vagts crop update vol 3 no 24 by 0bY3vD


									Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa

In this issue
    Cool and dry conditions forecasted                                                     Early soybean senescence: disease or drought
    Calculate the yield potential of the soybean crop                                       induced?
    Scout fields now for pest and soil problems                                            Give the alfalfa a rest

Cool and dry conditions are forecasted for the next 10 days. Crop development, although behind the last couple
of years, is on a near normal tract. Soybean yield potential can be estimated at this point, better estimates are
made with multiple samples. Now is a good time for final field scouting for disease, insect, weed, fertility and
soil compaction problems. Make note of problems to be corrected next year. Early senescence may be common
in area soybean fields, but is it due to early maturity, drought stress or disease? If in doubt, collect plant samples
for potential disease identification. Give the alfalfa a six week rest period before winter; that means no cutting
from now until mid-October.

A much cooler, but very dry 10-day forecast is in store for west-central and north-west IA. No appreciable rain
chance is seen in the next 7 to 10 days.

Row-Crop and Forage Development                                                                    Figure 1. GDD Acccumulation (September 1)
Figure 1 shows accumulated degree days (Y-axis) by planting date                            2400
(X-axis) and in turn attempts to predict growth stage of corn based                                            --2600 = Physiological Maturity--
on planting date. For a detailed discussion on how to interpret this
figure, refer to the May 5 newsletter                                                                --------------------------Full Dent--------------------------
      Degree-Day Weekly Accumulation
                                    12-Yr                                                   2250
                                                                       Accumulated DD50's

                         2003 2002 Ave
              Aug 25-31 149   155    148
     Forecasted Sept 1-7 106  162    132

Corn degree-days (Base 50). Last week’s degree-day                                          2100
accumulation was exactly normal, given the extreme
range in temperatures from the blistering hot at the                                        2050
beginning of the week to the very cool end of the
week. The 7-day forecast calls for a below normal                                           2000

degree-day week and much below last year. The area
is running a 230 to 260 degree-day deficit compared to                                          4/15    4/20    4/25     4/30 5/5 5/10          5/15    5/20     5/25
the last two years. Degree-days (average for NW                                                                        Planting Date
Iowa) can be accessed at this web site:                                                                   North                                    South

Estimate soybean yield potential.
    1. Estimate the number of plants per acre (measure an area 1/1000 of an acre and count the number of
       plants within the marked area.)
    2. Count the number of pods on ten randomly selected plants within the marked area and calculated the
       average number of pods/plant
    3. Calculate pods per acre by multiplying plants/acre by pods/plant
    4. Calculate seeds per acre by multiplying pods per acre by an estimate of 2.5 seeds/pod

                                                            Volume 3, No. 23
Crop Update Newsletter Prepared By:
Todd Vagts, ISU Extension Crops Specialist
Serving northwest Iowa
    5. Calculate pounds per acre by dividing seeds/acre by an estimate of 2900 seeds/pound
    6. Estimate yield by dividing pounds/acre by 60 lbs. per bushel.
    7. The formula is: [(plants/acre) x (pods/plant) x (2.5 seeds/pod)] / (2900*60) = Estimated Yield bu/acre

This formula uses several estimations and therefore may be variable depending on the final number of seeds per
pod and seed weight.
(Source: Corn and Soybean Field Guide, 1997; Purdue University Coop Extension Service)

Give alfalfa a rest before winter The timing of the last harvest of alfalfa affects its winter survival and next
spring's vigor. Alfalfa needs about six weeks of uninterrupted growth in the fall to become fully winterized. Dr.
Barnhart (ISU’s Extension Forage Specialist) recommends that for the best survival of the stand, attempt to take
the last summer cut by late August or very early September, and let regrowth stand in the field.

Dr. Barnhart further explains that during the cold-hardening process, plants accumulate carbohydrates and
proteins in storage organs, such as taproots and the crown in alfalfa. They convert these stored carbohydrates
into simpler molecules that may give the cells more "antifreeze" protection, and they lose some cellular water so
the cells do not rupture when intercellular fluids freeze. Interestingly, alfalfa's response to drought is very
similar to that for cold hardiness. So alfalfa plants are much better off going into the winter under dry compared
with wet conditions. For more in-depth information on winter survival of alfalfa, go to the following web site:

Plant Pest and Disease
Scout and evaluate corn and soybean fields now while you have time. Many disease, insect and weed pest
problems can be found in area corn and soybean fields now. Obtaining a positive ID on the disease, weed or
insect will be very important when making crop rotation, hybrid and variety selection decisions for next year. If
you, your crop consultant, seed or Coop agronomist cannot absolutely identify what the problem is, contact your
local extension office and send plant or insect samples to ISU for help. The ISU plant disease clinic, weed, insect and soil fertility specialists are
there to help you identify problems and develop remedial actions.

Soybean Maturity, Drought stress or Disease? A number of soybean fields have expanding yellow areas in
recent weeks. In some cases, the fields are reaching normal physiological maturity. But in fields that have
yellow patches and still have green pods, other factors may be occurring in the field that should be investigated.
A cumulative effect of drought stress, disease and soybean Cyst nematode may be the problem.

Further more, an unidentified disease problem may be in your fields. Dr. X.B. Yang (ISU Extension Soybean
Pathologist) has found an “unidentifiable disease” problem in some central IA fields. Symptoms described by
X.B include: 1) diseased plants were dead pre-maturely in patches similar to those of SDS. 2) Severely infected
plants wilted similar to charcoal rot or stem canker but no fungal fruiting boys could be found on diseased
plants. 3) Stems of diseased plants had discolorations resembling Phytophthora stem rot in early stage. Yet
there were no root rot or minimum root rot. 4) SDS like discoloration in cortex tissues of tap root and lower
stems. And 5) In some fields, piths of diseased plants had light BSR-like browning up to 4th nodes.

Although you can not do anything about field and disease problems this year, you can attempt to determine the
cause of the problem so it can be avoided in following years. When you walk your fields, dig plants and
examine the root system, the lower stalk area, split stems and collect soil and plant samples for further testing if
the problem is not easily identified.

Soybean Aphid Update: More information on soybean aphids can be found at:

                                                       Volume 3, No. 23
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
If you would like this letter to be emailed directly to you, please send an email with the desired email address to

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:        Fax: 712-792-2366
Web Page:

Provided to you by:


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

                                                  Volume 3, No. 23

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