Reducing the gap between attainable and potential yield in double rice-cropping systems of Zhejiang Province, China

The climate-adjusted, genetic yield potential of modern rice varieties in Zhejiang Province of China is currently about 10-12 t ha-1. Attainable yields in farmers' fields, however, have been maintained at only about 5.5-6.0 t ha-1 in both the early and late cropping seasons since 1985 despite the large-scale use of hybrid and other modern rice varieties and increasing fertilizer use. On-farm experiments involving 21 rice-growing farmer families began in 1997 in the central part of Zhejiang Province to assess the status of soil fertility and productivity under intensive rice-rice cropping. Low and variable recovery efficiencies of applied inorganic fertilizer nitrogen were identified as a major constraint to increasing plant nutrient uptake and grain yield in farmers' fields. A new, site-specific nutrient management strategy was developed to increase productivity and break the apparent attainable yield barrier of 6 t ha-1. Field- and season-specific fertilizer recommendations were calculated considering ndigenous soil nutrient supply, reasonable grain yield targets, and corresponding nutrient demand, nutrient balance, and nutrient-use efficiencies. The agronomic performance of SSNM was tested on the 21 farms against the farmer's fertilizer practice (FFP) in 1998 and 1999. Across seasons and years, grain yield was 8% greater with SSNM than with FFP (6.35 vs 5.90 t ha-1), whereas plant N, P, and K uptake increased by 8-14%. The gross return over fertilizer cost was about 10% greater with SSNM than with FFP (US$998 vs $905 ha-1 crop-1). In general, yields were about 1 t ha-1 or 19% greater in late rice (July to October, mostly hybrid cultivars) than in early rice (April to July, mostly inbred cultivars), but SSNM performed equally better than FFP in both seasons. Since fertilizer N, P. and K rates were lower in SSNM, especially in 1999, increases in grain yield and nutrient uptake were most likely due to improved timing and splitting of fertilizer N, which increases the average recovery efficiency of applied ertilizer N from 0.19 kg kg-1 in FFP to 0.29 kg kg-1 in SSNM. The average agronomic N-use efficiency (grain yield increase per kg fertilizer applied) was 80% greater with SSNM than with FFP (11.4 vs 6.3 kg kg-1). However, N-use efficiencies were generally low, thus indicating further scope for improving N management strategies.