Corn reproductive stages generally comprise about 60 days for full-season corn hybrids grown in Mississippi at normal planting dates. Early maturity hybrids (~110 day RM) will progress through reproductive stages slightly quicker, than our 118-120 day hybrids, but the difference will only be about 5 days. Therefore, this 60 day timeline is generally practical and accurate.
There are six commonly recognized corn growth stages noted with an R for reproductive growth, followed by a numeral designating subsequent stages.
R6: Physiological Maturity
My preference is to focus on reproductive growth stages in 20 day intervals, because this timeline mirrors some key physiological parameters and identification is more finite.
Silking is defined when silks emerge from the ear to receive pollen and begin the fertilization process. Tasseling immediately precedes this growth stage, but essentially must coincide for successful pollination to occur. Corn is extremely sensitive to any stress which limits photosynthetic capacity at this time. This is because the kernels have very little ability to draw energy reserves from storage, because they are exceptionally small. The pollination process typically occurs in a relatively short (5-8 day) period and silks emerge slightly later than pollen shed initiation. Corn’s sensitivity to stress during the 20 day period from silking to milk stage is very high and affects kernel number.
Milk stage (or commonly referred to as roasting ear) occurs about 20 days after silking as kernels develop dull yellow color and are full of milky white fluid. Stress from pollination through milk stage will readily cause kernels close to the ear tip to abort, as energy is prioritized to the base of the ear. Ear kernel number is generally determined by the milk stage. Later, kernels progress through the dough stage, when kernels gain consistency and size, but remain soft and very moist.
During the second half of the approximately 20 day period comprising R3 and R4, kernels transition through the dough stage, when kernels gain consistency and size, but remain soft and very moist. During the dough stage kernels become more yellow, and many will actually develop a dent in kernels, but remain a dull, matte appearance. Corn’s sensitivity to stress during the milk and dough stages is high and primarily affects kernel weight.
Dent stage occurs about 40 days after silking when nearly all kernels are fully dented and hard starch begins forming at the crown of kernels. If you grow an early-hybrid, this period may occur slightly quicker. Although it seems pretty simple, accurate identification of this stage can be confusing, which could create substantial issues with irrigation scheduling and termination, which can lead to yield loss. The best way to accurately identify dent stage is when the kernel crown turns the bright, shiny, dark yellow color of mature kernels, indicating the onset of hard starch development. Many kernels may begin denting during dough stage or R4, but full dent does not occur until hard starch forms at the top or crown of the kernel. Kernels mature from the outside-in (toward the cob) when hard starch begins developing at the crown. Kernels will continue to fill weight and develop for about 20 more days, if stress does not hasten maturity. Thus, this is far too early to terminate irrigation. Early irrigation termination might reduce yield as much as 15-20% if hot, dry weather prevails. Early irrigation termination may also reduce stalk strength and promote lodging, because plants will cannibalize energy from vegetative organs to fill kernels. Corn’s sensitivity to stress during the 20 day period from dent to physiological maturity is moderate and affects kernel weight.
Physiological maturity is when kernel development is finished and “the crop is made.” It normally occurs about 20 days after dent stage. It is often identified by a “black layer” which develops when hard starch progression to the base is complete and the kernel is physiologically mature. Grain weight accumulation is complete at this stage. Vegetation and stalks will quickly die if hot, dry weather persists. Further management practices, including irrigation, herbicide application, etc… have no effect on the plants.