Building Uncertainty Into N Recommendations for Maize: Addressing Insurance Applications.

Farmers often apply insurance fertilizer in addition to recommended nitrogen rates for maize.  This is a management strategy to deal with uncertainty in the actual optimum fertilizer rate due to many sourcs of variability including seasonal changes from weather and management factors.  The nonlinear response of maize yield to N fertilizer rate implies that the cost of under-fertilization is higher than that of over-fertilization, and therefore an upward adjustment of the fertilizer rate is required.  We modeled the required adjustments for different levels of uncertainty in optimum fertilizer rate, as well as different grain-to-fertilizer price ratios.  Results demonstrate that considerable adjustments to the optimum recommended rates are needed when the uncertainty is high, and those corrections are greater with lower fertilizer prices and higher grain prices.  For a uncertainty level with standard deviations of 50 kg ha-1, which is typical for standard university maize N recommendations, and a price ratio of 5.6, economic optimum rates are 33 kg ha-1 higher than when only the price ratio is considered.  For uncertainty levels of 40 kg ha-1, the upward N rate correction is 20 kg ha-1.  Profits under uncertainty of s=50 kg ha-1 are reduced by approximately $30 per acre.  In conclusion, uncertainty in the agronomic/economic optimum N rate for maize necessitates upward adjustments of the recoemmended rates, and reduces the returns to N application.