Crop reflectance sensors have been used to assess nitrogen (N) status in many crops. The SPAD chlorophyll meter and the GreenSeeker® sensor have been studied extensively to determine their ability to detect crop N status and predict crop N requirement. Mepiquat chloride (MC) is routinely used in cotton to manage vegetative growth. Pigment concentration often increases following application of MC and could confound results from GreenSeeker readings used to adjust N application. A field experiment was conducted in Georgia and North Carolina to determine the effect of MC on SPAD meter and GreenSeeker sensor readings. Treatments consisted of a factorial arrangement of multiple N and MC rates. Nitrogen was applied at the initial appearance of squares followed by MC application 14 to 21 d later. SPAD meter and GreenSeeker sensor readings were taken for 10 consecutive weeks beginning immediately before N application. The SPAD meter proved to be sensitive to MC and N, but was not associated with lint yield. The GreenSeeker sensor was sensitive to N, but not sensitive to MC. Furthermore, normalized difference vegetative index and lint yield responded similarly to N, suggesting that GreenSeeker might be a useful tool in assessing plant N status and predicting crop N requirement.
