Site-specific nutrient management enhances sink size, a major yield constraint in rainfed lowland rice

Nutrient management can increase crop yield and income, but its effects on yield components are rarely dissected in on-farm research. In this study, we compared aboveground biomass and yield components of rainfed lowland rice under site-specific nutrient management and farmer management from 69 demonstration sites prone to mild to moderate intermittent drought across 9 Philippine provinces over 3 years (the 2011-2013 wet seasons). The sink size (spikelets m(-2)) was most closely associated with grain yield in all years. Panicle size (spikelets per panicle) increased by 10.4% and 13.0% in 2011 and 2012, respectively, under site-specific nutrient management, with N application around the early reproductive stage of < 25 kg N ha(-1) in farmer nutrient management versus 30-33 kg N ha(-1) in site-specific management. Higher N application during seedling establishment in farmer nutrient management (55 kg N ha(-1)) than in site-specific management (22 kg N ha(-1)) did not increase panicles m(-2) in any year. Our results demonstrate the yield advantage of site-specific nutrient management in rainfed lowland rice in relatively fertile and less drought-prone environments: enhancing sink size should be the major target of nutrient management; it is unnecessary to apply high amounts of N during seedling establishment to secure a sufficient panicle number; and N application is most important during the early reproductive stage to increase panicle size.