When can I Safely Finish Irrigating my Corn?

Much of our March-planted corn crop has now dented and kernels are filling size and weight.  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,” this process is more complicated 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.

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: (20d – (20d x 25% milk line)) = 15 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 are definitely a tremendous help 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 at this time, if soils or compaction don’t limit water infiltration or root growth.
2. The daily rate of soil moisture loss moderates considerably after dent stage as the corn approaches maturity, compared to early reproductive stages.

Soil sensor chart showing a substantial decline in daily moisture decline as corn matures.

The new moisture sensor technology may allow you to monitor daily levels and better estimate whether you will have enough moisture when the crop reaches maturity. For instance, our sensor results are showing a fully charged soil profile may provide ample moisture for an irrigation cycle up to 15-18 days after dent stage, compared to only 9-12 days during peak water use. The other practical consideration we should employ is that corn’s sensitivity to drought stress decreases substantially near maturity, so we can justify using a considerably drier irrigation threshold at this time.

What are the consequences of improper irrigation termination on corn? I’m sure you have probably pulled ears prior to maturity and left them in the truck for a day or two. What do you notice when you go to toss them out? The kernels which were originally packed tight now wiggle somewhat when you grab the ear. This shrinking sensation illustrates how important the final days of grain filling are. Corn kernels develop about 25% of their potential size and weight after dent stage. Although, depletion of soil moisture won’t cut off grain fill as abruptly as pulling an ear, it wouldn’t be unreasonable to sacrifice 10-15% grain yield, depending on the duration and degree of water stress. Conversely, it is unprofitable to irrigate when it is not necessary, or too late for the crop to respond, when there is no benefit of having abundant soil moisture at physiological maturity or thereafter.