Many corn fields are showing signs of maturation. The first sign of maturation in a corn plant is natural death or senescence of the corn leaves. Leaf death is naturally timed with the corn plant reaching black layer or physiological maturity. In a natural order of events, bottom leaves would begin to die first and then progress towards the upper portion of the corn plant as maturity is reached.  However, many corn fields currently are experiencing death of leaves from the top down, or top die back prior to black layer.

 
There is considerable discussion in the industry as to why corn plants experience top die back. The question surrounds if top die back is caused by stress or a new "natural" maturation process for today's hybrids. How and when the corn plant dies can help distinguish the difference. Top leaf death surrounding black layer is probably normal senescence, but the rapid appearance of top leaf death prior to maturation would suggest the corn plant is shutting down prematurely.  
  

Effects of Die Back

 
Premature top die back is often the first symptom that indicates there is or has been severe energy stress on the corn plant during grain fill. The source of the stress can vary significantly and is not limited to but can include drought, excessive heat, leaf diseases, nitrogen deficiency or even high yield potential. The common factor of all of these stresses is the strain they put on the photosynthetic process of the corn plant.
 
Corn plants rely on photosynthesis to produce the energy it needs to live and produce grain. During grain fill there is a tremendous need for energy and anything that can limit the corn plants ability to produce energy via photosynthesis will put a huge stress load onto the corn plant. The common photosynthetic stresses of drought, excessive heat, rapid development/grain fill, leaf diseases and/or high yield potential affect the corn plant differently but all cause the same end stress load on the corn plant.
 
Drought Stress: Water is an essential element in the photosynthetic process and when water is limited so is the corn plants ability to produce sugars.
 
Excessive Heat Stress: Corn plants have a built in "thermostat" that regulates when the corn plant can "run" or when it needs to shut down to protect itself. When temperatures begin to creep above 86° F, the corn plant shuts down to limit the damage the heat can potentially causes the plant. Even when the plant shuts down, there is still the same need for energy to fill the ear. With the plant shutting down and a constant need for energy, the corn plant is forced to tap into its reserves.
 
Rapid Development/Grain Fill Period: The 2011 corn crop started late and then developed rapidly. A majority of corn planted in late May to June went from planting to tasseling in an amazing short time frame of about 60 days. Also the early stages of grain fill were pushed along rapidly due to ideal temperatures. Asking the corn plant to produce grain on a normal time schedule is difficult enough, but asking the corn plant to accomplish the same task in fewer days puts a significant strain on the photosynthetic process. The strain caused the corn plant to rely on the plants reserves to fill in the gaps of photosynthetic production.
 
Leaf Diseases: Leaf diseases do not directly impact the photosynthetic ability of corn plants, but rather the disease lesions reduce the amount of leaf area available to the corn plant for photosynthesis. The reduced area does not allow the corn plant to meet its energy needs and therefore must tap into its stored energy.
 
  
High Yield Potential: Surprisingly high yields do cause a huge stress on the corn plant. The energy requirements of an average yield with average weather conditions can be easily meet by the corn plant. But, when there are high kernel counts or high yield potential, the corn plant is forced to produce more and more energy to fill all those kernels. The added energy requirement means the corn plant cannot have any hiccups in photosynthesis and if there are hiccups then the plant must start mining its reserves to meet the increased energy needs.
 
The main energy reserves of the corn plant are the lower leaves, stalk and roots. When the gas tank is running low from photosynthesis, the corn plant begins to draw from its reserves. As the reserves are continually tapped into and depleted, the plant begins to become more vulnerable to pathogens and/or premature death. Whether top die back is caused by pathogens or premature death, the early death of the corn plant can lead to a reduction in yield potential. The following graph illustrates that the earlier premature death occurs the greater the effect on final yields.
 
Yield Loss Due to Premature Death
Kernel Stage        Yield Loss (%)
Milk                          80-90
Late Milk                  60-70
Early Dent                 45-55
Full Dent                   30-40
Half Milk Line             5-15
 
 
Premature Top die back is occurring throughout much of the state and the reasons vary from zip code to zip code. Different genetics will display top die back for different reasons and different degrees of severity. When checking fields for top die back keep in mind the different stresses the corn has undergone so an accurate comparison of genetic differences can be made. Planting dates and field conditions vary dramatically and can negate the genetic advantages of stress tolerant hybrids.
 
When top die back moves into a corn field there comes a concern for the overall and long term quality of the stalks in that affected corn field. Stay tuned for a more in-depth discussion on stalk quality and stalk rots in the next issue of the Agronomic Authority.