AA anhydrous ammonia EONR economic optimum nitrogen rate LP linear-plateau model MRTN maximum return to nitrogen PR price ratio of nitrogen to corn grain QP quadratic-plateau model RTN (net) return to nitrogen RTNEONR (net) return to nitrogen at the economic optimum nitrogen rate RTNMRTN (net) return to nitrogen at the appropriate Illinois guideline nitrogen rate UAN urea-ammonium nitrate Y0 predicted yield without nitrogen fertilizer YEONR yield at the economic optimum nitrogen rate YMRTN...