Active and Passive Reflectance Sensor Comparison in Cotton

Kevin F. Bronson

Texas A & M Univ. – Texas Agric. Exp. Stn., Lubbock, TX

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)

Study objectives

• To assess lint yields and N fertilizer use efficiency of UAN (32-0-0) and 28-0-0-5S injected between 1st square and mid bloom and 1st square and peak bloom

• To test the GreenSeeker and Cropscan spectroradiometers as in-season N status monitoring tools

Tr eat N Source Termination Other

1 32-0-0 Mid bloom a 90 lb N/ac

2 32-0-0 Peak bloom a 90 lb N/ac

3 28-0-0-5 Mid bloom a 90 lb N/ac

4 28-0-0-5 a 90 lb N/ac

5 32-0-0 Peak bloom 45 lb N/ac & reflect

6 Zero-N 1 rep/stn only aTreatments 1-4 based on 150 lb N/ac – lb NO3-N/ac in 0-24 in. soil - NO3-N in 12 inches of irrigation water)

Peak bloom

Treatments

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

1 Station 2

Station 3 Station 4 Station 5

Station 6

Station 7

Station 8

Station 9 Station 10 Station 11 Station 12 Station 13

GS 28MB 32MB 32EB 28EB 32MB GS 32MB Zero 28MB 32EB 28EB 28MB

Rep 1 Rep 1 Rep 1 Rep 1 Rep 1 Rep 2 Rep 2 Rep 3 Rep 2 Rep 2 Rep 2 Rep 2 Rep 3

101 105 109 113 117 201 205 209 001 213 217 301 305

121 221

102 106 110 114 118 202 206 210 002 214 218 302 306

122 222

103 107 111 115 119 203 207 211 003 215 219 303 307

123 223

104 108 112 116 120 204 208 212 004 216 220 304 308

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.

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

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60

Days after start of N fertilizer injection

Gre

en

ve

ge

tativ

e in

de

x

32-0-0 to mid-bloom

Reflectance-based

Zero-N

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

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60

Days after start of N fertilizer injection

Gre

en

ve

ge

tativ

e in

de

x

32-0-0 to mid-bloom

Reflectance-based

Zero-N

GVI_Cropscan R530 SPAD

GVI_GreenSeeker 0.30 NS 0.36

GVI_Cropscan 1 -0.90 NS

R550 1 NS

SPAD 1

GVI_Cropscan R530 SPAD

GVI_GreenSeeker 0.58 -0.50 NS

GVI_Cropscan 1 -0.81 NS

R550 1 -0.29

SPAD 1

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

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?