Corn degree-days (Base 50).  Last week’s degree-day accumulation was exactly normal, given the extreme range in temperatures from the blistering hot at the beginning of the week to the very cool end of the week.  The 7-day forecast calls for a below normal degree-day week and much below last year.  The area is running a 230 to 260 degree-day deficit compared to the last two years.  Degree-days (average for NW Iowa) can be accessed at this web site:  http://www.extension.iastate.edu/carroll/crops/degree-days-2003.htm

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

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.

The formula is: [(plants/acre) x (pods/plant) x (2.5 seeds/pod)] / (2900*60) = Estimated Yield bu/acre

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: http://www.ipm.iastate.edu/ipm/icm/1999/9-13-1999/alfalfaman.html

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:  http://www.extension.iastate.edu/carroll/crops/soybean-aphid.htm

For questions or comments please respond to vagts@iastate.edu

This page last updated on 09/02/03