By Jason Kelley, Extension Wheat and Feed Grains Agronomist
Many early planted corn fields are nearing or are at irrigation termination in South Arkansas. The goal of irrigation termination timing is to have adequate soil moisture to reach the R6 stage (black layer) without irrigating more than needed. This has been an exceptional year for many corn producers as most fields have been irrigated only 2-3 times. Many producers are irrigating now for the last time in South Arkansas, but many later planted fields and fields further north still may need an additional irrigation or two to reach maturity. Temperatures this week are forecasted to be the warmest of the season with little rain, so we don’t want to cut the irrigation off too quickly.
The easiest way to determine how close to corn is to irrigation termination is to look at the starch line development. The starch line begins at the top of the kernel (signified by a dent) and slowly progresses down the kernel over a 21-24 day period. 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. Figure 1. below illustrates a kernel with a starch line that has progressed 75% down the kernel and with good moisture, irrigation could be terminated on this field (furrow or pivot). If in doubt that the plant will have adequate moisture to reach maturity, it is advisable to irrigate once more, especially with the hot and dry conditions that are forecasted.
Once kernels have reached maturity, a black layer should be visible at the bottom of the kernel (Figure 2.) by scraping the tip off of 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 30-35%.
Measuring Growing Degree Days (GDDs) is another way to estimate corn maturity but should be considered only an estimate. Most corn hybrids grown in Arkansas require 2750-3000 GDD’s to reach maturity. A quick summary of accumulated GDDs for South Arkansas this year indicates that March 1st planted corn is nearing or at maturity, while corn planted later and/or further north still needs some time to reach maturity.