Nitrogen and Water Management for Optimized Sugar Beet Yield and Sugar Content

O. S. Walsh1, S. Shafian2, D. M. Jayawardena1
1University of Idaho, 2Virginia Polytechnic Institute and State University
Introduction
OBJECTIVE: To analyze the effects of water and N fertilizer rates on yield and quality.
 
Sugar beet (SB) production profitability is based on maximizing three parameters: beet yield, sucrose content, and sucrose recovery efficiency. Efficient nitrogen (N) and water management are key for successful SB production. Nitrogen deficits in the soil can reduce root and sugar yield. Overapplication of N can reduce sucrose content and increase nitrate impurities which lowers sucrose recovery. Application of N in excess of SB crop need leads to vigorous canopy growth, while compromising root development and sugar production.
 
Changes in SB varieties and management practices warrant re-evaluation of N management. The Amalgamated Sugar Company and the USDA-ARS found that in 60% of evaluated SB fields, application of N did not increase sucrose yield. This suggests that residual soil N from past applications and in-season N mineralization was adequate and indicates that growers could maximize sugar yield and save money by applying less N. Due to recommendation to have all N applied and plant-available by 4-6 leaves, it is imperative to determine the appropriate N application rates for N responsive fields early in the season. Appropriate irrigation amount and timing can optimize SB yields while minimizing disease pressure, water costs and N leaching. Excessive irrigation can increase SB root weight, but lower sugar content.
 
 
Materials and Methods
Location: Parma R&E Center; SB variety: BTS 2570;
Planting: April 2019; 5 inch seed spacing; 0.75 inch seeding depth
Plots: 4 rows per plot with 22 inch row spacing; 40 ft long at planting, 35 ft at harvest (cut 5 ft alleys between replications)
Treatment set-up: Split split-plot design with 4 replications; 4 blank rows between the plots to minimize water and/or N carryover
Nitrogen: 100, 200, and 300 lb N/ac (total: soil residual + added fertilizer); applied as urea (46-0-0) and incorporated into the soil immediately prior to planting using light tillage
Water: 100% ET and 50% ET; applied using subsurface drip irrigation system (7-inch depth). Daily reference grass-based ET (ETo) were calculated using data from the Parma AgriMet weather stations. Daily ETc was estimated by multiplying ETo by the SB crop coefficient (Kc).
Data collection: at 40 and 60 days after planting, and prior to harvest: 1) Plant height - 15 plants per plot; top leaf to the soil; 2) Plant dry matter determination (oven-dried at 220◦F for 24 h and weighed) and N content – 15 leaves and tops (0.8 in of taproot).
Harvest: In October, SB were scalped to a silver dollar sized disc and harvested for yield and root sugar content determination.
Data analysis: The response of yield and quality of SB to applied treatments will be assessed.
Summary and conclusion
100 ET significantly greater yield and ERS, compared to 50 ET
Future work: could water be cut down 25% (75 ET)?
For SB biomass: all spectral indices were excellent estimators of plant height and leaf N
Yield and ERS should have similar trend (further analysis)
Accuracy of yield and ERS prediction from UAV spectral indices improves substantially from June to July
Relationship becomes linear later in-season
Overall conclusion: UAV-based data can be successfully used to estimate SB yield and ERS in-season
Results: biomass, plant growth, yield, and quality
Plant height increased from early to mid-season as plant stand was establishing and biomass production accelerated, then declined late season due to changes in plant architecture as SB matured. Taller plants were associated with higher water and N inputs.
Biomass dry weight increased from early to mid-season as plants grew and developed. Early season biomass production was lower for 100 N treatments (50 and 100 ET); all treatments with higher N and water were statistically equal. Mid-season biomass increased as N and water inputs increased, with slight decline beyond 300 N + 50 ET.
Biomass N content early season was increased with higher N rates, and were comparable for 50 and 100 ET. N content declined throughout the growing season, as the taken up N got distributed among the developing plant biomass volume. Mid and late season biomass N content was comparable for all treatments.
Sugar beet root yield was maximized with 200 N + 100 ET treatment. Increasing N rate to 300 kg ha-1 did not enhance yield at 50 or at 100 ET level. Lower water inputs significantly reduced yield, especially at lower N rate. Lowest yield was obtained with lowest N + water input.
Sugar content was higher for 50 ET treatments, irrespective of N rate. Sugar content was lowest at 100 N + 100 ET, and maximized with 200 N + 50 ET treatment. Sugar content for 100 N + 50 ET was comparable to that of 200 N and 300 N treatments watered at 100 ET level.
Estimated recoverable sugar (combination of sugar beet root yield and sugar content) was maximized with 200 N + 100 ET; as with yield, increasing N rate to 300 kg ha-1 did not further enhance estimated recoverable sugar.
Early season biomass N content and plant height was strongly positively correlated with sugar content.
UAV set-up and image acquisition
Images imported to Micasense Atlas software (MicaSense, Inc, Seattle, WA) for mosaicking and calibration
 
Pix4Dmapper image analysis software (Pix4D SA, Lausanne, Switzerland) to create aligned and mosaicked images
 
Calibrated Reflectance Panel was used to calibrate images
In-season yield and quality prediction

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