1. Active and Passive Reflectance Sensor Comparison in Cotton Kevin F. Bronson Texas A & M Univ. – Texas Agric. Exp. Stn., Lubbock, TX

2. Introduction Previous work has shown that using spectral reflectance with well-fertilized plots/sufficiency index approach can improve agronomic N use efficiency in LEPA and subsurface drip irrigated cotton We hypothesized that reflectance based management can be used on long plots/stations with near daily N injection We built a 16-station SDI system that we can inject N daily as farmers do (not 30 lb N/ac doses)

3. Study objectives To assess lint yields and N fertilizer use efficiency of UAN (32-0-0) and 28-0-0-5S injected between 1 st square and mid bloom and 1 st square and peak bloom To test the GreenSeeker and Cropscan spectroradiometers as in-season N status monitoring tools

4. Treat NSource Termination Other 1 32-0-0 Mid bloom a 90lb N/ac 2 32-0-0 Peak bloom a 90lb N/ac 3 28-0-0-5 Mid bloom a 90lb N/ac 4 28-0-0-5 a 90lb N/ac 5 32-0-0 Peak bloom 45 lb N/ac & reflect 6 Zero-N 1 rep/stn only a Treatments 1-4 based on 150 lb N/ac– lb NO 3 -N/ac in 0-24 in. soil - NO 3 -Nin 12 inches of irrigation water) Peak bloom Treatments

6. Plot/station layout (plots are 8, 40-in. rows X 600 ft)

8. Reflectance methods Measured zero-N, reflectance-based (includes 30 ft over-fertilized plots), and 32-0-0 to mid bloom weekly between 23 June and 9 August Cropscan MSR 16 is a passive spectroradiometer. Has 16 upwards and downwards facing radiation tranducers/filters. Height of measurement was 48 inches (24–in. fov). Four spot measures (100 per) per GPS point (4 per 600 foot plot). GreenSeeker (Green) is an active spectroradiometer and calculated GNDVI and 1/GVI. Wavebands are 530 and 780 nm). Took 6 m of measurements (~100) per GPS point at 34-36 in above canopy.

15. Green vegetative index (r780/r530) from Cropscan MSR16, 48 inches above canopy, Lubbock, TX, 2005

16. Green vegetative index (r780/r530) from GreenSeeker, 36 inches above canopy, Lubbock, TX, 2005

17. Correlations of GVI from cropscan, GreenSeeker and SPAD meter readings at early bloom, and peak bloom, Lubbock, 2005

18. Summary/What next Reflectance-based had 65 lb N/ac injected, we hope for same lint yield as soil-test based of 90 lb N/ac. N Source or timing did not affect GVI. Will repeat study next year. May abandon small well-fertilized plots. Keep the same height above the ground for the GreenSeeker for all plots? Or maintain the same height above the canopy for GreenSeeker?