Using the Kernel Milk Line to Visually Monitor Grain Maturity in Maize1

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Abstract
Maize (Zea mays L.) growers use a variety of methods to estimate the maturity status of the grain. The objective of this study was to evaluate the usefulness of the kernel milk line as a visual means of monitoring grain maturity in maize. Five hybrids were field grown at St. Paul, Minn. in 1980 and 1981. Movement of the kernel milk line was followed in these hybrids and compared to moisture loss, dry weight accumulation, and black layer development in the kernels. Four of the hybrids were also defoliated at various stages of kernel development so that kernel milk‐line movement could be characterized under conditions of prematurity stress. The milk line was always a readily‐identifiable feature of maturing kernels. Under normal conditions, disappearance of kernel milk was coincident with blacklayer development and the cessation of kernel dry weight increase (physiological maturity). However, it was easier to determine the day on which milk was no longer present in kernels than to determine when the placental region was black. Movement of the milk line was useful in “timing” the maturity process. At the half‐milk stage, (milk line positioned half‐way between the tip and the base of the kernel) kernels had achieved about 90% of their final dry weight, contained approximately 40% moisture, and were 2 to 3 weeks from the optimal date to begin harvest. When plants were completely defoliated at the half‐milk stage, the kernels accumulated 95% of normal dry weight. We recommend using both development of the black layer and milk disappearance as indicators of maturity in maize. Development of the black layer was the more reliable indicator of physiological maturity. The milk line was more useful in monitoring grain maturation prior to physiological maturity.