Cultivar, nitrogen, and moisture effects on a rice-wheat sequence: Experimentation and simulation

Rice (Oryza sativa L.) followed by wheat (Triticum aestivum L.) is a dominant cropping sequence under a range of management regimes in South Asia, with variable productivity. Simulation models can be used to extrapolate experimental results to other sites of interest in variable environments. Experiments were conducted to quantify the responses of two rice cultivars to three N regimes and two soil moisture regimes, followed by the responses of the succeeding two wheat cultivars to three N regimes, two soil moisture regimes, and two dates of planting, on a Haplaquept noncalcareous brown floodplain soil at the Wheat Research Center, Nashipur, Bangladesh. CERES-Rice and CERES-Wheat models were validated using the experimental data set, and then were used to estimate the grain yields at two other sites in Bangladesh. In rainfed plots, grain yields of 'BR14' rice were reduced by 48.8, 43.4, and 39.3% relative to irrigated plots, and those of 'BR11' were reduced by 49.1, 43.8, and 42.2% (P < 0.05), for 0, 90, and 135 kg N ha(-1), respectively. The optimum N rate was between 90 and 135 kg ha(-1). In the succeeding wheat crop, at 0 N, there were no significant differences between the two moisture regimes but, at high N (180 kg ha(-1)), irrigation promoted greater yield. Relative to the high-N treatment, gain yields of irrigated 'Kanchan' wheat at ON were reduced by 73.0 and 71.6%, and of 'Sowgat' by 75.2 and 73.5%, for early and late plantings, respectively. Simulation investigated year-to-year variation in grain yields of both crops under their several N regimes. Rice yields tended to increase over years, but wheat yields had no definite trend. The simulation results indicated that, without N fertilizer application, N will be limiting across sites and long-term weather conditions for both rainfed and irrigated crops. Field experimentation quantified actual crop responses to a range of management practices and allowed validating the models for the rice-wheat sequence. The models were then used to extrapolate and generalize the results for two other sites in Bangladesh.