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Disaggregating Model Bias and Variability when Calculating Economic Optimum Rates of Nitrogen Fertilization for Corn

^a Iowa Soybean Assoc., 4554 114th Street, Urbandale, IA 50322
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50010
^c Dep. of Plant Science, Univ. of Connecticut, Storrs, CT 06269

View larger version (50K): [in this window] [in a new window]   Fig. 1. Relationships between corn yields and rates of N fertilization as observed in 54 trials having seven rates of N applied to corn grown after corn (c) or corn grown after soybean (s).

View larger version (21K): [in this window] [in a new window]   Fig. 2. Relationships between corn yields and rates of N fertilization found when data from 54 trials were analyzed as a single category and as separate categories based on the previous crop. EOR, economic optimum rate; EXP, exponential model; LRP, linear response and plateau model; QRP, quadratic response and plateau model; QUAD, quadratic model; SR, square root model.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Model residuals (observed minus predicted yields) for five models fit to mean yields for corn after soybean crop category. EXP, exponential model; LRP, linear response and plateau model; QRP, quadratic response and plateau model; QUAD, quadratic model; SR, square root model.

View larger version (15K): [in this window] [in a new window]   Fig. 4. Relationships between mean yields for individual trials and rates of N fertilization when data from 54 trials were analyzed as a single category and as separate categories based on the previous crop. EOR, economic optimum rate; EXP, exponential model; QUAD, quadratic model; SR, square root model.

View larger version (21K): [in this window] [in a new window]   Fig. 5. Relationships between mean yields for individual trials and rates of N fertilization when data from 54 trials were analyzed as a single category and as separate categories based on the previous crop. EOR, economic optimum rate; EXP, exponential model; LRP, linear response and plateau model; QRP, quadratic response and plateau model; QUAD, quadratic model; SR, square root model.

View larger version (16K): [in this window] [in a new window]   Fig. 6. Relationships between mean yields for individual trials and rates of N fertilization found in the near-optimal range when data from 54 trials were analyzed as a single category and as separate categories based on the previous crop. EOR, economic optimum rate; EXP, exponential model; QUAD, quadratic model; SR, square root model.