Many Arkansas corn growers are anxious to get to irrigation termination as it has been a long hot and dry summer for many. This year in particular many producers who are watering from surface water or reservoirs are getting increasingly nervous at the low water levels and are some are being forced to decide which crop to water (corn, soybean, or rice) and in some instances there is not enough water to water all crops at the full levels that are needed.
Fortunately much of our corn is nearing irrigation termination and some early planted fields, especially in south Arkansas have likely have been watered for the last time. However cutting off irrigation too soon can have consequences as kernels are still adding weight until full maturity. For those who are contemplating terminating irrigation sooner than normal, keep in mind that even at R5.5 (starch line 50% down kernel) the kernel has only accumulated 90% of its final weight, so there is still yield to be preserved by maintaining adequate water to the plant until maturity. The goal should be to maintain soil moisture until maturity.
The easiest way to determine how close to corn is to irrigation termination is to look at the starch line development on kernels from the middle of the ear as there will be some differences from the top to the bottom of the ear. 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. Keep in mind that there may be differences in maturity from the top to the bottom of the field this year due to the uneven early growth that was seen in many fields.
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%.