Mepiquat Chloride Confounds Sensor Based Nitrogen Recommendations

Nitrogen is often the most limiting nutrient in cotton production and is used rather inefficiently in cotton; nitrogen use efficiency (NUE) values are typically below 50%. Precision agriculture offers a means to improve NUE by real-time adjustments of N rates based on estimation of crop needs. These nitrogen “prescription” rates are based on current leaf N status as well as in field yield estimates that incorporate nitrogen rich vegetative strips. Chlorophyll content in leaves and canopy are often a good indication of leaf N concentration and can be correlated to plant greenness. The SPAD-502 meter, an optical chlorophyll meter, and the GreenSeeker, a crop reflectance sensor that calculates normalized differential vegetative index (NDVI), are two sensors that can measure greenness. Recent attempts have been made to develop a sensor based nitrogen rate calculator utilizing GreenSeeker NDVI output. Since GreenSeeker output is in part a measure of greenness, it is sensitive to biotic and abiotic factors that affect greenness including the application of Mepiquat Chloride (PIX). PIX has been shown to increase the chlorophyll content in cotton leaves with little impact on yield; therefore, PIX applications may have huge consequences in GreenSeeker and SPAD readings. Altered sensor readings without a corresponding change in yield may negatively affect sensor based nitrogen “prescription” rates. Consequently, a study was conducted to determine the effects of PIX applications on SPAD and GreenSeeker readings. PIX was applied at early bloom and nitrogen was applied at the match head square (MHS). SPAD readings and GreenSeeker scans were conducted weekly beginning one week prior to MHS and continued through defoliation, a total of 10 weeks. Preliminary results indicate that SPAD meter and GreenSeeker readings are not as highly correlated as desired and PIX applications significantly affect sensor readings with little impact on yield.