Dying to Live: How Corn Puts All It Can into Filling Its Kernels

 As corn marches towards maturity, an inevitable situation occurs: the plant essentially pulls nutrients from its reserves in leaves and stalk to remobilize into its grain, causing its own demise so that its offspring can live on. 

This may sound a little morbid, but this is part of the cycle of life.  Annuals like corn only get one year to exist and develop seed that will grow in future seasons. 

Let’s examine further what goes on with corn at the last stage of its development, in R5 or Dent stage. 

What Happens? 

This stage is indicated when a small indentation appears on kernels.  This is due to hard starches replacing soft starches in the crown of the kernel.  In some cases, especially under high management programs with more-than-sufficient fertility, the dent is not as visible.  Nonetheless, moisture is moving out of the kernels.  Initially, moisture content is around 55-60% and moves down from there.  R5 is the longest stage in the Reproduction portion of the corn development process, taking approximately 330-360 growing degree units (GDUs) to accomplish. 

During R5, a “milk line” becomes visible in the kernels.  The hard starch that creates the dent moves down the kernel towards the cob, replacing the soft starch.  The cob acts as a carrier of excess water out of the kernel. 

Staging during this time is based on the percentage the milk line appears to be.  R5.25 indicates the milk line is about 25% of the kernel’s length from the top of the kernel.  R5.5 means the milk line is halfway, and R5.75 describes the milk line as about three-quarters from the top. 

Starch makes up about 70-75% of the composition of the completed kernel.  Proteins, containing nitrogen, make up 8-10% of the overall mass of the kernel.  Much of this nitrogen comes from remobilization within the plant.  Phosphorus, potassium, and other macro and micronutrients are also being transported into the kernel during this time. 

R5 Dent stage is an important time in the plant’s life cycle: At the beginning of R5, 45% of the grain’s final mass exists in the ears.  Another 55% is to be put into grain.  The whole-plant biomass doesn’t change much during R5, but a large shift of biomass is made from leaves and stalks to grain at R5.  This growth stage is primarily a time of remobilization of existing nutrients and biomass, not additional uptake. 

Environmental Considerations 

Knowing that R5 is a time of transition for the plant, not so much of active growth and development but one of remobilization and movement within the plant, we may think that the plant is on auto-pilot and doesn’t need attention for diseases or other pathogens. 

However, energy is still needed to facilitate the movement of nutrients, and anything, like gray leaf spot to southern rust, that reduces the effectiveness of the leaves “solar collection system” will affect the final yield.  While kernel count and plant count have already been determined by R5, seed mass is still to be determined, and late-season diseases can affect the remobilization and fill of the kernels by lowering energy needed for nutrient transport. 

If diseases are confined to lower leaves, little to no yield loss will be experienced.  Upper canopy infestation will cause the most harm to the overall grain fill during R5, which can last up to 30 to 35 days. 

On the other hand, cooler temperatures during the last grain filling growth stage can help add more seed mass to the kernels.  This is due to the duration of R5 being extended.  360 GDUs for R5 can be as short as 12 days at 30 GDUs per day or as long as 40 days at 9 GDUs per day.  Both are extreme, but the longer it takes to complete maturity, the more opportunity there is to remobilize nutrients into the kernels.  Hot days towards the end mean quick maturity and less opportunity for remobilization. 

In the end, corn is doing what it has done over previous centuries; creating the best offspring it can for future life.  During its final days, corn will proceed to remobilize nutrients from itself and give all it can to its offspring, causing its own demise so that the next generation can live on.