Active and Passive Reflectance Sensor Comparison in Cotton

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Active and Passive Reflectance Sensor Comparison
in Cotton

• Kevin F. Bronson

Texas A M Univ. Texas Agric. Exp. Stn.,
Lubbock, TX
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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)

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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

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Treatments
Tr
ea
t

N
Source

Termination

Other

a

1

32
-
0
-
0

Mid bloom

90
lb N
/ac

a
2

32
-
0
-
0

Peak bloom


90
lb N
/ac

a
3

28
-
0
-
0
-
5

Mid bloom


90
lb N
/ac

a
Peak bloom
4

28
-
0
-
0
-
5



90
lb 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
-
N/ac in 0
-
24 in. soil

3
-

NO
-
N
in 12 inches of irrigation water
)

3

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Plot/station layout (plots are 8, 40-in. rows X
600 ft)
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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 (24in. 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.

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Green vegetative index (r780/r530) from Cropscan
MSR16, 48 inches above canopy, Lubbock, TX,
2005
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Green vegetative index (r780/r530) from
GreenSeeker, 36 inches above canopy, Lubbock, TX,
2005
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Correlations of GVI from cropscan, GreenSeeker
and SPAD meter readings at early bloom, and peak
bloom, Lubbock, 2005
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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?