Corn kernel development begins with silking (R1) and is marked by the blister stage (R2), milk (R3), dough (R4), and dent (R5) stages. The final stage called black layer formation (R6) marks the end of kernel development. The corn kernel accumulates weight in a sigmoidal pattern over a 55-60 day period beginning with a 7-10 day "lag" phase and ending with a 7-10 "maturation" phase (Figure 1). The linear phase of the sigmoidal curve lasts about 40 days.
|Figure 1. Kernel weight accumulation pattern of corn.|
For a 200 bushel per acre yield level about 5 bushels per day (200 / 40) accumulates during the linear phase of kernel development. About 60% of the starch that accumulates within the kernel is produced by the ear leaf. Leaves above and below the ear are also important sources for developing kernels, but as the distance from the ear increases less starch is translocated to kernels and more to other plant parts. The stalk serves as a temporary storage organ during the day and photosynthate will be translocated to the kernels throughout the night.
Photosynthesis is maximized at about 1/3 of full sunlight, so even cloudy days can produce the starch needed to sustain accumulation in the kernel. Other plant parts (leaves, stalk and roots) demand photosynthate for respiration and are competitors with kernels. Temperatures that are comfortable for us (65-80 degrees F during the day and 50-65 degrees at night) provide the best trade-off between maximizing photosynthesis production and minimizing respiration in corn.
About 0.25 to 0.30 inches of water is being transpired by the plant during August. Every day that corn plants are stressed can lower yields 5% per stress day. Nutrients (N-P-K) are still being taken up by the plant until about the R3 to R4 stages. Brace roots are acting as a nutrient scavenger system in the upper layers of the soil profile, while roots deeper in the profile are used primarily for water uptake. During August it is important to protect the ear leaf since that is the plant part where most of the photsynthate is produced for a developing kernel.