How to Safely Terminate Irrigation in Corn

Much of our March-planted corn crop has now dented and kernels are filling size and weight. However, heat unit accumulation has lagged this year, meaning crop development is likely to be about five days later than normal, and we have a lot more later plantings as well. Thus, crop management is still critical, since brutally hot temperatures will certainly stress your crop, especially when adequate moisture is not available to finish the crop out.  Although you have likely heard it said, “corn should be irrigated until physiological maturity or black layer,” the process can be a little more complex than that. This article will help you make difficult irrigation decisions as your corn approaches physiological maturity.

As the corn crop nears maturity, knowing when you can safely terminate irrigation timing, while optimizing kernel development and yield potential is likely the most important management decision you will face. In order to make this decision, you need to be able to estimate when the crop will reach maturity, relative to how much moisture is present in your soil profile. Both crop maturity and soil moisture reserves must be assessed to make a prudent decision. Improper timing will either limit yield potential or unnecessarily waste money and labor. Fortunately, we can outline the steps needed to help make this process accurate and reliable.

Corn kernels continue to gain size and weight until physiological maturity, so you need to make sure to not terminate irrigation or other types of management early when stress will limit grain fill.  Luckily, as kernels progress through the latter stages approaching maturity, there is a method to definitively monitor kernel development.

The milk-line has progressed 50% of the way through this ear.

Kernels mature from the outside-in when hard starch begins forming at the dent stage. The kernel crown will become hard and attain the bright, shiny, golden yellow color of mature kernels. This starch accumulation will steadily progress towards the base of the kernel (where it attaches to the cob) taking about 20 to 24 days to complete. This progression of maturity can be monitored by movement of the milk-line or hard starch. The milk-line represents the borderline between where hard starch has occurred and where the kernel has a soft, doughy consistency and development is still occurring. You can find the milk-line by observing the transition in kernel color from a bright yellow color, to the light buttery tint on the bottom side of the kernel.

To monitor the milk line, break an ear in half and observe the cross-section of the top half of the ear, so you are looking at the side of kernels opposite the embryo. If you have difficulty seeing this color disparity between layers, you can confirm its location by simply poking the seed coat with your fingernail into the soft, doughy layer near the kernel base and repeating progressively toward the crown of the kernel, until you feel the hard starch.

You can use the milk-line development to estimate when the crop will reach physiological maturity. The milk-line progression through the entire kernels lasts about 20 to 24 days. Alternatively, each quarter of the kernel fills starch over 5-6 days. Therefore, if your milk-line has progressed one-quarter of the way through the kernel, you have about 15 to 18 more days to maturity. The formula used to calculate this example is: (24d – (24d x 25% milk line)) = 18 days to maturity.

Once you estimate how long it will take the crop to reach physiological maturity, you must also evaluate your soil moisture reserves to see if you have enough to carry the crop to maturity. You can evaluate moisture using a shovel, probe, auger or other basic methods, but soil moisture sensors serve extremely well for this purpose. We are confirming or learning several things from soil moisture sensors relevant to this objective through our efforts in the Corn Verification Program:

1. Mississippi grown corn is fully capable of drawing moisture from at least 36-inches deep during late reproductive stages, if soils or compaction don’t limit water infiltration or root growth.
2. The daily rate of soil moisture loss diminishes considerably after dent stage as the corn approaches maturity, compared to early reproductive stages.

Soil sensor chart confirming substantial decline in daily moisture use as corn matures.