Gross primary productivity (GPP) is the rate per year of energy
capture in photosynthesis.  Net primary productivity (NPP) is another
rate - GPP minus energy required for life processes (respiration), or
in other words, the energy left in the plant tissue.  Both are
expressed in the units kCal/m^2/year, or grams carbon/m^2/year  I am
looking for the ratio (NPP:GPP) for oats, either an accepted value
(maybe for a specific region), or a study on a specific farm or test
site.  There are many factors that can change the ratio (moisture,
temperature, sunny days) but any generalization or case study would be
fine.  Also, if you can't find it for oats, any agricultural grain
would suffice.  The answer will probably be between about 0.35 and
0.75.  Thanks!
Emily