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 20

Northwest IA Crop Update, August 5, 2002
(Word Document)

In this issue 

DD50 accumulation and Corn development
Estimate corn grain yield
Soybean pod fill begins
Heat and moisture stress effects on crops
2nd generation ECB
2nd generation Bean Leaf Beetle

Introduction

Weekend rain showers brought much needed relief too many areas, yet did not reach all crop acres.  Corn is progressing rapidly and will begin denting within the next 7 to 10 days.  The soybean crop is holding its own against the hungry grasshopper; recent rains will greatly benefit the seed fill period.  Extended periods of heat and moisture stress most likely have reduced yield potential as photosynthate production may have been reduced.  2^nd generation ECB scouting continues in late developing corn and grasshopper and bean leaf beetle control will begin in soybean acres.

Weather update
Once again, much of west-central Iowa was reaching a critical moisture stress period as hot and windy days depleted moisture from recent rains from the upper soil profile.  Yet the area’s crops were relieved with very general rains over the weekend.  S.W. Iowa is still dry as the rain did not venture south of Carroll, Crawford and Monona counties.  The 7-day forecast calls for cooler temperatures and scattered showers.

Growing Degree Day Accumulation and Crop Development
Corn development is at the R4 (Dough) in many west-central IA corn fields.   R4 Stage (Dough) commonly occurs 24-28 days after silking.  Continued starch accumulation in the endosperm has now caused the milky inner fluid to thicken to a pasty consistency.  The reduced fluid and increased solids within the kernel at this time produce a doughy consistency. Just prior to R5, kernels along the length of the ear begin to dent or dry on top.

Moisture stress induced nitrate concerns may be alleviated with the recent rains.  The corn plant (with the additional moisture) will mobilize the accumulated nitrates in the lower stalk to the leaves or developing grain where it will then be converted into amino acids

Estimate corn grain yield

{(Ear #) x (# kernel rows) x (# kernels per row)}/90 = Estimated Yield bu/acre

Soybean Development has moved into the seed fill period in many fields.  This period is characterized by rapid seed filling and redistribution of dry weight and nutrients within the plant.  At initial R5 (the R5 stage is reached when one pod in the uppermost four nodes contains a seed 1/8 inch wide) reproductive development can range from flowers just open to pods containing seeds 8 mm long.  Root growth and expansion slows; therefore it's important that water be available where the developed roots are -- no longer can the tap and branch roots grow to where the water is.  At R5.5 the plant attains its maximum height, node number and leaf area.  The nitrogen fixation rate peaks and begins to drop.  The seeds at this point will begin a rapid period of dry weight and nutrient accumulation.  As development approaches R6, nutrients accumulated in the leaves and petioles begin to be redistributed to the seed.  

As mentioned earlier, this is a very critical period to maintain good moisture status in the soil.  If irrigation is available, make sure the soil water profile was full at R3 and again at R5.  The developing seeds need moisture throughout development.  

Heat and Water Stress to crops has been a common problem throughout the region this summer.    High daytime temperatures can be harmful, high nighttime lows can be even more damaging.  As long as corn has adequate moisture and does not experience any moisture stress, high day-time temperatures should not have any appreciable affect on yield.  Due to physiological differences, soybeans and alfalfa are much less able to cope with high temperatures and heat stress.  

Respiration (energy use) in the plant (night respiration and daytime [photo] respiration) uses up assimilated energy that is gained during the day from photosynthesis.   The warmer the plant temperature is, the greater the respiration rate.  Plant canopy temperature is determined by air temperature, humidity and moisture status.  When plants are under moisture stress, they are less able to cool themselves via transpiration, resulting in an increased plant canopy temperature.  During the night, leaf stomata close, effectively shutting down the mechanism by which the plant cools itself.  When nighttime temperatures are high (mid 70’s to low 80’s), plant canopy temperature remains high and nighttime respiration burns up even more assimilated energy.  As more energy is lost through respiration there is less energy to go into seed fill (soybeans), plant growth and root energy storage (alfalfa). 

Pest Management
Corn Insect Pests - 2^nd generation ECB scouting should continue in late maturing seed-production and popcorn fields.  Fields that have green silks and are shedding pollen are most susceptible to second-generation infestation.  Each egg develops a black spot just before hatching. Best control is realized when application is timed to the first significant egg hatch and when young larvae are still located in the leaf axils.  As the plant approaches blister stage and beyond, potential economic benefits of an insecticide application rapidly decline.

Yield loss from the 2nd generation ECB occurs mainly from physiological yield loss (reduced yield from reduced water and nutrient uptake through the stalk).  Several worksheets (some interactive on the web (http://www.ianr.unl.edu/forms/forms.skp/ecb_2nd.html) have been developed to help you make the decision on whether to treat the pest.  These worksheets can be access online at http://www.ipm.iastate.edu/ipm/icm/1996/8-5-1996/ecbcostben.html or http://www.ianr.unl.edu/pubs/insects/nf365.htm

Western Bean Cutworm

Monitor current degree-day accumulations and trap catch numbers at the following web page:  http://extension.iastate.edu/carroll/crops/wbc-2002.htm

Soybean Insect Pests - Bean Leaf Beetle (2^nd Generation) will begin emergence in the next 7 to 14 days.  If 1^st generation population levels exceeded thresholds (for control of 2^nd generation) begin scouting and make treatments as necessary.  Consider defoliation potential of both Grasshopper and BLB when making control decisions.

Dr. Marling Rice (ISU Extension Entomologist) found 2nd generation Bean leaf beetles emerging in central Iowa. New beetles are soft and gray. As they get older they will turn yellow or red. Small beetles, even the small ones, are full grown.  He suggests (estimates) that the best time to spray will be the end of this week and the beginning of next week in the south half of the state. The best time might be next week for the northern half. 

(Word Document)