Time to Remobilize

As corn transitions from vegetative to reproductive stages, nitrogen within the plant also changes direction.   

During vegetation the plant’s duty was to build out the photosynthetic collection system – deploy leaves, filled with chlorophyll, to capture solar energy and continue to build. 

But now in the reproductive phase, energy is directed to maintaining and filling the pollinated kernels on the ear(s).   

Yield has three components – Plants per acre, kernels per plant, and mass per kernel.  The first is already set at emergence; the next is set at R2, once pollination is completed, but the final step of kernel mass is made during the last days of reproduction.  Generally, corn grain will contain 0.53 to 0.65 pounds of nitrogen per bushel, or approximately 1.1 to 1.3% nitrogen concentration on a dry matter basis. 

This nitrogen must come from somewhere.  While the roots and shoot still functions to capture and transport nitrogen through the plant, and later season mineralization of soil organic matter can continue to contribute, oftentimes the nitrogen to fill these kernels comes from the plant itself through remobilization. 

The Remobilization Drive 

During remobilization, lower leaves are often the first to be asked to give up their nitrogen for the greater good; this is allowable, as these lower leaves contribute the least to the overall photosynthetic capture for the plant.  Proteins in these leaves are broken down into amino acids and translocated to the ear.  As the plant moves further along in the reproductive stages, this will become more common. 

In extreme cases where nitrogen is not available, remobilization will move up the plant’s leaves, eventually reaching the ear leaf and potentially above.  This will start to create negative feedback for grain fill; reduced energy collection from lower photosynthesis activity causes lower nitrogen uptake from the soil, thereby creating a further demand for local nitrogen storage near the ear, and so on. 

Towards the end of the reproductive phase, if sufficient nitrogen is still not available from the leaves, it may be removed from the stalk as well.  This can create stalk lodging under high wind events and makes the harvest very difficult. 

Premature Senescence of Nitrogen in Lower Canopy at R3

Sufficient Nitrogen in Lower Canopy at R3

What You Can Do 

As you scout your fields during reproductive stages, look at the lower canopy to see if early senescence is occurring.  Leaves will turn brown from the inside out (iNside out, get it?) as nitrogen is pulled out.  At earlier reproductive stages (R2 and R3) this is concerning, but more likely to occur in R4 and R5. 

By observing any early senescence in the lower canopy, you can use this information for future nitrogen programing guidance.  In the above photos, the image with early senescence was in a no-till field with a heavy stand of pennycrest winter annual at emergence.  The image on the right with sufficient nitrogen was in a shallow tilled field without weeds at emergence that also benefited from the tillage’s quick mineralization burst of nitrogen. 

Premature senescence may also be due to poor drainage (leading to denitrification) or sandier soils (potential for leaching).  As we know, water and nitrogen often play together. 

Ensuring sufficient nitrogen gets into the grain by physiological maturity is key to producing the best yields.  As your corn plants reach that level, it will naturally pull from its own reserves through remobilization.  Having sufficient nitrogen within the plant (and some from the soil) helps the plant put the final effort into filling out its kernels to obtain the highest seed mass and thereby tip the scales towards best yields.