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 24

Northwest IA Crop Update, September 10, 2002
(Word Document)

Introduction
I’ve been on vacation for ten days and it is unreal how things can change so quickly when you are gone.  Last week’s heat wave really advanced crop development.  Those chopping corn for silage were probably caught off guard as crop development rapidly moved forward with the warm and dry conditions.  According to degree-day accumulation, most corn south of I-20 planted before May 5 should be at or near physiological maturity.  Corn north of I-20 will be nearing physiological maturity within the next week.  Soybeans have also progressed quickly towards maturity.  I’ve seen several fields that have dropped all leaves.  Harvest of both corn and soybeans is on the minds of all producers.  Let’s hope the fall conditions remain conducive for a rapid and clean harvest.

Growing Degree Day Accumulation and Crop Development
The area picked up 163 degree days last week, quickly advancing much of the corn crop past physiological maturity (Figure 1).  Physiological maturity is the point in which maximum dry weight accumulation has occurred in the grain.  This can often be determined by the formation of the black layer at the base of the corn grain kernel.  Cooler temperatures over the next 5 to 7 days should help to slow crop development and allow producers to prepare for harvest.  Total degree day accumulation has surpassed last year’s accumulation for this calendar day and is well above (200) the 12-year average (figure 2)

Stalk rot scouting should be going on now. Scouting involves looking for lower stalk discoloration and test stalk firmness by pinching the lower internodes between your thumb and forefinger. Healthy stalks are firm and won't compress easily. If a node can be compressed or otherwise feels soft, rotting has started that plant is a good candidate for lodging. Check at least 100 plants per field, in representative locations. Fields with different tillage systems that are in different rotations, planted to different hybrids or with different fertility histories should be scouted separately. If more than 10-15 percent of the stalks show rot damage in a field, significant lodging is likely.  If possible, harvest high lodging potential fields early. And don’t forget to record the stalk rot incidence in field records so you can use this year’s problems to avoid future problems.

Dry down rates of corn.  Now that many fields of corn across the area are near physiological maturity, the rate of in-field dry down will determine when harvest maturity is reached.  Peter Thomison from Ohio State University describes in the CORN Newsletter (http://www.ag.ohio-state.edu/~corn/index.html) the rates in which corn grain dries. The following dry down rates were derived from research in western Ohio, therefore they may be different for northwest IA, but they are a good baseline to work from.  Starting point for this is black layer, of course.

·        Corn will normally dry approximately 3/4 to 1 point of moisture per day during favorable drying weather (sunny and breezy) during the early warmer part of the harvest season from mid to late September 

·        Early to mid-October, dry-down rates will usually drop to 1/2 to 3/4% per day.

·        Late October to early November, field dry-down rates will usually drop to 1/4 to 1/2% per day.

·        Mid November, probably 0 to 1/4% per day. By late November, drying rates will be negligible.

Estimating dry-down rates can also be considered in terms of Growing Degree-Days (GDDs). It takes about 30 GDDs to lower grain moisture each point from 30% down to 25%. Drying from 25 to 20 percent requires about 45 GDDs per point of moisture.

Additional discussion of corn dry-down can be obtained from a 2001 article prepared by Dr. Bob Nielsen, extension agronomist at Purdue - in the "Chat n' Chew Café."  It's available on-line: http://www.agry.purdue.edu/ext/corn/news/articles.01/Grain_Drydown-0826.html

Fertility Management
Sampling time for the end-of-season cornstalk test to evaluate nitrogen management is rapidly approaching. Basic information for doing the test is provided in extension publication Pm 1584, Cornstalk Testing to Evaluate Nitrogen Management

Forages
Give alfalfa a rest before winter.  When you take your 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 last summer cut by late August or very early September, and let regrowth stand in the field (no late cut or grazing). If you have not fertilized in the summer, topdress any needed potassium in late August or early September.

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

(Word Document)