The Two Main Phases of Corn Growth
To understand at what stage does corn stop growing, it's essential to first grasp the two main developmental phases: vegetative (V) and reproductive (R). The vegetative phase focuses on the growth of the stalk, leaves, and root system, culminating in the tasseling stage (VT). The reproductive phase begins with silking and is dedicated to kernel development and grain filling, ending with physiological maturity.
Vegetative Stages (VE to VT)
These stages are defined by the number of visible leaf collars on the plant, from emergence (VE) to tasseling (VT). During this period, the plant establishes its root system and accumulates biomass, but its height and kernel row number are already being determined. Critical management decisions regarding fertilization and pest control are often made during this time to set the plant up for a successful reproductive phase.
Reproductive Stages (R1 to R6)
The shift to the reproductive phase is marked by the emergence of silks (R1), which must be pollinated to develop kernels. This phase is a critical period for yield determination, as the plant directs its energy toward grain fill. A healthy leaf canopy is crucial for producing the carbohydrates needed to fill the kernels.
The Critical Stage: Physiological Maturity (R6)
The final reproductive stage, R6, is physiological maturity, the definitive point where corn stops growing. This stage signifies that the kernels have reached their maximum dry weight and no further dry matter accumulation will occur. A key visual indicator of this stage is the formation of the "black layer." This black layer is a dense, compressed layer of cells that forms at the base of the kernel where it attaches to the cob, sealing it off from the plant's vascular system.
Once the black layer forms, the kernel is essentially cut off from the plant's nutrient and moisture supply. Any subsequent moisture loss from the kernel is simply through evaporation, a process known as dry-down. Environmental conditions after this point have a much lesser impact on overall grain yield, though they can affect harvest moisture and stalk integrity.
Factors Influencing the End of Growth
While genetics primarily determine the timeline for reaching physiological maturity, external factors can play a significant role. Severe stress, such as a frost or disease, can trigger the formation of the black layer prematurely, halting grain fill and reducing final yield. Conversely, good plant health can prolong the grain-filling period, potentially increasing kernel weight.
Here are some factors that can influence the end of growth:
- Hybrid Selection: Different corn hybrids have different maturity ratings and will reach physiological maturity at different times based on their genetic makeup.
- Growing Degree Days (GDD): This is a measure of heat accumulation that helps predict the timing of different growth stages. The total GDD required to reach maturity can vary based on planting date and hybrid type.
- Stress Factors: Drought, heat stress, disease, or insect damage can all induce an earlier onset of physiological maturity and black layer formation.
Reproductive Stages: A Comparison
| Stage | Timeframe After Silking | Key Characteristics | Impact of Stress |
|---|---|---|---|
| R1 (Silking) | Starts when silks emerge | Pollination occurs; silks capture pollen | High impact; drought reduces pollination and kernel set. |
| R2 (Blister) | ~10-12 days | Kernels are white, watery, and blister-shaped | High impact; stress can cause kernel abortion. |
| R3 (Milk) | ~18-20 days | Kernels turn yellow, filled with milky fluid | Moderate impact; can reduce kernel size and weight. |
| R4 (Dough) | ~24-26 days | Kernel fluid thickens to a dough-like consistency | Moderate impact; can produce shallow kernels. |
| R5 (Dent) | ~31-33 days | Kernels develop a dent as starch hardens | Lower impact; mostly affects kernel weight. |
| R6 (Physiological Maturity) | ~55-65 days | Black layer forms, maximum dry weight reached | Low impact; yield is set, though stalk integrity can be affected. |
For more detailed information on corn grain fill stages, consult resources from Purdue Agronomy Grain Fill Stages in Corn.
Conclusion
The answer to "At what stage does corn stop growing?" is definitively at the R6 stage, known as physiological maturity. This final developmental milestone is marked by the formation of the black layer at the base of the kernels, signaling the completion of dry matter accumulation. While factors like genetics, growing degree days, and environmental stress can influence the precise timing, the black layer is the definitive visual cue for a completed growth cycle. Understanding this process is vital for farmers and gardeners to make informed decisions about crop management and harvest timing, ensuring maximum yield and quality.
