Seed viability vs seed vigor

​Crop agriculture is dominated by multiple environmental and biological factors. Everyone participating with corn seed attempts to define and control these interactions.  Breeding procedures can mostly assure that a hybrid is genetically uniform, production methods are intended to maintain this purity and testing methods can evaluate for level of genetic purity.
 
Seed viability is affected by environments during seed production in the field and after harvest.  Each individual seed within a seed lot has a distinct experience with this process, ultimately affecting the ability to germinate with viable shoot and root tissue.  Individual seed may have some damage to membranes within cells that require metabolic repair before being able to elongate the root (radicle) part and shoot part of the seed embryo after imbibition.  This may be affected by genetics, often of the female seed plant and perhaps of the mitochondria in the female parent.
 
Germination tests to identify seed viability, usually defined as a seeds ability to produce a shoot and root when placed in a controlled environment can be done with reasonable repeatability.  Results are determined after a specific time with specific definition of a root and shoot. Defining and characterizing differences among the seed’s vigor, or the time it takes for that individual seed to produce a root and shoot is more difficult.  The seed analyst may see differences in vigor among germinating seed but communicating these differences becomes a major problem.  
 
How to characterize a seed lot that has a high percentage of seed that meet the definition of viability but does not germinate uniformly in test conditions?  Generally, those seed lots with delayed germination in warm conditions have lower germination percentages when tested under cold conditions (50°F) but there are exceptions to that as well.  
 
The ultimate goal is to reduce the possibility that seed viability and vigor affect hybrid performance in the grower’s fields where environments present their own variables.  It is understood that late emerging seedlings, regardless of cause, have difficulty in competing with adjacent corn plants.  They often remain less vigorous because competitors reduce light on leaves and outcompete the late-emerging plant’s roots for minerals and water.  Often late emerging plants produce ear shoots later than most adjacent plants resulting in poorly pollinated ears. Genetics of hybrids probably differ in ability for late emerging plants to remain nearly fully pollinated and thus the detriments of lack of uniformity is not exactly the same for all hybrids. 
 
Everyone in corn agriculture wants maximum performance from the seed.  We attempt to remove known variables by measuring viability and vigor and by preparing planting conditions.  There remain uncontrollable environments and difficulties in defining and communicating seed vigor.  Late emerging corn plants detract from maximum yield potential of a hybrid, but how late is the emergence and how much is the yield reduction?  Like most of life’s experiences, we wish for clear definition but often the variables make that difficult. 
 
Politicians can sum up ambiguity in a simple phrase.  The rest of mere humans must only attempt to evaluate and communicate what we think is happening within a corn seed lot sample.