Can we increase economic return from Sugarbeet with slow-release nitrogen additions?

Optimizing sugarbeet root yield and sugar content depends on successful management of fertilizer nitrogen (N). Low soil N availability can reduce yield, but the excess supply of N can reduce the sugar content without increasing the root yield (Anderson & Petereson, 1988). As sugarbeet growers are paid according to recoverable sugar yield (RSY), the decisions made on fertilizer N source and rate are critical. Slow-release N fertilizers delay the release of N by slow hydrolysis of water-soluble polymer coatings and better match the crop N uptake pattern relative to readily soluble urea (Guertal, 2009; Cahill et al., 2010). Use of slow-release N fertilizers has the potential to increase N use efficiency.

For the Red River Valley of North Dakota and Minnesota, the current fertilizer N recommendation suggests a single application rate of 130 lb N/ac, adjusted for soil-available nitrate (NO₃^–)-N to a depth of 4 ft and a root yield potential of 20 ton/ac (Franzen, 2018). However, the current sugarbeet yield had almost doubled due to the introduction of glyphosate-resistant cultivars and resulting improvement in weed management (Tarkalson, 2012; USDA-NASS, 2018). Fertilizer N studies between 1970 and 2011 indicated a narrow N supply range of 160–180 lb N/ac was needed to maximize sugarbeet yield (Tarkalson, 2018). Slow-release N fertilizers may offer growers the opportunity to safely increase N application rates with less risk of loss in RSY, but limited research has been conducted in the Red River Valley to determine the effect of slow-release N on sugarbeet production.

The field experiment was conducted to compare Environmentally Smart Nitrogen (ESN) to urea at less-than-recommended (90 lb N/ac), close-to-recommended (120 lb N/ac), and higher-than-recommended (150 lb N/ac) N rates on sugarbeet tissue N concentration, yield, sugar, RSY, and economic return in the Red River Valley of MN, USA during the 2017 and 2018 growing seasons. The hypothesis was that longer N availability might negatively impact sugarbeet sugar content without increasing yield.

Field Trials and Data Analyses

Field trials took place at a grower’s field near Ada, MN during the 2017 and 2018 growing seasons. Site location and basic soil properties are presented in Table 1. Monthly rainfall data of the growing season were collected from the North Dakota Agricultural Weather Network website (https://ndawn.ndsu.nodak.edu/) and are presented in Figure 1. Before planting, representative bulk soil samples from 0- to 6-, 6- to 24-, and 24- to 48-inch depth increments were analyzed for available nitrate N (NO₃–N) while samples from the 0- to 6-inch depth only were analyzed for available phosphorus and available potassium using soil test methods recommended for the Northern Great Plains (NCR 221, 1998).

Fields were chisel-plowed, and the previous crop was soybean in 2017 and spring wheat in 2018. Seven treatments—a control (without fertilizer N) and a factorial combination of two fertilizer N sources, urea and ESN (44% N, Nutrien Ltd., Saskatoon, Canada), each at three application rates, 90, 120, and 150 lb N/ac—were laid out in a randomized complete block design with four replications. Fertilizer N application rates were adjusted for initial soil N levels to a depth of 4 ft and N coming from monoammonium phosphate. Recommended phosphorus and potassium were applied in the form of monoammonium phosphate and muriate of potash, respectively, and according to the recommendation based on soil test values. The individual plot was 30 ft long and 11 ft wide (six rows at a 22-inch row spacing). Sugarbeet seeds were planted 4.75 inches apart using a John Deere Maxemerge planter.

Roundup herbicide was applied twice, at rates of 32 and 24 fl oz/ac, respectively for weed control, and Quadris at the rate of 10 fl oz/ac was applied at the four- to six-leaf stage and again three weeks later to control Rhizoctonia root rot. Three fungicides—Inspire, Topsin, and Headline—were applied at the rates of 7, 7.6, and 10 oz/ac, respectively, for Cercospora leaf spot control.

The middle two rows of the plot (108 ft²) were mechanically defoliated and harvested and weighed for root yield determination. Subsamples (10-15 beets) were analyzed for sugar content (sucrose), α-amino-N, sodium, and potassium concentrations. Sugar content was determined polarimetrically using aluminum sulfate–clarified brei samples (McGinnis, 1982). Sugar loss to molasses was calculated using the modified Carruther’s equation (Carruthers et al., 1961):

Relevant information to calculate return was obtained from Tyler Grove, General Agronomist at the American Crystal Sugar Company in Moorhead, MN (personal communication). Amino N was determined by the copper method using a spectrophotometer at a wavelength of 610 nm (International Commission on Uniform Methods for Sugar Analysis, 2007), and potassium and sodium were determined by flame photometry.

Sugarbeet leaf samples were collected at the middle of the growing season and late in the growing season, June 19 and August 22 in 2017 and June 19 and July 16 in 2018. At each sampling time, 10 fully expanded recently mature leaves from each plot were dried at 140°F, followed by sulfuric acid-salicylic acid digestion, distillation, and titration with 0.1N HCl to determine total N concentration (Horneck & Miller, 1997).

Tissue N concentration, yield parameters, and economic return were analyzed using the generalized linear modeling procedure (SAS Institute Inc., Cary, NC) for fertilizer N source and fertilizer N application rate factors with four replications and growing season as the repeated measure. Mean separation was done using least square means at 95% significance level.

Influence of Growing Year, Nitrogen Source, and Rate on Sugarbeet Production

The growing season had a significant effect on sugarbeet late-season tissue N concentration, root yield, sugar, RSY, and economic return (Table 2 and Figure 1). May to July rainfall was below normal for both growing seasons. In 2017, May rainfall was lower than in 2018. Chatterjee et al. (2018) also found a significant effect of growing condition and site characteristics on root yield and sugar concentration in the Red River Valley of North Dakota and Minnesota.

Fertilizer N application rate had a significant effect on the late-season tissue N concentration in 2017. Application of ESN at 90 lb N/ac had the higher late-season tissue N concentration than ESN application at 120 lb N/ac. Under optimum N supply, tissue N concentration declined at the late growing season due to translocation of N to storage root. Low supply of N, at 90 lb N/ac, might have reduced the top growth and increased the tissue N concentration compared with higher N application rates. Sugarbeet growth rate primarily depends on internal supplies of sugar produced in the photosynthesis (Loomis et al., 1971).

Neither N source nor application rate influenced root yield. During 2017, yield ranged from 33.4 tons/ac with the urea N at the rate of 90 lb N/ac to 37.8 tons/ac with urea N at the rate of 150 lb N/ac. However, exactly the opposite trend was observed in 2018; the lowest yield of 38.3 tons/ac was achieved with urea N at the rate of 150 lb N/ac, and the highest yield was recorded in response to urea N at the rate of 90 lb N/ac. The fertilizer N source had a significant effect on sugar content in 2017; ESN at the rate of 150 lb N/ac had the highest sugar content and was significantly higher than the ESN application rate of 90 lb N/ac as well as urea at all N application rates. For both growing seasons, N source and rate did not affect RSY. In 2017, ESN had higher RSY than urea at the same application rate, and RSY increased with increasing application rate for both N sources. In 2018, the highest RSY was observed with the lowest N application rate (90 lb N/ac) for both urea and ESN. Several researchers reported that application of ESN did not increase yield or nutrient uptake. Rajkovich et al. (2017) did not find any difference in yield and N uptake pattern between ESN and the non-ESN treatment, UAN, for the wheat production in North Carolina. In Michigan, blending of urea with polymer-coated urea did not improve sugarbeet root yield or quality but also did not significantly reduce N uptake or plant development (Steinke & Bauer, 2017).

Fertilizer N source and rate did not significantly affect economic return. Although not significant, economic return was higher with ESN than urea and increased with N application rate in 2017; but in 2018, economic return was reduced with increasing N application rate for both urea and ESN. Due to extreme variability in rainfall, slow-release N fertilizers designed to reduce N losses might not consistently offer positive returns, but it might be advisable to apply them to fields prone to N losses (Steinke & Bauer, 2017).

Lack of sugarbeet response to slow-release N over urea might be due to much N being taken up in early crop development, rapidly and predominantly from topsoil (Jaggard et al., 2009). Application of urea N at the rate of 150 lb N/ac, higher than the recommendation, might reduce the quality by increasing the sugar loss to molasses and amino-N without significantly increasing in root yield. Chatterjee et al. (2018) reported application of 100 lb N/ac achieved the maximum yield, sugar, and economic return. Excess soil N availability at the late-growth stages increases crop N uptake. Higher tissue N concentration late in the growing season had a negative influence on root yield or sugar concentration, which might lead to less RSY and economic return sometimes.