Jefferson County corn growers are anxious to get to irrigation termination as it has been a long hot and dry summer for many. Growers are starting to ask when can they stop watering corn fields, according to a news release.

“Fortunately, much of our corn is nearing irrigation termination in some early planted fields,” Kurt Beaty, Jefferson County Extension Agent-Agriculture, said. “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, they should keep in mind that even at R5.5 (starch line 50 percent down kernel) the kernel has only accumulated 90 percent 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, Beaty said.

“The easiest way to determine how close 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,” he said. “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 percent or more and you have good soil moisture, irrigation on furrow irrigated fields could be terminated,” Beaty said.

At the 50 percent starch line movement and good soil moisture, the plant should have enough moisture to reach maturity. For pivot irrigated fields, the starch line needs to be 75 percent or more down the kernel before irrigation is terminated.

“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,” he said. “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 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 percent,” Beaty said.

For details on corn irrigation termination, contact the Jefferson County Cooperative Extension Office at 870-534-1033 or email

The local Extension service is part of the University of Arkansas System Division of Agriculture which offers all its Extension and Research programs and services without discrimination.