If you were to measure protein (and yield) during harvest, you would begin to understand better how much and where your land requires nutrients.  Measuring protein is the most important as kernel protein content responds linearly to N application where yield levels out.  Accordingly, where you measure protein below the critical %level of your variety, you could add more N (and boost yield) next growing season, and where protein is above the critical %level, you should apply less so as not to waste.