Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa
In this issue




Manure solid settling highlighted at field day
August or October?
Corn and Soybean advancing slowly
Soybean aphid numbers increase


The SAGE Models predicts aphid population
growth
WBC moth catch drops dramatically
Introduction
Cool temperatures this week will greatly slow crop development but will benefit late summer pasture conditions.
Moisture conditions across west-central IA remain positive, allowing moisture-stress free grain fill. The
soybean crop has entered the critical pod filling stage in many fields. The cool temperatures may limit
additional node development and may promote pod abortion on lower nodes. Soybean aphid numbers are on the
increase in N.W. Iowa soybean fields. The cool temperatures (and high humidity) will help to slow aphid
population growth, but the lack of precipitation in the forecast may benefit the aphid. Use the University of MN
SAGE Model to help predict soybean aphid population growth potential. Western Bean Cutworm moth flight
has decreased in most of N.W. Iowa. Plan to attend the ISU Western Research and Demonstration farm field
day near Castana. The main topic of discussion will be on settling solids from manure runoff.
Western Research and Demonstration Farm Field Day, Castana, IA. August 24
Manure Solids Settling Demonstration - The Iowa Department of Natural Resources (IDNR) has been checking
open beef feedlots as part of their follow-up to the Iowa Feedlot Plan. What is the minimum required of a feed
yard producer? Current IDNR rules require all open feedlots (regardless of size) to remove settleable solids
before any liquid is allowed to leave the feedlot. If you would like to see how solids settling works, Iowa State
University Extension is featuring solids settling as a part of the ISU Experimental Farm Field Day near Castana
on August 24 from 1:00 to 3:30 p.m. The key topic is controlling feedlot runoff.
The Experimental Farm has a 100-head open beef feedlot that features solids settling, vegetative filter strips and
spreaders. The vegetative filter strip uses a grassed terrace to improve the quality of the feedlot runoff. The
spreaders are 2-in x 10-foot boards set on edge at spaced intervals in the terrace. Spreaders are used to slow
down the runoff and prevent channeling and erosion in the filter strips.
Other topics for the field day will include the use of condensed distillers’ solubles in stocker and feedlot cattle
rations, soybean rust, and producing pigs for niche markets. Pre-registration is not necessary. For directions to
the Experimental Farm, call 712-885-2802.
Row-Crop and Forage Information
Table 1. Degree-Day Weekly Accumulation
2004 2003 2002 13-Yr Ave
Growing Degree Day A cold mass of air will settle
over the region this week bringing with it nightAugust 02 - 08 134
141
153
149
Forcasted
Aug
09
15
time lows in the 40’s and daytime highs in the 60’s,
87
149
135
141
more indicative of October than August. The 7-day
forecast shows only 87 degree-days, 62% of the average of 141. Below normal degree-day accumulations
through August usually benefits corn and soybean yield potential as it allows more time for grain fill, but this
week may be a bit extreme. Current degree day accumulations for April and early May planting dates will loose
more ground this week and remain at below average levels (88 to 92% 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 slowly advancing through the milk and early dough in the most advanced fields. R3 Stage
(Milk) commonly occurs 18-22 days after silking. R4 Stage (Dough) commonly occurs 24-28 days after silking.
Continued starch accumulation in the endosperm causes the milky inner fluid to thicken to a pasty consistency.
08/09/2004
Vol. 4 No. 22
Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa
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.
2000
Figure 1. Degree-day accum ulation and Corn
Developm ent by Planting Date (N.W. IA)
13-Yr Averages
Degree-Days (base 50) and Growth Stage
1900
20-April
30-April
1800
10-May
Dough----------------------------------------
1700
1600
1500
---------------------------------------------------------------------1400
Current Forecasted
13
-A
ug
8Au
g
3Au
g
1300
29
-J
ul
Soybean Development has moved into the seed
fill stage in many fields. This stage 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. 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.
Current Forecasted
April 10
Date
What affect will this week’s cool temperature have on soybean development and yield? Dr. Palle Pedersen (ISU
Extension Soybean Specialist) thinks we may observe plants with fewer nodes and the loss of pods from the
bottom five nodes; ultimately reducing the potential for outstanding soybean yields.
Crop Growth Model Statistics: The most recent model runs (August 01) suggest yield potential for corn and
soybean is still strong, indicating greater than 100% of the previous 4-year (model) average for all locations.
Observe the daily development and yield potential “trend” for corn and soybean plants from estimates given by
growth simulation models using local weather data at this web address:
http://www.extension.iastate.edu/carroll/crops/crop-modeling.htm
Pest Management
Soybean Aphid Aphid populations are becoming much easier to find in area soybean fields, yet they still remain
below the 250 aphid/plant threshold. Minnesota’s pest survey (August 6) has now found 42 fields (out of 165
sampled) in the state with (low) aphid numbers, still much below last year. Crop scouts in N.E. Nebraska are
finding increasing aphid numbers as well. Joel DeJong (ISU Extension Crop Specialist) in far N.W. Iowa stated
this morning that aphid numbers are approaching treatable levels in several fields in his area. This is definitely
the time to be vigilant. Current and forecasted weather conditions (cool and dry) could still allow SA
reproductive potential to be high. Interestingly though, temperatures for this week are below optimum and
projected aphid population growth (SAGE Model) is less than if temperatures were warmer.
Estimating aphid population growth. The University of MN recently released their aphid population growth
calculator called the SAGE model which can be used to help predict the growth potential of an existing aphid
populations based on current aphid numbers and the 7-day weather forecast. The SAGE model can be accessed
at this URL: http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm Click on "Soybean aphid
growth estimator v1-2.xls" following the second paragraph. This is a Microsoft Excel program.
08/09/2004
Vol. 4 No. 22
Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa
The developers of the SAGE model state that the model will predict an average rate of population build up
(expressed as the time required for populations to double). The model will also predict when the population is
expected to reach a threshold of 250 aphids per plant. The model is likely to over-estimate aphid populations in
the field. Use this model as a guide to determine when to resample a field (preferably at least 2-3 days before
threshold is reached).
Weather data can be obtained from www.weather.com (input your location into the local forecast box).
Soybeans are currently at R4 (3/4 inch pod on one of upper 4 nodes) to R5 (beginning pod fill) across much of
the region. The University of MN researchers (developers of the 250/plant threshold) still recommend using the
250 aphids/plant threshold through R5.
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
Use pictures at this web address (http://www.extension.iastate.edu/carroll/crops/soybean_aphid_leaf_counts.htm) to help
estimate aphids per leaf. Print the pictures and take them to the field.
The SAGE Models predicts (for Carroll, IA) aphid populations to double every 2.8 days with the extended 7-day
forecast (August 9 – 15). Results from the SAGE Model with varying aphid numbers are given in the following
table.
Soybean Aphid Estimated Population Growth
Location: Carroll, IA
Current Date:
08/09/2004
Today's Population
Population in 7 days
5
28
10
56
Aphids/Plant
20
111
40
376
80
446
Days to Threshold
16
13
Days
10
7
5
Western Bean Cutworm WBC moth flight appears to have peaked in most counties in N.W. Iowa. The Ida
County light trap is now picking up only a few moths per night. Scouting for eggs and hatched larva is still
recommended in corn that is pre or early post tassel (replant or specialty corn). Moth catch data for the state of
IA can be seen at this web address: http://latrodectus.ent.iastate.edu/westernbeancutworm/. Graphed data for N.W. Iowa
can be seen at this URL: http://www.extension.iastate.edu/carroll/crops/wbc-2004.htm
08/09/2004
Vol. 4 No. 22
Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa
For further information pertaining to this newsletter; please contact me or any of the county extension offices.
This newsletter can also be accessed on-line at http://extension.iastate.edu/carroll/crops/newsletter_2004.htm. If
you would like this letter to be emailed directly to you, please send an email with the desired email address to
vagts@iastate.edu.
******************************************************************************
Todd Vagts
Iowa State University Extension
Field Specialist, Crops
1240 D. Heires Avenue
Office: 712-792-2364
Carroll, IA 51401
Cell:
712-249-6025
Email: vagts@iastate.edu
Fax: 712-792-2366
Web Page: http://extension.iastate.edu/carroll/crops/homepage.html
Provided to you by:
IOWA STATE UNIVERSITY OF SCIENCE AND TECHNOLOGY
COOPERATIVE EXTENSION
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
Iowa State University and U.S. Department of Agriculture cooperating
Extension programs are available to all without regard to race, color, national origin, religion, sex, age or
disability.
08/09/2004
Vol. 4 No. 22