The Timing of Sugar Beet Harvesting Significantly Influences Roots Yield and Quality Characteristics

Abstract

The accumulation of sugar beet (Beta vulgaris L.) root yield across Central and Eastern Europe typically occurs mostly from July to September but can vary substantially depending on precipitation patterns. When summer rainfall is insufficient, the period of intensive yield accumulation may be delayed, often affecting the technological quality of the roots. Conversely, as light and thermal conditions deteriorate in autumn, growth processes slow down, and each cultivar’s response to late-season conditions may vary. To investigate these dynamics, we examined nine sugar beet cultivars (Zeltic, Pacific, Mariza, Everest, BTS 2205N, Jaromir, Jantar, Eliska KWS, and Klara) under three harvest dates (8–10 September—first date; 7–8 October—second date; and 3–5 November—third date) during the 2020–2021 growing seasons. Both cultivar and harvest timing had a significant impact on root yield, sucrose content, and the concentrations of molasses-forming elements (K, Na, and α-amino N), though the magnitude of these effects strongly depended on weather conditions. In 2020, which was characterized by very high precipitation in June and August, harvesting in early September resulted in optimal yield for most cultivars, with no significant benefit from delaying harvest. However, in 2021, when precipitation was moderate and more evenly distributed, later harvest dates enhanced both yield and sucrose content in several cultivars (e.g., Eliska KWS and Jantar). Among all cultivars tested, Eliska KWS consistently demonstrated high root yield and sucrose content. The sucrose content in the roots was strongly influenced by weather conditions in each study year. In 2021, which had average water availability, sucrose content was high, and delaying the harvest led to an increase in sucrose content while reducing molasses-forming elements in the roots. In contrast, in 2020, during summer rainfall, the effect of harvest date on quality traits was significantly weaker and largely dependent on the cultivar. These findings underscore that choosing the optimal harvest date is highly site- and season-dependent, shaped by precipitation distribution, late-season temperatures, and cultivar genotype. In practical terms, these results can help producers and sugar processors align harvest schedules with local conditions to optimize both root yield and technological quality.

1. Introduction

Sugar beet (Beta vulgaris L.) is the second most important global source of sucrose, with Europe accounting for 67.7% of sugar beet-based production over the last decade [1]. Due to its high sucrose concentration, sugar beet contributes approximately 14% of worldwide sugar output [2]. Root yield and quality are influenced by numerous factors, including cultivar, agronomic practices, weather conditions, phytopathological pressures, and harvest timing [3,4,5,6]. The technological quality of sugar beet is typically defined by sucrose content and the concentration of molasses-forming elements (e.g., K, Na, and α-amino N), which negatively impact extraction efficiency in factories [7,8].

Many studies have demonstrated that delaying sugar beet harvest can enhance both root yield and sucrose content [9,10,11,12,13,14]. However, excessively late harvests may lead to increased concentrations of molasses-forming elements—such as K, Na, and α-amino N—offsetting the gains in sucrose content [15]. The extent to which roots benefit from extended field time is cultivar-dependent and strongly influenced by climatic conditions [15,16]. For farmers, the primary goal is to maximize root yield and sucrose content, whereas processors prioritize high extractable sucrose levels and minimal impurities [17,18]. Extended growing periods and delayed harvests generally reduce K and Na concentrations in the roots, improving juice purity [19,20,21]. However, these processes are highly weather-dependent [22,23]. Water shortages restrict sucrose accumulation and hinder plant development, leading to reduced root mass and lower overall yield. While strong sunlight promotes sucrose accumulation, when combined with high temperatures, it creates favorable conditions for Cercospora beticola infection and accelerates sucrose degradation [24]. Photosynthesis, which drives sucrose synthesis, continues until the end of the beet’s vegetative phase. The rate of sucrose accumulation depends on both the duration and intensity of leaf exposure to sunlight [25,26].

In Central and Eastern Europe, the sugar beet harvest period extends from mid-August to December, primarily dictated by the need to supply factories with fresh raw material [27,28,29]. Optimal harvest timing is determined by the rate of yield and sucrose accumulation, which is strongly affected by seasonal precipitation and temperature [30]. In rainy years, the primary growth phase may shift or extend, whereas dry summer periods can delay peak yield accumulation until autumn [9,27]. Cultivars differ in drought tolerance, disease resistance, and assimilation capacity, requiring cultivar-specific insights to determine the ideal harvest window for both farmers and processors [31].

Despite extensive research, limited information is available on how modern sugar beet cultivars adapt to different precipitation patterns and delayed harvest scenarios in Central and Eastern Europe. Therefore, this study aimed to examine yield formation and technological traits (sucrose content and molasses-forming elements) in nine sugar beet cultivars, assessing the impact of harvest delays from early September to early November under variable seasonal conditions.

2. Materials and Methods

2.1. Experimental Site and Weather Data

Field experiments were conducted during the 2020–2021 growing seasons at the Agricultural Research Station of the Bydgoszcz University of Science and Technology in Minikowo (53°10′ N, 17°44′ E), Poland. According to the World Reference Base (WRB) system, the local soil is a loamy sand Luvisol (LV) [32]. Soil analysis revealed 143.9 mg·kg⁻¹ available P, 240.7 mg·kg⁻¹ available K, and a pH_KCl of 6.3.

Long-term climate data (2000–2019) indicate a mean annual air temperature (MAAT) of 8.9 °C and mean annual precipitation (MAP) of 526.6 mm. Regarding the study period, 2020 was 0.8 °C warmer than the 20-year average, with total precipitation of 557.9 mm, whereas 2021 was 0.4 °C cooler than average, with 430.4 mm of total precipitation. Monthly precipitation and temperature data were obtained from the Chrząstowo meteorological station, part of the official Polish state network operated by the Institute of Meteorology and Water Management (IMGW-PIB)—located 8 km from the experiment site.

2.2. Experimental Design and Agronomic Operations

A split-plot experimental design with four replicates was used. The first factor was cultivar, with nine sugar beet cultivars: Zeltic, Pacific, Mariza, Everest, BTS2205N, Jaromir, Jantar, Eliska KWS, and Klara. These cultivars were selected from five different breeders to evaluate genetically diverse lines under uniform conditions. Introduced to the market between 2016 and 2020, they displayed various declared genetic resistances to diseases and nematodes, thereby allowing the assessment of how different genotypes responded to varying harvest dates. The second factor was harvest date:

I: 8–10 September;

II: 7–8 October;

III: 3–5 November.

Each plot measured 26.64 m², with plant density of 11 sugar beet plants·m⁻² and 45-cm row spacing. Sowing was carried out in early April each year (6 April 2020 and 5 April 2021) to ensure timely emergence and a sufficient growing season.

Fertilization consisted of 130 kg N ha⁻¹, 80 kg P₂O₅ ha⁻¹, and 120 kg K₂O ha⁻¹. Approximately 60% of the nitrogen was applied before sowing as urea, with the remainder applied at the 4–6 leaf stage in the form of ammonium nitrate. Weed management involved an initial pre-emergence application of a metamitron-based herbicide (1400 g active ingredient (a.i.) ha⁻¹), followed by three post-emergence treatments targeting broadleaf weeds with a mixture of 700 g a.i. ha⁻¹ metamitron, 300 g a.i. ha⁻¹ ethofumesate, 160 g a.i. ha⁻¹ phenmedipham, 100 g a.i. ha⁻¹ clopyralid, 50 g a.i. ha⁻¹ lenacil, and 10 g a.i. ha⁻¹ triflusulfuron applied when newly emerged Chenopodium album L. plants were at the cotyledon stage. Grass weeds were controlled at the 2-leaf stage using quizalofop-p-ethyl at 50 g a.i. ha⁻¹. Fungicides containing 125 g a.i. ha⁻¹ azoxystrobin and 125 g a.i. ha⁻¹ difenoconazole were applied twice (late June and mid-July) to manage Cercospora beticola and other foliar diseases, following local recommendations and label guidelines. These agronomic practices were implemented and uniformly applied across all plots to isolate the effects of cultivar and harvest timing.

2.3. Sample Collection and Laboratory Analyses

At each harvest date, a 2.22·m⁻² area per plot was manually harvested from four replicates (n = 4). Tops were removed using a cutting tool, and roots were weighed in the field to determine root yield (Mg·ha⁻¹). A random subsample of 20 roots per plot was then transported to the Kutno Sugar Beet Breeding Company laboratory for quality analysis according to International Commission for Uniform Methods of Sugar Analysis (ICUMSA) guidelines.

2.3.1. Sucrose Determination

Sucrose content was determined using the polarimetric method with an aqueous aluminum sulfate digestion of beet pulp, as described in ICUMSA Method GS6-3 [33]. Results are reported as a percentage of fresh weight.

2.3.2. Determination of Melassogenic Elements

The Na and K content in sugar beet pulp was determined using the flame photometry method [34], which is commonly used to measure Na and K concentrations in pulp after aqueous extraction, with aluminum sulfate serving as a clarifying agent.

The principle of the method is based on measuring the intensity of radiation emitted by excited atoms of the analyzed elements. In practice, the elements present in the solution after pulp digestion are brought into an atomic state by the thermal energy of a flame. This excitation causes the free atoms to emit radiation at characteristic wavelengths within the visible range (K: 766.5 nm; Na: 589 nm). The intensity of this emitted radiation is directly proportional to the concentration of the analyzed element in the solution.

The α-amino N content, which serves as an indicator of the soluble nitrogen level in sugar beet pulp, was determined using the ICUMSA GS6-5 method [35]. After digesting the sample, a solution of copper nitrate and sodium acetate was added to the extract, maintaining a pH of 6.0. These reagents react with α-amino N, forming a characteristic blue complex. The absorbance of this complex is then measured at a wavelength of 610 nm, allowing for the determination of the α-amino N concentration in the sample. The concentration value was calculated based on a previously prepared calibration curve, using glutamine as the reference standard.

2.4. Statistical Analysis

Data from 2020 and 2021 were analyzed separately before being combined for joint analyses. A two-way analysis of variance (ANOVA) was performed, with cultivar and harvest date as factors, following a split block design in four replications. It was assumed that the distribution of results within the analyzed groups approximated a normal distribution, which was verified using the Shapiro–Wilk test. The homogeneity of the variances was confirmed by using Levene’s test. Means were compared by Tukey’s Honestly Significant Difference (HSD) test at p < 0.05 when the results of the F-test were statistically significant. Pearson correlation coefficients were calculated to assess relationships between yield and quality components (sucrose, K, Na, and α-amino N). All statistical computations and figures were performed using STATISTICA v.13 (TIBCO Software Inc., Palo Alto, CA, USA) [36].

3. Results

3.1. Weather Condition

Precipitation in 2020 was highly unevenly distributed (Figure 1). After near-normal winter months and a moderately wet February (42.1 mm vs. the multi-year average of 27.2 mm), April was extremely dry (4.2 mm vs. 26.1 mm), which may have negatively impacted sugar beet emergence and initial growth. However, rainfall increased significantly in June (165.5 mm vs. 73.0 mm) and August (104.7 mm vs. 77.0 mm), partially compensating for earlier moisture deficits. As a result, the 2020 season was characterized by periods of both drought and excessive moisture at key stages of plant development.

In contrast, 2021 brought above-average rainfall as early as January (49.2 mm vs. the multi-year average of 35.8 mm), followed by higher-than-average precipitation in April and May, which theoretically met the crop’s spring water requirements. However, from June to September, precipitation was notably below average. For instance, in July, August, and September, rainfall remained well below the water requirements of sugar beet, with precipitation amounts of 45.7 mm, 34.1 mm, and 25.0 mm, respectively, compared to the crop’s required 70–80 mm per month. As a result, plants experienced prolonged water stress during the critical period of intensive root mass accumulation.

In 2020, winter temperatures (January–March) remained notably above the multi-year average (Figure 2). The most pronounced positive anomaly occurred in February (3.7 °C vs. the typical −0.3 °C). However, spring (April–May) proved cooler than usual: in April, temperatures averaged 8.0 °C (compared to 8.5 °C normally), and in May, 10.8 °C vs. the long-term average of 13.2 °C. During the summer of 2020, temperatures fluctuated around typical regional values: June was slightly warmer (17.7 °C vs. 17.3 °C), July was cooler (18.2 °C vs. 19.4 °C), and August slightly exceeded the norm (19.7 °C vs. 19.1 °C). In autumn (September–November) and in December 2020, average monthly temperatures were generally higher than long-term means; for instance, November recorded at 5.7 °C compared to the multi-year average of 4.3 °C, resulting in mild thermal conditions at the end of the year.

In contrast, early 2021 was characterized by below-average temperatures. January and February (−2.7 °C vs. −0.3 °C) were colder than the multi-year average. April and May were also significantly cooler (5.8 °C and 11.6 °C, respectively, vs. norms of 8.5 °C and 13.2 °C). The summer of 2021 was characterized by high temperatures, particularly in June (19.1 °C vs. 17.3 °C) and in July, where the monthly average (22.5 °C) significantly exceeded the multi-year norm (19.4 °C). August, however, was cooler than usual (17.1 °C vs. 19.1 °C), while September and October returned to near-normal values. November was slightly warmer than average (4.9 °C vs. 4.3 °C), whereas December was once again colder than usual (−1.4 °C vs. 0.82 °C).

3.2. Root Yield

In 2020, high root yields were recorded, averaging 87.67 Mg ha⁻¹. The highest yields were obtained for BTS 2205N (91.3–113.6 Mg ha⁻¹) and Eliska KWS (102.2–109.0 Mg ha⁻¹), with similar yields recorded for Jaromir, Everest, and Zeltic. In contrast, Jantar and Pacific produced significantly lower yields, except at the earliest harvest date. Overall, sugar beet root yield was influenced by harvest timing (Figure 3). However, individual cultivars showed distinct responses each year.

In 2020, delaying the harvest from 10 September to 8 October resulted in decreased yields for BTS2205N, Jaromir, and Klara. The remaining cultivars did not show significant yield changes at this later date. The yields of Zeltic, Pacific, Mariza, Everest, and Jantar remained unaffected by the harvest date in the first year. Extending the harvest from 8 October to 5 November increased yields in BTS2205N and Klara, with no significant changes in the other cultivars. None of the studied cultivars yielded better on 5 November than on 10 September.

In 2021, the average root yield was approximately 6% lower than in the first year, yet it remained higher than typical production yields in the study region. The most productive cultivar was Eliska KWS (86.0–115.2 Mg ha⁻¹), while Pacific and Jantar produced significantly lower yields. Similarly, Zeltic, Mariza, Everest, BTS2205N, Jaromir, and Klara also had lower yields at the third harvest date. During the second year, each delay in harvest significantly increased the yields of Eliska KWS. Jantar yielded significantly higher in November than in September, whereas Klara, Jaromir, and Pacific presented improved yields when harvest was postponed from September to October. The other cultivars showed no significant yield response to harvest timing in 2021.

Over the entire study period, Eliska KWS consistently produced the highest root yield, which was comparable to that of Zeltic, Everest, BTS2205N, and Jaromir at the first and second harvest dates. However, at the third harvest date, yields of all cultivars were significantly lower. Delaying the harvest date generally did not affect the two-year average root yield in most cultivars; only Eliska KWS and Jantar showed a significant yield increase when harvest was postponed from early October to early November.

3.3. Sucrose Content

Sucrose content in sugar beet roots depended on both the cultivar and the harvest date but was primarily influenced by weather conditions in each study year (Figure 4). In 2020, sucrose content ranged from 14.5% at the first harvest date to 14.9% at the third, and from 14.2% in the BTS2205N, Jaromir, Klara, and Zeltic cultivars to 16.0% in Eliska KWS. Regardless of harvest date (except for Jantar at the earliest harvest), Eliska KWS showed a significantly higher sucrose content than the other cultivars. For Mariza, Everest, and Jaromir, sucrose content at the third harvest date was significantly higher than at the first and second. In Eliska KWS, sucrose content was significantly higher at the second and third dates than at the first, while in Zeltic, it was significantly higher at the second date compared to both the first and third. In contrast, the Pacific, BTS2205N, Jantar, and Klara cultivars showed no significant effect of harvest date on sucrose content in 2020.

In 2021, the effect of cultivar on sucrose content was less pronounced, while the impact of harvest date was more significant than in 2020. All cultivars had the highest sucrose content at the final harvest and the lowest at the first. However, some cultivars (Mariza, Everest, BTS2205N, Jaromir, and Jantar) did not show an increase in sucrose content when harvest was postponed from September to October. Jaromir had significantly lower sucrose content than Pacific and Klara at each harvest date, as well as lower sucrose content than Eliska KWS at the second and third harvest dates.

Across the two-year study period, Eliska KWS consistently demonstrated the highest average sucrose content, which was comparable to that of Pacific, Mariza, Everest, BTS2205N, Jantar, and Klara at the first harvest date. Delaying the harvest from September to October resulted in a significant increase in sucrose content for Zeltic and Eliska KWS, whereas postponing it from October to November led to a significant increase in sucrose content in Pacific, Mariza, Everest, BTS2205N, Jaromir, Jantar, Eliska KWS, and Klara.

3.4. Potassium Content

The K content in sugar beet roots was significantly influenced by both experimental factors and varied between the two study years (Figure 5). In 2020, the Jantar cultivar had the lowest K content in storage roots at the second and third harvest dates. At the first harvest date, the K content was similar in most cultivars, but Zeltic had a significantly higher level. Harvest timing had a distinct effect on each cultivar. In BTS2205N, Jaromir, Jantar, Eliska KWS, and Klara, delaying the harvest from September to October led to a reduction in root K content, whereas no significant changes were observed in the remaining cultivars. Extending the harvest from October to November resulted in an increase in K content in Klara only, while other cultivars did not exhibit a significant response.

In 2021, postponing the harvest from September to October caused a significant decrease in K content across all tested cultivars. A further delay led to an additional reduction in K content only in Mariza and Klara. At the first harvest date, the lowest K levels were recorded in Pacific, BTS2205N, and Klara, while Zeltic, Everest, and Jaromir had significantly higher values. Jaromir also maintained elevated K content at the second and third harvest dates compared to most other cultivars, except Mariza and Eliska KWS at the second harvest date.

Averaged over both years, the lowest K content was recorded in Pacific and Klara, whereas Zeltic and Jaromir had significantly higher levels. Delaying the harvest from September to October significantly reduced K content in most cultivars, except for Pacific and Jaromir. Postponing the harvest from October to November further reduced K content only in Eliska KWS cultivar.

3.5. Sodium Content

The Na content in sugar beet roots was significantly influenced by both experimental factors (Figure 6), although plant responses to these factors varied across the study years. In 2020, Eliska KWS had the lowest Na content (5.0 mmol kg⁻¹) in storage roots, while Jaromir and—at the second and third harvest dates—Jantar had the highest. A significant interaction was observed between cultivar and harvest date. In Zeltic, Pacific, Mariza, Everest, and Jantar, delaying harvest from September to October led to an increase in Na levels, whereas the other cultivars were not significantly affected. Extending the harvest from October to November resulted in a reduction in Na content in Everest and Eliska KWS roots, but it did not significantly influence the remaining cultivars.

In 2021, postponing the harvest from September to October markedly decreased Na content in the roots of all cultivars (Figure 6). An additional delay further reduced Na content only in Everest and Eliska KWS. At the first harvest date, Pacific and BTS2205N had the lowest Na levels, while Jantar and Eliska KWS had significantly higher values. At the second harvest date, Eliska KWS still contained significantly more Na than Pacific, BTS2205N, Zeltic, Everest, and Klara. By the third harvest date, BTS2205N and Everest had the lowest Na content, whereas Zeltic, Jaromir, Jantar, and Eliska KWS had significantly higher levels.

Across the two-year study period, Eliska KWS had the lowest average Na content, which was comparable to that of Zeltic, Pacific, Mariza, Everest, BTS2205N, and Klara at the first harvest date. Delaying the harvest from September to October significantly reduced Na levels in Everest, BTS2205N, and Eliska KWS, while postponing the harvest from October to November further decreased Na content in the same three cultivars. At the second and third harvest dates, Eliska KWS again had the lowest Na content, followed by Zeltic, Pacific, Mariza, and Klara, while Jaromir and Jantar had the highest value.

3.6. Content of α-Amino Nitrogen

The α-amino N content in sugar beet roots was significantly influenced by both cultivar and harvest date each year (Figure 7), with plant responses varying across the study years. In 2020, Jantar had the lowest α-amino N levels (14.4 mmol kg⁻¹), whereas Zeltic had significantly higher levels (22.2 mmol kg⁻¹). The remaining cultivars generally did not differ significantly from either of these extremes. A significant interaction was observed between cultivar and harvest date: in Jaromir, Jantar, and Eliska KWS, delaying harvest from September to October led to a reduction in α-amino N content, while no such effect was observed in the other cultivars. Extending the harvest from October to November did not significantly affect α-amino N levels in any of the tested cultivars.

In 2021, as in the first year, Jantar had the lowest α-amino N content (12.8 mmol kg⁻¹), while BTS2205N had the highest (23.2 mmol kg⁻¹). Eliska KWS and Klara also had relatively low α-amino N levels, followed by Zeltic and Pacific at the second and third harvest dates. Delaying harvest from September to October significantly reduced α-amino N content across all cultivars, and an additional postponement further decreased α-amino N levels in BTS2205N, Jaromir, and Eliska KWS.

Over the two-year study period, Jantar consistently had the lowest α-amino N content, which was comparable to that of Eliska KWS and Klara at certain harvest dates (first and third). BTS2205N had the highest α-amino N levels overall. Delaying harvest from September to October resulted in a notable decrease in α-amino N content for most cultivars (except Mariza), while postponing it from October to November led to a further reduction in α-amino N content in BTS2205N, Jaromir, and Eliska KWS.

3.7. Correlations Among Yield and Quality Traits

In the current study, various relationships were observed between root yield (Y) and quality traits of sugar beet, including sucrose content (S), potassium content (K), sodium content (Na), and α-amino nitrogen content (αN). Additionally, significant correlations were found between the quality traits themselves (

Table 1. Correlation coefficients (Pearson correlation) of sugar beet yield and quality characteristics depending on the year and harvest date.

Harvest Date	2020			2021
	I	II	III	I	II	III
Y × S	ns.	0.4286	ns.	ns.	ns.	ns.
Y × K	0.3662	0.4648	ns.	0.5041	ns.	ns.
Y × Na	ns.	−0.4457	−0.5270	ns.	ns.	ns.
Y × αN	ns.	ns.	ns.	ns.	ns.	ns.
S × K	ns.	ns.	−0.3833	−0.5845	−0.7283	−0.8294
S × Na	−0.5289	−0.6068	−0.4975	−0.4878	ns.	−0.4679
S × αN	ns.	ns.	−0.4449	ns.	ns.	ns.
K × Na	ns.	−0.3528	ns.	0.6288	0.6044	0.5401
K × αN	0.6965	0.8361	0.7916	ns.	ns.	ns.
Na × αN	ns.	ns.	ns.	ns.	ns.	−0.6878

Harvest dates: 8–10 September—first date; 7–8 October—second date; and 3–5 November—third date. Y—root yield; S—sucrose content; K—potassium content; Na—sodium content; αN—α-amino nitrogen content; ns.—non-significant correlation. r-values are significant at the 0.05 level, Pearson correlation.

). In 2020, root yield was positively correlated with sucrose content at the second harvest date. A positive correlation was also found between root yield and K content at the first and second harvest dates, while the relationship between root yield and Na content was negative at the second and third harvest dates. In 2021, root yield was positively correlated with K content only at the first harvest date and showed no significant relationship with Na, α-amino N, or sucrose content.

A negative correlation was observed between sucrose and Na content, where an increase in Na was generally associated with a decrease in sucrose content. A negative relationship was also found between K and sucrose content, but the strength of this correlation varied depending on the year and harvest date. In 2020, a negative correlation between K and sucrose content was observed only at the third harvest date, but the strength of this relationship was weak. In 2021, at the first harvest date, the correlation was moderately negative, while at the second and third harvest dates, the correlation between sucrose and K content was strongly negative. A very strong positive correlation was observed between K and α-amino N, particularly in 2020, suggesting that the accumulation of these components often occurred simultaneously. The correlation between K and Na varied over the years. In 2020, a negative correlation was recorded at the second harvest date, whereas in 2021, a positive correlation was observed across all harvest dates. Additionally, in 2021, Na and α-amino N content were negatively correlated at the third harvest date.

4. Discussion

The timing of cultivar harvest, in combination with year-specific weather patterns, plays a key role in determining both sugar beet root yield and technological quality, aligning with previous reports [25,38,39]. Particularly favorable conditions in 2020 resulted in high root yields, averaging 87.67 Mg ha⁻¹. This value exceeded the 2021 average by approximately 6%, with the difference at the first harvest date reaching 26%, primarily due to abundant rainfall in June and August 2020. The strong impact of weather factors, particularly total rainfall in July and August and temperatures in September and October, on root yield and sucrose content has also been highlighted by Bönecke et al. [40], Rempelos et al. [41], and Macholdt et al. [42], who reported that climatic factors can overshadow agronomic inputs. The present findings confirm that year-to-year variability remains a major driver of production outcomes, consistent with previous research. Fitters et al. [43] also observed fluctuating yields under variable rainfall conditions, supporting the observed trend that differences in rainfall distribution and timing relative to critical growth stages significantly influence yield variation.

Furthermore, previous studies emphasize that sugar beet genotypes differ in their capacity to adapt to diverse climatic conditions, thus explaining the distinct patterns of yield accumulation observed among cultivars under different weather scenarios [44,45,46]. The present investigation is broadly consistent with these findings, but it also reveals that even cultivars considered adaptable (e.g., Eliska KWS) can experience large yield fluctuations when rainfall occurs outside their most responsive growth phases. This genotype × environment interaction, although discussed in prior studies, is rarely explored in the context of extreme monthly differences in precipitation.

Significant intraspecific differentiation was observed. Eliska KWS produced the highest yields, reaching up to 115.2 Mg ha⁻¹, while at specific harvest times, Zeltic, Everest, and BTS2205N also achieved high yields. These results align with studies indicating that yield stability and resistance to environmental stress depend on genotype-specific traits and interactions with local conditions [47]. In contrast, Mohammadi-Ahmadmahmoudi et al. [48] reported narrower yield gaps among modern cultivars under optimal conditions. However, the present study indicates that in years featuring climatic extremes (e.g., excessive rainfall in certain months), yield gaps can widen substantially, particularly at early or late harvest dates, emphasizing more pronounced genetic differentiation than might be observed under uniformly favorable conditions.

Meanwhile, certain cultivars (e.g., Jantar and Pacific) exhibited lower yields, particularly at later harvest dates, suggesting a limited capacity for continued root growth at the end of the growing season, especially under conditions favoring early biomass accumulation, such as those in 2020. These cultivars appear more sensitive to specific weather and phytopathological challenges. Such genotype-specific vulnerabilities align with previous studies [49,50], which demonstrated that water stress and high fungal disease pressure can substantially reduce yields in susceptible cultivars.

The response of individual cultivars to postponed harvest dates varied depending on the year. In 2020, BTS2205N, Jaromir, and Klara exhibited yield reductions when harvest was delayed from September to October, whereas in 2021, Eliska KWS and Jantar recorded significant yield increases following similar delays. The different yield accumulation dynamics across cultivars likely resulted from variations in rainfall distribution. In 2020, exceptionally high rainfall in June and August accelerated yield accumulation, leading to an average yield of 92.8 Mg ha⁻¹ by September 5, surpassing all other harvest dates in both study years. Weather patterns also influenced fungal pathogen pressures, notably with a high incidence of Cercospora beticola in August 2020, which significantly reduced leaf area and inhibited further yield growth in September. This decline in root yield was particularly pronounced in cultivars BTS2205N, Jaromir, and Klara, previously characterized by low resistance to Cercospora infection [51]. Similar findings were reported by Pavlů et al. [12], who demonstrated that the length of the growing season significantly affects both biomass accumulation and root quality parameters. However, the cultivar × disease interaction observed in this study appears to be more pronounced, likely due to a concentrated outbreak period in August, which may differ from the conditions examined by Bhuyian et al. [52], who analyzed a broader range of infection windows.

In line with the present study, Pavlů et al. [12] also demonstrated that genetically identical cultivars can yield contrasting results in terms of root mass and sucrose concentration, depending on the growing environment and harvest timing. In this study, under the relatively lower but more evenly distributed rainfall of 2021, the average yield at the first harvest date was only 73.7 Mg ha⁻¹ across all tested cultivars. However, for most cultivars (Pacific, Jaromir, Jantar, Eliska KWS, and Klara), yields increased significantly when harvest was postponed from September to October.

Sucrose content in roots also varied significantly across cultivars and harvest dates, but it was strongly influenced by weather patterns. In 2020, which was characterized by high rainfall totals in August, sucrose content averaged 14.7% and was largely unaffected by harvest timing. In contrast, in 2021—when rainfall was low and sporadic—sucrose levels were, on average, 4 percentage points higher, increasing significantly with later harvest dates. These findings corroborate earlier research [38,53], which suggests that reduced water availability and higher temperatures can enhance sucrose concentration, albeit sometimes at the expense of root yield. In 2020, unusually high rainfall in June and August stimulated rapid biomass accumulation but did not proportionally increase sucrose accumulation. This discrepancy may have been caused by subsequent disease pressure or nutrient imbalances. This interplay of climatic and phytopathological factors underscores the context-specific nature of root growth and sucrose accumulation [54].

Results concerning the accumulation of molasses-forming components (K, Na, and α-amino N) indicate that their levels in roots were primarily influenced by environmental conditions (year) and cultivar. The tendency of these elements to decrease with later harvest dates aligns with previous research on fertilization and its impact on the technological parameters of sugar beet [55]. However, in 2020, some cultivars exhibited unexpected responses, similar to the pattern observed for sucrose content. Zeltic, Pacific, Mariza, Everest, and Jantar had higher Na contents when harvest was postponed from September to October, whereas in the remaining cultivars, harvest timing did not significantly affect Na levels. A similar trend was observed for other molasses-forming elements in 2020, where harvest timing generally had no effect, although Jaromir, Jantar, and Eliska KWS behaved as expected. In 2021, all cultivars showed a decrease in Na, K, and α-amino N levels, as harvest was postponed from September to October, confirming the well-established negative correlation between Na and K levels and sucrose concentration in roots. Previous research also suggests that a longer growing period enhances the translocation and processing of nitrogen forms, thereby reducing molasses-forming α-amino N concentrations [56].

The present study indicates that the relationship between root yield and sucrose content is not straightforward. In 2020, sugar beet yield was positively correlated with sucrose content only at the second harvest date, whereas no significant correlation was observed at other dates or in any harvest date in 2021. Past studies have reported conflicting results. Aly et al. [57] observed a negative correlation between root yield and sucrose concentration, but a positive correlation between root yield and sucrose yield. Conversely, Tsialtas and Maslaris [58] found that higher root yields can coincide with increased sucrose content, provided that nitrogen fertilization is adequately managed, as excessive nitrogen application reduces sucrose concentration. The present findings partially align with both perspectives. In certain conditions (e.g., the second harvest date of 2020), higher yields were associated with increased sucrose content. However, in other scenarios, particularly under disease pressure or erratic rainfall, this correlation was not observed. Deviations from the commonly cited negative yield–sucrose correlation may also be attributed to Cercospora beticola infection, which reduces leaf area, forcing the plant to redirect resources toward foliage regeneration at the expense of sucrose accumulation in roots [59,60,61]. Thus, while broadly consistent with existing research, the current results emphasize the complexity introduced by localized weather extremes and pathogen outbreaks, which can override or obscure typical yield–sucrose relationships.

The present findings align with those of previous studies: in 2020, sugar beet yield was positively correlated with sucrose content at the second harvest date. The lack of correlation at other harvest dates may be attributed to Cercospora leaf spot outbreaks or excessive nitrogen fertilization, as indicated by elevated α-amino N content observed at the first harvest date in 2021. Notable differences in correlations between K and Na contents and root yield were also observed, varying by year and harvest date. In 2020, root yield correlated positively with K content at the first and second harvest dates, whereas in 2021, this relationship was observed only at the first harvest date. Additionally, in 2020, Na content was negatively correlated with yield at the second and third harvest dates. These findings are consistent with the results of Yasin [62], who reported that higher potassium fertilization enhances yield and technological quality, while excess sodium can negatively impact sucrose content. Similarly, Tsialtas and Maslaris [58] highlighted that preferential K uptake over Na, which is critical in saline soil conditions, is associated with higher sucrose levels and improved root quality. The present study confirms these broader trends, while also emphasizing that cultivar-specific responses can vary significantly, particularly under extreme conditions such as heavy rainfall or disease outbreaks, which were less pronounced in some of the previously cited studies. Our analysis of the present dataset revealed a negative correlation between sucrose content and both Na and K; however, the strength of this relationship varied by year and harvest date. These findings are in agreement with those of Ata [63], who demonstrated that elevated Na and K concentrations reduce sucrose content and juice quality. Furthermore, Bocianowski et al. [64] confirmed that higher Na and α-amino N levels lead to increased sucrose losses during extraction, thereby diminishing the raw material’s quality for sugar processing.

A strong positive correlation was identified between K and α-amino N contents in 2020, consistent with the findings of Maslaris et al. [65], who suggested that α-amino N accumulation is associated with higher K levels in soil and plants, particularly under intensive nitrogen fertilization. Meanwhile, research by Rehab et al. [66] indicates that appropriate fertilization with boron and humic acids can reduce α-amino N and Na levels, thereby enhancing the technological quality of sugar beet processing. This suggests that the accumulation of these molasses-forming elements is closely linked to the overall nutritional status of the crop. While the direction of these relationships aligns with the existing literature, the magnitude observed in this study—particularly in a wet year—suggests that interactions among fertilization, rainfall, and disease pressure may be even more substantial than previously reported in drier or more moderate environments.

The potential impacts of climate change—including more frequent droughts, extreme rainfall events, and increased disease pressure—may significantly affect sugar beet production. The current study, along with previous reports [41,43,52], indicates that cultivars differ not only in yield potential but also in their resilience to abiotic (e.g., water deficits and temperature fluctuations) and biotic (e.g., fungal diseases) stresses. The high yields observed in Eliska KWS under certain conditions demonstrate how breeding programs can develop cultivars with relatively stable performance, even in unpredictable weather scenarios. In contrast, cultivars such as Jantar and Pacific appear more vulnerable to late-season stress, highlighting the importance of careful cultivar selection in regions prone to water shortages or pathogen outbreaks.

With ongoing climatic changes, farmers will benefit from selecting cultivars with proven resilience to diverse environmental conditions—whether through tolerance to early-season drought, resistance to leaf-spot pathogens, or the ability to maintain productivity under fluctuating soil moisture [67,68]. In many countries, official recommended lists increasingly emphasize adaptability traits, helping growers choose cultivars better suited to local or regional climate forecasts [69,70,71]. However, beyond cultivar selection, optimized agronomic practices—including precise fertilization and timely plant protection—are equally essential for sustaining yields and preserving sucrose content [72].

In the long term, breeding programs should place greater emphasis on improving water- and nutrient-use efficiency, along with enhanced disease resistance, to address the challenges posed by a changing climate. Since sugar beet production relies on both high biomass accumulation and high sucrose content, an integrated approach that combines genetic, agronomic, and environmental management strategies will be essential. By optimizing cultivar selection and refining cultivation practices, the sugar beet sector can better mitigate the risks associated with climate variability and ensure the long-term economic viability of production. Overall, the present findings highlight the complexity of managing sugar beet harvest timing in relation to weather patterns, disease pressure, and cultivar-specific traits. As climate extremes become more frequent, selecting cultivars that combine high yield potential with strong stress tolerance will be critical to maintaining stable and profitable sugar beet production.

5. Conclusions

Both cultivar selection and harvest timing had a significant, albeit variable, impact on sugar beet root yield across the study years. In a season characterized by very high total precipitation in June and August, maximum root yield was already achieved at the earliest harvest period (8–10 September). Delaying harvest to 7–8 October or 3–5 November did not significantly affect yield in most cultivars; however, in BTS 2205N, Jaromir, and Klara, later harvest dates resulted in a marked reduction in root yield. In contrast, under moderate and more evenly distributed rainfall, the root yield of most cultivars increased when harvest was postponed from 8–10 September to 7–8 October. For Eliska KWS, yields continued to rise even with further postponement to 3–5 November.

Weather conditions strongly influenced sucrose content in sugar beet roots each year, although both experimental factors (cultivar and harvest date) also played a significant role. Among the tested cultivars, Eliska KWS consistently had the highest sucrose content. Delaying harvest increased sucrose content only in 2021, and only in specific cultivars (Pacific, Jaromir, Jantar, Eliska KWS, and Klara). In a year with average water availability, postponing harvest from September to October reduced the concentration of molasses-forming elements in sugar beet roots. However, in a year characterized by high summer rainfall (2020), the effect of harvest timing on these compounds was significantly weaker and varied depending on the cultivar.