Improving Nitrogen Use in California Rice.

California rice (Oryza sativa L.) growers commonly apply topdress N fertilizer at panicle initiation (PI) stage. However, these applications are often without consideration of crop N status, and can result in losses both economically and environmentally. Although, some tools are available to guide in-season fertilization decisions, generally these techniques are time consuming, cumbersome, and limited by their small-scale sampling methods. Thus, a comprehensive method to assess in-season crop N status and develop sustainable fertilizer recommendations is needed for CA rice systems. Preliminary studies have shown that remotely sensed Normalized Difference Vegetation Index (NDVI) can predict in-season N status for several crops. Therefore, to investigate its potential in CA rice, on station and on farm N response trials were established over a 3-year period (7 site-years) across the Sacramento Valley rice growing region. The objectives were to determine how NDVI correlates with PI N status in rice, establish the critical level of N required by PI to achieve maximum grain yield, and predict the response in grain yield to adding topdress N across different levels of PI N status. Initial results indicate that NDVI measured with a handheld GreenSeeker correlates strongly with PI aboveground N uptake (biomass x N content). Moreover, PI aboveground N uptake is a strong predictor of final grain yield, and results show adding topdress fertilizer to a N sufficient crop can lead to significant yield loss. These findings demonstrate that NDVI can provide CA rice growers the information needed to make well informed, sustainable N fertilization decisions.