The Figure below illustrates the complete life cycle of Corn from germination through to maturity and harvest. The growth stages are explained in detail below.
Germination and emergence
(stages VE to V2 in Figure 15)
When Corn seed is sown in soil with a temperature above 21°C and adequate moisture, it rapidly absorbs water and emerges within 2 or 3 days. If the soil temperature is low (less than 18°C), germination slows and radicle emergence may take as long as six to eight days. In addition, radicle emergence is slow if the depth of sowing is deeper than 8 cm. On the other hand, under rainfed conditions when the seed is sown in dry soil awaiting rain, high soil temperature and inadequate moisture can cause the seed to die. Nutrient reserves in the seed feed the emerging seedling for the first week until the primary roots develop and begin to supply the plant with water and nutrients from the soil. The stem’s first internode grows rapidly until eventually the seedling emerges, usually 4 or 5 days after sowing, provided there is enough moisture in the soil and temperature is
optimal.
Early vegetative development
(stages V3 to V10 in Figure 15)
The adventitious root system develops from the first stem node below the soil surface and takes over the main root function approximately 10 days after emergence (stages V3 to V4 in Figure 15). All the leaves the plant will ever produce are formed by a single growing point below the ground during the first 2 to 3 weeks. As the growing point is below the ground, young Corn plants are susceptible to damage from water logging, especially when combined with high temperatures. However, if later conditions are favourable, the plant can recover well
from damage during this stage.
Three weeks after emergence the growing point is at the soil surface and, having formed all the leaves, develops an embryonic tassel (stage V5). At this stage, leaf formation is at its fastest stage of production and at 4 weeks eight leaves are fully emerged (stage V8).
Late vegetative development
(stages V11 to V16 in Figure 15)
This is one of the most critical stages in the development of the Corn plant. The plant grows and the stem elongates rapidly, with a high demand for water and nutrients nitrogen (N), phosphorus (P) and potassium (K). Leaf enlargement is complete by 5 weeks (V12) and the roots quickly fill most of the root zone.
Ears begin to form within the plant soon after tassel initiation (V5); however, over a 2-week period in weeks 5 to 7 (V11 to V16), the highest one or two ears start rapidly developing and ear size is determined. The number of rows per ear is determined first, then kernels per row. At about 7 weeks the tassel reaches full size (V16).
Any adverse effect suffered at this stage, such as nutrient or water shortage, insect damage, or too high a plant population, will significantly affect yield. Furthermore, damage to pollen or ear structures in this period will be permanent, with little chance of compensation later.
Flowering
(stage R1 in Figure 15)
At this stage plants will have finished producing all 20 leaves. Tassels fully emerge (R1) and pollen sheds 40 to 50 days after emergence, with the length of time depending on variety and environmental conditions. Silks emerge from the uppermost ear and sometimes from the second ear. Pollination and fertilization of the ears occurs. During this period there is a high demand for water, and the uptake of N and P is rapid, although K uptake is almost complete.
As pollen supply is abundant, poor seed set is usually due to nutrient or water deficits that either delay silking or result in kernel abortion after pollination. If Corn is flowering during hot, dry weather this places extra stress on the plant’s resources and the silks may wither and burn off before the pollen reaches the ear. Hence fertilization does not occur for all kernels and seed set is greatly reduced. This is commonly referred to as pollen blasting.
The corn cob begins as just a little bunch of leaves with silk coming out of the top. The cob and the kernels will not grow and mature until the kernels are pollinated.
Cob and kernel development
Cobs, husks and shanks are fully developed by day 7 after silking. The plant is now using significant energy and nutrients to produce kernels on an ear. Initially the kernels are like small blisters containing a clear fluid; this is referred to as the kernel blister stage. As the kernels continue to fill, the fluid becomes thicker and whiter in color. This is called the ‘milk stage’. Next is the ‘kernel dough stage’, at which point the fluid within the kernels becomes thicker as starch accumulates. During these kernel filling stages N and P uptake continues at a rapid rate. As the number of ears and kernels has already been determined, it is the kernel size that is affected by conditions during this stage. A low kernel weight will reduce yield. Denting of the grain occurs around 20 days after silking; this is an indicator that the embryos are fully developed. Initially at denting a line can be seen which slowly moves to the tip of the kernel through until physiological maturity. This line is called the ‘milk line’ and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels.
Maturity
Approximately 30 days after silking the plant has reached the maximum dry weight, a stage called physiological maturity. This is where a ‘black layer’ (a dark mark) is noticeable at the tip of each kernel, where cells die and block further starch accumulation into the kernel. At this stage the milk line has completely disappeared. Kernel moisture at physiological maturity is around 30%. The grain and husks begin losing moisture while healthy stalks remain green. Eventually the leaves will dry off. Harvesting can commence when grain
moisture is below 20% . The grain is dried down to 14% for delivery to storage or market.
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