Kernels are arranged in even numbers of rows along the rachis or cob of the ear (Figure 13). Several husk leaves protect the ear from environmental effects such as disease infection.
Grain filling is in three stages:

Maturity occurs in two stages:

Blister stage. After fertilization, silks wilt and turn brown. Carbohydrates and nutrients rapidly accumulate in the developing kernels in the form of clear fluid. About 10 days after flowering, kernels shaped like small blisters appear. This is the blister stage (stage R2). The embryo can be seen at this stage.

Milk stage. The milk stage (stage R3) begins 3 weeks after silking (that is, R1). The kernels are filled with a white, milky fluid. The fluid has a high sugar content and the kernels are most suitable for consumption as fresh maize. Following the milk stage, the sugar content decreases and the starch content increases. Also, water content decreases as dry matter content increases.

Dough stage. The last stage of actual grain-filling may be divided into two phases:

At soft-dough, kernels are filled with a white paste. The embryo occupies half the width of the kernel. At hard-dough, the white paste in the kernel gradually solidifies to starch, starting from the top part of the kernels.

A 'milk line' becomes apparent on the side of the kernel opposite the germ. The milk line separates the mature, starchy area from the milky region near the base of the kernel. The milk line moves toward the base of the kernel as the grain continues to mature.

Noting the position of the milk line is a useful way of monitoring the maturity of the kernels and anticipating time to harvest. When an ear of corn is broken in half, the milk line is seen on the exposed kernels on the tip half of the ear. When the milk line has moved about halfway down the kernel, about 95% of the final yield is attained, and the maize should be ready for harvest in 2-3 weeks. Grain moisture content is about 40%.

Note that grain moisture content is expressed on a wet basis:
% moisture = (weight of water/total grain weight) x 100.

In a dent variety, the kernels start to dent. Vegetative plant tissues senesce in the late grain-filling stages as carbohydrates and nutrients are translocated to the developing kernels.

Dent stage. In dent types, 50-90% of the kernels are fully dented. The milk line is now close to the base of the kernel or may no longer be visible from the outside of the kernel.

Physiological maturity. The embryo reaches full size about 7 weeks after flowering. By the following week, the kernel is at maximum dry weight and the kernel moisture content is about 35%. When transport of assimilates to the kernel ceases, a 'black layer' (abscission layer) forms at the base of the kernel. Kernels in the middle region of the ear are the first to reach the black layer, followed by those at the tip. Kernels at the base mature last.

The ear is considered to be physiologically mature when 75 % of the kernels in the central part of the ear reach the black layer.

Another way to determine maturity of the ear is by looking for milk at the base of the kernel. When liquid can no longer be expressed by pressing the kernel with a knife or fingernail, the ear is at or near physical maturity.

It is advisable to use both the disappearance of the milk line and black layer formation as indicators of physiological maturity. In general, kernels will be free of milk 2 days before the black layer begins to form. However, if maize is stressed, the black layer may form prematurely, indicating the end of dry matter accumulation, and hence physiological maturity.

Kernels at the tip of the ear are smallest. They are pollinated last and mature first, and therefore have less time to accumulate dry matter.

Although grain filling and maturity are usually categorized into discrete stages (R2-R6), these stages are sometimes difficult to identify precisely. Grain dry matter accumulation is in fact a continuous process that can be divided into:

The three phases are shown in Figure 14.

Lag phase. This phase begins immediately after fertilization and continues until the onset of linear dry matter accumulation, about 10 days after fertilization (depending on genotype and temperature).

Linear phase. The linear phase is the period of rapid grain filling during which about 90% of total grain dry matter is accumulated.

Final phase. Dry matter accumulates at a decreasing rate, ending in physiological maturity and black layer formation.

Kernels continue to lose water after physiological maturity. Variety and weather determine rate of drying.

Typically in the lowland tropics, moisture loss from the grain occurs at a rate of 2-3% per week immediately after black layer formation. The rate declines exponentially until an equilibrium moisture content is reached. The equilibrium moisture content ranges from about 14% at the end of the rainy season, to 18% in a humid lowland environment.

Grains must be well dried to maintain viability in storage (no more than 14% moisture).

Mature kernels do not require a dormancy period. They can germinate as soon as growing conditions become favorable.