Numerical optimization of nitrogen application to rice. Part II. Field evaluations

The MANAGE-N model (Part I; Ten Berge et al., this issue) was tested by comparing predicted and measured final crop biomass production for 48 rice cultivars under application of 0, 30-40, 60-80 and 90 to 120 kg urea-N per ha at Cuttack, India, during seven consecutive wet seasons. The overall coefficient of correlation between predicted and measured values was 0.93 (p < 0.01). The root mean squared prediction error was 1110 kg biomass per ha in long, 769 kg/ha in medium and 1210 kg/ha in short duration cultivars. Data sets from three other experiments at Nanchang (Jiangxi, China), Jinhua (Zhejiang, China) and Cuttack were used to (a) evaluate model-recommended N management (dose D(M), timing T(M)) against local state-recommended management (dose D(s), timing T(s); and (b) determine the yield prediction accuracy of the model. Under dose D(s), timing T(M) increased grain yields significantly (p <0.05) by 5 to 10% at all sites, relative to T(s). Application of dose D(M) resulted in further yield increases at Jinhua and Cuttack. Negative effects of high N dose on yield were observed but not predicted at Nanchang and Jinhua. Agronomic efficiency (mean yield increment per kg N applied) was at all sites higher in D(s)T(m) and D(M)T(M) than in D(s)T(s). Predicted grain yields differed from observed values by less than the least significant difference between field treatments (LSD, p =0.05) at Nanchang and Jinhua. Differences were larger than LSD at Cuttack. Simulations further indicate that the time window for N application narrows as N input decreases. Simulated effects of crop duration on yield, N response and optimum N dose are discussed.