Determination of optimal nitrogen rate for rice varieties using a chlorophyll meter

Site-specific N management (SSNM) such as real-time N management (RTNM) and fixed-time adjustable-dose N management (FTNM) improve fertilizer-N use efficiency of irrigated rice. This study was conducted to compare the N response and fertilizer-N use efficiency of Shanyou63 (SY63) and Liangyoupeijiu (LYP9) under a wide range of N rates. SY63 and LYP9 were the most popular hybrid varieties in the late 1980s and late 1990s in China, respectively. The overall objective was to develop optimal N management for LYP9 using SSNM. The two varieties were grown under eight and six N treatments in 2004 and 2005, respectively, in Hubei, China. N treatments included a zero-N control, an FTNM, and several RTNM with different chlorophyll meter (SPAD) thresholds. SPAD readings were taken weekly on the topmost fully expanded leaves. Grain yield, yield attributes, total N uptake, and components of fertilizer-N use efficiency were measured in both years. Both FTNM and RTNM can be used to improve N management for SY63 and LYP9, but the optimal SPAD threshold for determining the timing and rate of N application was 2 units higher in LYP9 than in SY63 because LYP9 has thicker leaves. The two varieties required a minimum total N rate of 120–150 kg N ha −1 for producing maximum grain yield. The difference in maximum grain yield was very small between the two varieties, although LYP9 had the potential to produce higher grain yield than SY63. The two varieties did not show a clear difference in fertilizer-N use efficiency. When excessive N was applied (195–275 kg N ha −1), grain yield of LYP9 was 13% higher than that of SY63 in both years because they responded differently to the high N inputs. Our study suggests that the planting of a variety insensitive to high N input such as LYP9 would lead to over-application of fertilizer-N by rice farmers if knowledge-intensive N management technology such as FTNM and RTNM were not used.