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08
Jul
2016
Corn growth stage and heat unit accumulation for irrigation termination
Author: Jason Kelley, Wheat & Feed Grains Extension Agronomist

By Jason Kelley, Wheat and Feed Grains Agronomist

Much of our early planted corn is progressing towards maturity and has made it to the dent stage (R5).  One would think that once corn has reached the dent stage, maturity (R6) would be soon to follow, however it is a slow process and generally takes 21-24 days with normal temperatures for our full season hybrids to go from R5 to R6.  During the R5 stages, the kernel is still adding a great deal of weight and yield potential.  At the beginning of the R5 stage, only about 45% of the total kernel dry weight has been accumulated.  By the R5.5 stage (starch line 50% down), approximately 90% of the kernel dry matter has accumulated and by the R5.75 stage (starch line 75% down) 97% of the kernel dry matter has accumulated.  Maintaining adequate soil moisture through the R5 stages is needed to maximize kernel weight and yield potential.

The goal of irrigation termination timing is to have adequate soil moisture to reach the R6 stage without irrigating more than needed.  There are several ways to determine corn maturity and whether irrigation can be terminated.  The most obvious is to look at the kernel and see how far down the starch line has moved.   The starch line begins at the top of the kernel (signified by a dent, Figure 1.) and slowly progresses down the kernel (Figures 2, 3).

Figure 1.  (left) Dent Stage (R5)Figure 2.  (right) Starch line 15-20% down kernel

Figure 1. (left) Dent Stage (R5) Figure 2. (right) Starch line 15-20% down kernel

Once the starch line has moved half way down the kernel, you are approximately 10-12 days to maturity with normal July temperatures.  If the starch line has moved down the kernel 50% or more and you have good soil moisture, irrigation on furrow irrigated fields could be terminated.  At 50% starch line movement and good soil moisture, the plant should have enough moisture to reach maturity without running out of water.  For pivot irrigated fields, the starch line needs to be 75% or greater down the kernel before irrigation is terminated since less water is likely applied during each irrigation.  If in doubt that the plant will have adequate moisture to reach maturity, it is advisable to irrigate once more, especially if hot and dry conditions are present.

Figure 3.  Starch line has moved approximately 75% down the kernel and corn is approximately 7 days from maturity.  With adequate soil moisture, furrow or pivot irrigation could be terminated on this field.

Figure 3. Starch line has moved approximately 75% down the kernel and corn is approximately 7 days from maturity. With adequate soil moisture, furrow or pivot irrigation could be terminated on this field.

Keep in mind that maturity may be variable across a field, so it is advisable to check several spots in a field to get an accurate measurement of maturity.  Also on any given ear, maturity will vary from the top to the bottom of the ear.  I recommended looking at kernels from the middle of the ear for reference.  Once the kernel has reached the R6 Stage (Figure 4) a black layer should be readily seen by scraping the bottom of the kernel.  The black layer formation will occur over a few days and will begin as a tan/gray color and slowly increase in size and become black in color.  At true black layer no further moisture enters the kernel and grain moisture is approximately 35%.

Figure 4.  R6 stage corn.  A fully developed black layer is clearly visible on the right two kernels.

Figure 4. R6 stage corn. A fully developed black layer is clearly visible on the right two kernels.

 

Keeping track of Growing Degree Day (GDD) accumulation or often called heat units can also provide a relative estimation of corn maturity.  GDD’s for corn are calculated as:  GDD = [(Min Temp+ Max Temp)/2] -50.  Most corn hybrids grown in Arkansas are full season hybrids and have a relative maturity of 110-120 days and require approximately 2700-2950 GDD’s from planting to reach maturity.  With accurate and local high and low temperature data, keeping track of accumulated GDD’s can accurately track corn maturity.  In corn that I have this year, silking dates were nearly identical to published GDD’s required for silking.

In past years maturity was also very well predicted by accumulated GDD’s.  Current GDD accumulation across Arkansas for late March planted corn is running 2130-2300 GDD’s.  For an example, corn planted on March 28th in Dumas, AR has reached approximately 2300 GDD’s.  If the hybrid required 2800 GDD’s to maturity, that corn would be mature in approximately 16 days with average temperatures that allow for 30 GDD accumulation per day.  Keeping track of heat units can help provide an estimation of when maturity will occur.  For a weekly summary of accumulated heat units for selected areas of Arkansas see the weekly corn and grain sorghum verification report at: http://www.uaex.edu/farm-ranch/crops-commercial-horticulture/corn/#

There are also several computer based programs and phone/ipad apps that will estimate accumulated GDD’s and growth stage based on your planting date and location.   These programs are quick and easy to use and are another useful tool to estimate corn maturity.

Contact Information: 

Please contact me by email at jkelley@uaex.edu, if you have questions or comments regarding this newsletter.  This article and other timely articles on Arkansas crops can be found at: www.arkansascrops.com


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