When we enter the dog days of summer, water management is normally a high priority for corn producers with irrigation capability. Commonly asked during this time is “When can I cut off the water?” Improper termination will either cut yield potential or increase expenses, so correctly scheduling irrigation is essential for profitable corn production.

Irrigation should be continued long enough to meet corn moisture demand until physiological maturity is reached. Physiological maturity is signified by the formation of a dark-colored abscission layer, referred to as the black layer, at the base of kernels.

The black layer may be found by shelling kernels from an ear and gently scraping away the seed coat to expose the abscission layer. The black layer formation occurs progressively from kernels at the tip of the ear to the base.

The black layer forms when hard starch accumulation completes its progression from the top to bottom of a kernel. This abscission layer stops water and dry matter transfer into the kernel, meaning seed weight accumulation is complete.

Kernels will usually have a moisture content of 28 to 35 percent at this stage.

Mississippi-grown corn planted during the suggested planting dates normally reaches physiological maturity the last week of July or in early August.

Physiological maturity usually occurs about 20 days after dent stage or 60 days after silking. Hard starch begins forming at the kernel tip when denting occurs. This hard starch layer gradually progresses to the kernel base over the next 20 days.

The best method for growers to monitor kernel maturation is by observing progression of the milk line, rather than looking for the black layer. The milk line is the borderline between the bright, clear yellow color of the hard seed coat outside the hard starch layer, compared to the milky, dull yellow color of the soft seed coat adjacent the dough layer.

To observe the milk line, break a corn cob in half and observe the cross-section of the top half of the ear (which is the bottom side of the kernels — the side opposite the embryo).

Growers can monitor maturity by observing milk line progression, compared to the complete time interval between dent stage and physiological maturity, which is 20 days.

If the milk line is halfway down the kernels, it will take about another 10 days to reach physiological maturity (20 — (20 * 1/2) = 10). Thus, the field needs to be irrigated enough to supply moisture for 10 more days.

If the milk line is three-fourths of the way down the kernel, the corn is only about five days from physiological maturity (20 — (20 * 3/4) = 5). In this case, further irrigation may not be required if the soil is somewhat moist.

One common irrigation error is terminating irrigation before physiological maturity (black layer) occurs. Early irrigation termination will accelerate maturity, prohibiting kernels from reaching their full potential size and weight.

Although corn water use will drop to about 1 inch per week at the dent stage and steadily decline until physiological maturity, this is not the time to terminate irrigation. Potential kernel weight is only about 75 percent complete at the dent stage. Thus, termination of irrigation at the dent stage can reduce grain yields as much as 15 to 20 percent.

Erick Larson is a grain crops specialist with Mississippi State University. e-mail: elarson@pss.msstate.edu