Topdress strategies and remote sensing for nitrogen management in processing carrots

Improved nitrogen (N) fertilizer management strategies are required to optimize yield and quality in processing carrots (Daucus carota L.) while minimizing risks of N loss. A 2-year field study was conducted in commercial carrot fields near Hesperia and Pentwater, MI, USA, to investigate the effects of N fertilizer rate (29, 67, 135, and 202 kg ha(-1) N), topdress N frequency (single vs. three split applications), and timing of split N applications on carrot production and N utilization. In addition, the potential for remote sensing-based vegetation indices (VIs) to guide in-season N topdress decisions was evaluated relative to conventional methods including petiole sap nitrate testing. Both carrot root yield and shoot biomass increased with greater N rates but did not plateau. Split-applied topdress N and timing of topdress applications did not affect yield. However, greater N rates, single front-loaded applications, and both earlier and later than typical topdress timings exhibited potential to increase N loss depending on the year. While VIs explained at most 66% and 29% of the variation in yield in 2019 and 2020, respectively, the indices consistently explained greater variation compared to petiole sap nitrate (6%), shoot N concentration (8%), and carrot root (10%) and shoot (14%) weights. Hypothetical N topdress decisions made using VI-based sufficiency indices recommended N 26% less often than petiole sap nitrate, but more research is needed to evaluate reference and threshold selections. Despite labor and technological tradeoffs, remote sensing may increase accuracy, resolution, and scalability of N decision support in processing carrot production.