Weather
Widespread precipitation last week covered much of IA, but totals once again varied dramatically from l/2 inch to over 4 inches.  The northwest and southeast quadrants faired the best.  The late season rain could potentially help some late developing (green) soybean fields.  I would guess most corn fields are beyond the point of getting much help from the rain though.  Temperatures have been mild and will continue to be so over the next 10 days.  Corn degree-day accumulations will be less than normal.

Row-Crop and Forage Development
Figure 1 shows accumulated degree days (Y-axis) by planting date (X-axis) and in turn attempts to predict growth stage of corn based on planting date.  For a detailed discussion on how to interpret this figure, refer to the May 5 newsletter http://www.extension.iastate.edu/carroll/crops/vol_3_no_07.htm

Corn degree-days (Base 50).  Degree-day accumulation was below normal last week (109 vs. 128) and is forecasted to be only 66% of normal this week (75 vs. 113).  Based on an April 20 planting, 2003 degree day accumulation (September 15) is 91% of 2001 and 2002 accumulations at the same date and 1% greater than the 12-yr average for the date.  Later corn planting dates follow a very similar trend when compared to previous years and the 12-yr average.

Corn development is at or nearing physiological maturity in many fields, but also ranges to full dent.  Physiological maturity is when the grain kernel reaches its maximum dry weight.  Physiological maturity in corn can be determined by finding the “black layer” at the base of the kernel.

Soybean Development likewise is at or near physiological maturity in many to most fields across the region.  Unlike corn, soybean pods reach physiological maturity over a range of time on each plant depending on its location on the plant and when the pod was formed.  Physiological maturity of individual pods may be obtained over a period of three weeks for an individual plant.  When drought stress, disease or insects have been a problem, determining physiological and harvest maturity can be a challenge.

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

Determining soybean maturity can be challenging.  Yet determining when soybeans are mature (both physiological and harvest maturity) helps to schedule harvest operations and reduces in-field harvest losses.  Physiological maturity is when the soybean has accumulated its maximum dry weight.  The growth stage classification is R7 - R8.  With an indeterminate soybean, all pods will not reach physiological maturity at the same time.  There is a period of about two to three weeks from the time the 1st pod reaches physiological maturity to the last pod reaching physiological maturity.  The average seed moisture at the time the pod is physiologically mature is 55%.  When all pods on the soybean plant have reached physiological maturity, the average seed moisture for the plant is usually around 44%.

Two main indicators can be used to determine when individual pods reach physiological maturity:

• Seed Shrinkage: Once the seed has attained its maximum dry weight and size, it will begin to shrink. 

• Pod Color: A short time prior to seed shrinkage, the pods will begin to lose their green color and will take on a yellowish cast.

Harvest maturity indicates when all beans on the soybean plant are ready to be harvested.  This is usually when the average seed moisture is 13%.  Harvest maturity is usually reached about one to two weeks after the entire plant has reached physiological maturity, depending on temperature and humidity. 

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

This page last updated on 09/15/03