1.  VRS is appropriate when you know there is nothing else to do to correct the yield variability...i.e. on steeper side slopes (less water holding) vs. "flatter" areas (more water holding).

2.  VRS is NOT appropriate say in the flip side of above....that being you might have an area of the field that is way to "flat" causing yield problems and the best way to address that would be to shape the field to let that area have a consistant slope albiet pretty flat.  You can use RTK vertical control to remove any reverse grade issues and keep water moving rather than standing.

3.  On hybrid selection the more the hybrid will ear flex the less appropriate variable rate seeding would be.  A fixed size ear of course would need more plants to acheive maximum yield were appropriate.

>>>>In summary....generally speaking more population would need relatively more support with more fertilizer.  BUT overall one needs to keep the following in mind when considering any type of variable rate application.  IF you can not objectively determine the cause of yield variability with a good GIS correlation of various data layers to the yield layer then how can you variable rate with confidence?  As I stated above briefly let me offer this example up.  I have in many cases fixed my yield variability with land shaping micro "flat" topography rather than chasing my tail with variable rating......and on macro "steeper" topography that may not be affordable to land shape I deal with that by not throwing money at an area of the field that currently will not make even field average.

In short determine the true cause of yield variability before chasing the problem with VRS or any other variable rating excercise.

Arnie