Early irrigation termination on corn is a common mistake and it can hurt your yield depending on when you cut the water off. Although kernels outwardly appear mature and corn water use begins declining at the dent stage, this is far too early 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 15-20% when hot, dry conditions persist. Early irrigation termination will also likely reduce stalk strength and promote lodging, because plants will cannibalize energy from vegetative organs to fill kernels when they are stressed. So water corn until black layer…
I have been questions about checking milk line in corn. Corn kernels mature from the outside-in when hard starch forms beginning at the top of the kernel at dent and steadily progresses towards the base of the kernel (where it attaches to the cob). This final stage of grain development normally takes about 20 days to complete.
The most reliable method for you to monitor kernel maturity for irrigation scheduling purposes is to observe this progression of the milk-line (or hard starch layer) between dent stage and black-layer or physiological maturity.
The milk-line is more relevant than the black-layer, because it indicates maturation progress, before the black layer is evident. The milk-line is the borderline between the bright, clear yellow color of the hard seed coat outside the hard starch, compared to the milky, dull yellow color of the soft seed coat adjacent the dough layer.
To observe the milk line, break a corn ear in half and observe the cross-section of the top half of the ear (the side of kernels opposite the embryo). If you have difficulty seeing this color disparity between layers, you can find it by pressing your fingernail into the soft, doughy seed, starting at the kernel base and repeating this procedure progressively toward the tip, until you feel the hard starch.