Nitrogen Consumption and Productivity of Cotton under Sensor‐based Variable‐rate Nitrogen Fertilization

Precision N management based on crop reflectance can be used to reduce over‐ and under‐application of N fertilizer to cotton ( Gossypium hirsutum L.). The objective of this study was to evaluate the effects of variable‐rate nitrogen management systems (VNMS) based on optical sensing of crop canopy relative to a farmer's uniform‐rate nitrogen application system (FUNS) on N fertilizer consumption, post‐harvest soil N level, and cotton productivity. Eight field strip‐plot trials were conducted on cotton across West Tennessee during 2012 to 2014. Four sensor‐based VNMS were compared with FUNS in a randomized complete block design with three replicates. Two VNMS were based on optical sensing of canopy alone with different algorithms, while the other two were developed from optical sensing and crop yield maps of previous years but differed in algorithms. The variable and uniform fertilizer application systems had similar leaf N concentrations between early bloom and mid‐bloom and similar canopy normalized difference vegetation index (NDVI) values at mid‐bloom. The VNMS produced similar lint yield as FUNS. The VNMS reduced N consumption by 10.3 to 16.9% and improved nitrogen use efficiency (NUE) by 8.3 to 11.0% compared with FUNS. Both VNMS and FUNS had similar post‐harvest N levels in soil profile. Benefits from incorporating yield maps into sensor‐based VNMS were not noticeable. Thus, the in‐season sensor‐based VNMS are reliable in precision N management for cotton resulting in comparable lint yield, less N consumption, and higher NUE, which could lead to environmental and economic benefits. Core Ideas Leaf N concentration and canopy normalized difference vegetation index were comparable under variable‐rate N management systems relative to a farmer's uniform‐rate N application system. Variable‐rate N management systems produced similar lint yield but better fiber quality compared with a farmer's uniform‐rate N application system. Variable‐rate N management systems reduced N use by 10.3 to 16.9% but increased nitrogen use efficiency by 8.3 to 11.0% over a farmer's uniform‐rate N application system. No effect of variable‐rate N management systems was observed on post‐harvest soil N level relative to a farmer's uniform‐rate N application system. Benefits with incorporating yield maps into sensor‐based variable‐rate N management systems were not noticeable.