How widespread are yield declines in long-term rice experiments in Asia?

This paper analyzes yield trends in 30 long-term experiments (LTEs) conduct ed at 24 sites with intensive rice monoculture or rice-upland clop systems in tropical and subtropical regions of Asia. The objectives of the analysis were to (1) investigate how widespread yield declines are in experiments w ith constant levels of input and (2) study the cross-sectional relationship between the magnitude of yield trends and the initial yield level in the e xperiment. In rice-rice (-rice) systems, yields in the most favorable clima tic season (dry season) have remained stable in many LTEs conducted outside of IRRI, mostly at yield levels of 4-7 Mg ha(-1) with recommended rates of N, P and K applied. Statistically significant yield declines at p less tha n or equal to 0.05 were observed in only two of 21 data sets outside of IRR I, although all thr et: LTEs at IRRI showed yield declines. One dry season experiment showed a statistically significant yield increase. In the less f avorable climatic season (wet season), only two of 14 data sets outside of IRRI showed a statistically significant declining yield trend, while three of four at IRRI showed statistically significant yield declines. In rice-up land crop (typically wheat) LTEs, a significant yield decline for rice was found in only one of ten data sets. For wheat. there were no statistically significant yield declines, although there was one statistically significan t yield increase. There was a negative correlation between the magnitude of yield decline and the level of initial yields, but it was not statisticall y significant. The data suggest that while yield declines exist in some LTE s, they are less common than previously thought, particularly at moderate y ield levels. Where yield declines occured, they were related to soil proper ties affected by prolonged soil wetness or soil nutrient depletion. (C) 200 0 Elsevier Science B.V. All rights reserved.