Refer to the trap catch data to determine if and when you should begin scouting in your area.

Weather update
Elwynn Taylor (ISU Climatologist) indicated that the high pressure that has been hanging over Iowa seems to be moving to the Detroit area.  Slightly above normal rainfall is expected during the July 4th - 14th period.  We may be seeing some rainfall in SW Iowa by Tuesday and general rainfall over the state by July 4th. Much of the state now has a 4" rainfall deficit for the season.  El Nino is of slight strength and sitting off the coast of Peru.  It is not strong enough to affect the Midwest with cooler summer temperatures. 

Growing Degree Day Accumulation and Crop Development
Northwest IA picked up 190 Degree-days (base 50) last week and is forecasted to pick up 199 this week (Figure 1).  NW Iowa has been accumulating greater than normal DD50’s over the last five weeks and is well above 11-yr average trend line for this time of the year.

Corn Development   The majority of corn is at or shortly past the V-14 leaf stage.   Nutrient uptake through the 14^th leaf stage of corn is N = 38%, P = 25%, K = 40% of the year’s total.  K demand has been large compared to the other primary nutrients, and will be at 80% of the year’s demand in the next 7 to 10 days.

The earliest planted corn in the area will be approaching tassel in 7 to 10 days.  As mentioned previously, the number of rows of kernels per ear has already been established, but the determination of the number of kernels per row will not be complete until about one week from silking or about V17.   Area corn fields are in the most critical period (to avoid moisture stress) in determining corn yield, typically two weeks pre and two weeks post tassel.  Water stress during this period can have major impacts on final yield.  Corn at the V12 stage may take up 0.24 inches of water per day and corn at early tassel may take up to 0.28 inches of water per day.  From now on we need 1 ¾ inches of rain per week to maintain current soil moisture status.  Ideally we should to go into tassel with a full soil profile (for those with irrigation potential, this should be your goal!), then the crop can work with the given rainfall and pull off of soil reserves to fill the grain. 

Soybean Development is moving 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. 

Pest Management
Bean Leaf Beetle  Begin scouting for first generation Bean Leaf Beetles next week in the earliest planted soybeans.  Peak 1^st generation emergence will occur around 1212 GDD’s following soybean emergence. ISU has developed a new control strategy (for 2^nd generation beetles) based on 1^st generation population numbers.   Based upon a high density of first generation beetles, it is expected that the second generation will exceed the economic threshold.  This new management concept stops the second-generation beetles long before they reach the old economic threshold. 

The first thing to do is determine the week soybeans emerged, then refer to the accumulated GDD table and match dates.  Begin sampling fields 1 week after peak emergence.  If the sample is below threshold, scout again the next week.  If the sample is still below threshold, scout 1 more week.  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

Western Bean Cutworm The time for Western Bean Cutworm (WBC) moth flight is soon approaching.  University of Nebraska researchers indicate that 25, 50 and 75 percent moth flights begin at 1319, 1422, and 1536 degree-days (base 50) respectively starting from May 1.  As of July 1, the average degree-day accumulation for NW Iowa is 1017 and I forecast the accumulation to be 1179 by July 7.   Most of the eggs are laid during the peak moth flight in mid July.

Several pheromone Traps have been placed across NW Iowa that will hopefully detect the beginning of the moth flight, if it occurs at all.  Field scouting should be initiated when western bean cutworm moths are first noticed (NebGuide G98-1359-A).  Hopefully NW Iowa will not experience a heavy flight of WBC moths this year.  But with the trap monitoring system in place, we should be able to better predict where outbreaks may occur.  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

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