About a week after the corn kernels on the ear have dented, a distinct horizontal line appears near the dent end of a split kernel and slowly progresses to the tip end of the kernel over the next 3 weeks or so. This line is called the “milk line”and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels. Severe stress as a result of drought, disease, and or frost during this phase can severely limit grain yield, test weight, and stalk health. Severe stress after physiological maturity has little effect on grain yield, unless the integrity of the stalk or ear has been compromised.
Premature death of only the leaves prior to physiological maturity results in yield losses because the photosynthetic “factory” output is greatly reduced. The plant may remobilize stored carbohydrates from the leaves or stalk tissue to the developing ears, but yield potential will still be lost. Approximate yield losses due to premature death of leaves, but not stalks, range from 36, 31, and 7% when the leaf death occurs at R4 (dough), R5 (full dent), and half-milk line stages of kernel development, respectively.
The premature death of the whole plant results in greater yield losses than if only leaves are killed because the death of the entire plant prevents any further remobilization of stored carbohydrates to the developing ear. Whole plant death that occurs before normal black layer formation will cause prematureblack layer development, resulting in incomplete grain fill and lightweight, chaffy grain. Grain moisture at that point will be greater than 35%, and field drydown will be significantly slower than if it were a healthy plant. Approximate yield losses due to premature whole plant death range from 50, 39, and 10% when the whole plant death occurs at R4 (dough), R5 (full dent), and half-milk line stages of kernel development, respectively.
Side note:A common misconception is that kernel black layer formation sometimes fails to occur following a frost or other late-season severe stress. Not true. FAKE NEWS! (Sorry, could not resist.) The kernel black layer always develops. Any severe stress that occurs during the grain fill period will cause premature kernel black layer formation and is related to the reduction in or termination of sucrose (photosynthate) availability to the developing kernels.
This chart is a quick and dirty way to “ball park” the relationship between early plant death and yield loss.