Utilization of Crop Sensors to Detect Cotton Growth and N Stress.

Crop reflectance utilizing tractor mounted sensors has the potential to direct fertilizer N applications.  However, there is a need for a more precise definition of the relationship between sensor-observed reflectance, cotton plant height, and cotton N status.  The objective of this study was to examine the YARA N Sensor (Yara International ASA, Oslo, Norway) for its effectiveness in differentiating cotton (Gossypium hirsutum L.) plant height and leaf N.  Field trials were conducted from 2008-2010 at the Plant Science Research Farm, Mississippi State, MS. Fertilizer N rates of 0, 40, 80, and 120 lb N/acre were applied to establish wide growth differences.  The Yara N Sensor was utilized to collect canopy reflectance across visible and near-infrared wavelengths at several physiological stages.  Plant height and leaf N were also determined at these stages.  Several ratios, wavelengths, and techniques were analyzed relative to sensitivity of cotton N status.  Relationships between GNDVI and leaf N and GNDVI and plant height generally strengthened throughout the growing season to peak flower.  A combination of leaf N and GNDVI data from each year demonstrated the consistency of readings across growing seasons, however strong relationships for GNDVI occurred at a date past which a fertilizer N application would impact yield.  Derivative analysis of the spectral reflectance signatures showed consistent shifts in red edge inflection point (REIP) across N rates, suggesting REIP to be an indicator of early season cotton N status.  In 2009, first week of square REIP ranged from 702 nm to 709 nm as influenced by a leaf N range of 3.2% to 4.5% (r²=.864).  At third week of square, REIP ranged from 709 nm to 714 nm with a leaf N range of 3.6% to 4.8% (r²=.805).  Relationships between REIP and leaf N were stronger than relationships between GNDVI and leaf N at first week of square (r²=.684) and third week of square (r²=.488).  Although GNDVI readings may be restricted to directing sampling or N applications in the following growing season, REIP has the potential to provide cotton N status early enough to drive a yield impacting fertilizer N application.

Acknowledgements

Funded by Cotton Incorporated project number 08-433.