Sensor Readings in Corn as Affected by Time of Day Peter Scharf University of Missouri With thanks to Larry Mueller, Joao Medeiros, Bettina Coggeshall, and Amanda Coggeshall

Why consider time of day? Most of our readings were collected within one hour in our calibration research Our method requires comparison with a reference area If spectral properties change during the day, the time between measuring the reference area and fertilizing may introduce error There is some evidence in the literature that spectral properties change during the day

Literature: color vs. time Hoel & Solhaug, 1998: Wheat SPAD highest at dusk & dawn SPAD dropped when plants were transferred from low to high intensity light in growth chambers 5 units in 30 minutes in shade plant 6% in one hour in wheat SPAD goes back up when light intensity is dropped Brugnoli & Bjorkman, 1992: Sunflower transmittance higher at higher light intensity Due to chloroplast movement (arrangement & orientation)

Methods Experiments conducted in 2004 near Columbia Sensors were mounted in place over individual plants Sensor measurements were taken hourly from 6am to 9pm (approx. weekly) Experiment 1: June 16 (V8) to June 29 (V12) Experiment 2: July 7 (V6) to July 28 (V14)

NDVI vs. time: green is most sensitive

Green NDVI consistently drops a lot in the middle of the day

Green NDVI consistently drops a lot in the middle of the day

Regular NDVI also consistently drops in the middle of the day

Yellow NDVI also drops in mid-day, but perhaps the least of the three

What’s going on?--NIR

What’s going on?--VIS

What’s going on?--SPAD

Why were NDVI values lower in the middle of the day? SPAD suggests that these may change late in day Ideas: Change in canopy architecture Change in internal leaf structure Change in leaf pigment concentration or arrangement Influence of shadows (readings were in north-south rows: shadows in the background change dramatically during the day)

Why were NDVI values lower in the middle of the day? Ideas: Change in canopy architecture Change in internal leaf structure Change in leaf pigment concentration or arrangement Influence of shadows (readings were in north-south rows: shadows in the background change dramatically during the day) We were curious about this and decided to test it

Influence of shadows Greenseeker green & red, Crop Circle, stationary over soybean canopy Take reading in sun, then move shade over sensor & canopy, take reading, repeat Ten pairs afternoon 8/9/2005 Ten pairs morning 8/10/2005 Photo & measurements: Larry Mueller

Influence of shadows Greenseeker green NDVI was consistently much higher when measured in shadow Mean 0.684 in sun, 0.761 in shade Different with p = 0.0001 Greenseeker red NDVI was consistently higher when measured in shadow Mean 0.742 in sun, 0.769 in shade Different with p = 0.04

Influence of shadows Crop Circle yellow NDVI was not different measured in sun or shade Mean 0.707 in sun, 0.700 in shade p = 0.27

What about visible/NIR ratio? Visible/NIR ratio can be calculated exactly from NDVI—the exact same information is contained in both parameters So I won’t show vis/NIR vs. time of day But proportionally, differences are larger in vis/NIR than in NDVI analogs

Summary We thought that with active light sensors, we would not have problems with reflectance measurements changing as light conditions changed (time of day, clouds) With all three sensors, NDVI values were lower in mid-day and higher near dawn and dusk This pattern was much more pronounced with the Greenseeker green sensor than the other two

Summary Some of the diurnal change in NDVI may be due to actual changes in the crop SPAD is higher in evening Canopy architecture may also change Change in value for NDVI (or vis/NIR) during the course of the day is an obstacle to making good N rate decisions Need to solve this to be a useful on-farm tool Frequent reference reading is one possibility (N applied across rows with airplane?) Another possibility is to use a correction function

Summary We also observed differences in sensor readings in sun vs. shade Biggest in green Greenseeker Small in red Greenseeker No problem in Crop Circle Change in sensor readings when a cloud comes over is also an obstacle to making good N rate decisions We also have data from passive sensors in these experiments—not yet analyzed, but we will eventually compare them with active sensors