Industrial Hemp Variety Performance in Latvia Under Baltic Sea Climate

Abstract

As the world shifts towards more sustainable and eco-friendly practices, industrial hemp (Cannabis sativa L.) is gaining recognition as a versatile crop with numerous applications. The Baltic Sea region is well-suited for hemp cultivation, with its temperate climate and varied soil types. This study evaluates the suitability of various hemp varieties for the region, focusing on their ability to produce high-quality biomass, fibers, seeds, and dual-purpose products. The findings will contribute to the development of a thriving hemp industry in the region. Five years of research was conducted to investigate the productivity of 12 industrial hemp varieties, including 7 varieties mainly developed for seed production and 5 varieties mainly designed for fiber production. The results showed significant differences in yields among the varieties, with ‘Bialobrzeskie’ exhibiting the highest biomass yield (47.2 t ha⁻¹) and ‘Futura 75’ producing the highest fiber yield (10.8 t ha⁻¹). ‘Henola’ demonstrated the highest seed yield (3.5 t ha⁻¹), while ‘KA-2-2011’ and ‘USO-31’ were identified as dual-purpose varieties suitable for fiber (3.4 and 6.4 t ha⁻¹, respectively) and seed production (2.2 and 1.3 t ha⁻¹, respectively). The calorific value of hemp shives is comparable to wood fuels, indicating their potential as a viable fuel source. The results offer farmers a crucial tool for selecting the best-suited varieties for their specific region, promoting sustainable agriculture practices.

1. Introduction

Industrial hemp (Cannabis sativa L.) has gained significant attention in recent years due to its versatile uses in various industries such as textiles, paper, biodegradable plastics, construction materials, food and beverages, medicine products, and even cosmetics, as well as biofuels and biopesticides [1,2,3,4,5,6,7,8]. Figure 1 shows the broadest areas of use of different parts of hemp. Hemp cultivation is also environmentally friendly as it requires minimal water, pesticides, and fertilizers compared to other crops [9]. Moreover, hemp also has the potential to sequester carbon dioxide, making it a promising candidate for sustainable agriculture practices [4,10,11]. According to research-based calculations, hemp produces 4 to 10 times more biomass than wood biomass per hectare per year. The high cellulose content of hemp stems, which is 2–3 times higher than that of wood [12], and wood-equivalent cellulose content in the by-product of hemp fiber production—hemp shives [13]—makes them an attractive source of pulp.

Furthermore, the functionalization of hemp fibers and its products is a growing trend [15,16,17,18], offering a new avenue for sustainable economic growth based on renewable resources [9,14]. Also, due to the rapid growth in biomass use, its high CO₂ capture, and high energy potential, hemp is a good source of an alternative, renewable fuel [19]. As an eco-friendly crop, hemp cultivation can potentially contribute to sustainable economic development [20]. Despite the numerous benefits of hemp cultivation, the yield and quality of fiber and seeds can vary significantly depending on several factors, such as the varieties, environmental conditions, soil type, applied fertilizers, and agricultural practices [10,11,21].

In countries bordering the Baltic Sea, hemp cultivation has gained interest due to the region’s suitable climate and high-quality soil [22]. However, there is limited research on hemp varieties’ fiber and seed productivity, specifically under Baltic Sea conditions. Therefore, this publication evaluates the fiber and seed productivity of industrial hemp varieties grown under Baltic Sea conditions. The Baltic Sea region, consisting of countries such as Denmark, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Germany, experiences a temperate maritime climate with relatively mild temperatures and moderate rainfall. These conditions are conducive to hemp cultivation, which requires a temperate climate with well-distributed rainfall throughout the growing season. Additionally, the region’s fertile soils provide optimal conditions for hemp growth, ensuring high yields of fiber and seeds [23]. Equally important is that northern European countries have markedly longer days and shorter nights during summer, which are very important for vegetative growth and photosynthesis [24,25].

Data on agronomic parameters such as plant height, biomass yield, fiber content, seed yield, and oil content have been collected to evaluate the performance of each variety. The full research data can be accessed in an open format at the following link—DOI: 10.5281/zenodo.13908645.

In addition, the use of hemp shives—a by-product of hemp fiber production—for energy production has been evaluated. Therefore, we analyzed the selected varieties to understand their adaptability and potential for commercial cultivation in the Baltic Sea region.

This information is valuable for farmers, breeders, and policymakers to make informed decisions regarding hemp cultivation and the selection of varieties that are best suited for the region. Industrial hemp cultivation has the potential to thrive under Baltic Sea conditions, providing sustainable solutions for various industries and contributing to environmental conservation. In Latvia, while the primary focus has been put on utilizing hemp seeds, a more sustainable strategy involves leveraging the full potential of the hemp plant through a comprehensive biorefinery process. This approach emphasizes the simultaneous utilization of all hemp plant parts to generate high-value products. For instance, farmers can harvest seeds and fibers concurrently by selecting dual-purpose hemp varieties. The hemp shives can also serve as bedding for livestock or can be converted into an alternative solid fuel source. Extracts from the leaves and flowers have applications in pharmaceuticals and perfumery, while any leftover biomass can be utilized for biofuel production. To effectively implement this full-cycle biorefinery approach, further research is essential to optimize each stage of the process and maximize overall output.

This paper summarizes a five-year (2019–2023) field trial study. Oil yield has been analyzed over four years (2020–2023). Part of this five-year study, as a 3-year investigation (2020–2022) analyzing the effects of climate and weather on the phenological characteristics and yield of hemp, has been published by Stramkale et al. [26]. The complete research data related to this publication can be accessed through the following link—DOI: 10.5281/zenodo.13908645.

This research shows a shift in focus from the previous study. While the earlier investigation was focused on phenological traits and crop yield in relation to weather conditions, this analysis delves into the suitability of specific hemp varieties for cultivation in the Baltic Sea region. It specifically assesses the yield potential of varieties intended for seed and/or fiber production. Unlike prior studies, which reported annual yield values heavily influenced by climatic factors, this study evaluates the average yields across five years. This approach affords a more comprehensive and long-term evaluation of each variety’s adaptability to the region. Notably, this research includes a broader range of hemp varieties, featuring five that were developed in the Baltic States: ‘Adzelviesi’, ‘KA-2-2011’, and ‘Loja’ from Latvia, ‘Estica’ from Estonia, and ‘Austa’, a fiber hemp variety from Lithuania. To the best of our knowledge, these varieties have not been documented in any peer-reviewed literature prior to this analysis.

2. Materials and Methods

A field experiment was conducted at the Agriculture Science Center of Latgale in Vilani (56°34′10″ N, 26°58′01″ E, elevation 115 m), Eastern Latvia, over five growing seasons from 2019 to 2023. The study was conducted in an area with loam humic gleyic podzol soil typical of the Latvian Eastern region. The main agrochemical parameters of the arable soil layer were characterized as having a 7.0 ± 0.2% content of organic compounds, 182 ± 13 mg kg⁻¹ P₂O₅, 139 ± 16 mg kg⁻¹ K₂O, and a pH at 7.2 ± 0.2. The study utilized 12 different hemp varieties, including varieties which were originally intended for obtaining the seeds ‘Adzelvieši’, ‘Pūriņi’, ‘KA-2-2011’, ‘Loja’ (Latvia), ‘Finola’ (Finland), ‘Henola’ (Poland), and ‘Estica’ (Estonia), and varieties initially developed for fiber production: ‘USO 31’ (Ukraine-France), ‘Futura 75’, ‘Futura 83’ (France), ‘Bialobrzeskie’ (Poland), and ‘Austa’ (Lithuania).

Except for ‘Henola’, the hemp seed varieties analyzed in this study are dioecious. In contrast, the seed hemp variety ‘Henola’ and the fiber hemp varieties investigated are monoecious. In dioecious hemp varieties, male flowers began to bloom approximately 40 to 50 days after planting, while female flowers appeared 50 to 60 days following sowing. In contrast, monoecious hemp varieties initiated flowering between 70 and 80 days after sowing. Regardless of the variety, the overall flowering duration lasted between 50 and 60 days.

The field trials were arranged in a randomized complete block design with four replicates for each variety for each year. Each plot size was 25 m². Hemp varieties were sown at the beginning of May (7.-13.05. depending on the year) using an experimental sowing machine (SN-16) (Vologda, Vologda Oblast, Russia) with an inter-row spacing of 12.5 cm and a seeding rate of 60 kg per hectare. After the first soil cultivation, 300 kg per hectare of Yara Mila NPK(S) 18-11-13(7) (Yara, Vilnius, Lithuania) complex fertilizer was applied. Additionally, 35 days after sowing, 60 kg per hectare of nitrogen fertilizer AN 34 N (ammonium nitrate, AB AChema, Jonalaukis, Lithuania) was added to the plots based on recommendations from a previous study [4,27].

2.1. Growing Condition

This publication provides average temperature and rainfall data for the entire five-year study period, thereby minimizing the impact of annual weather variations on hemp fiber and seed yield. The average temperatures for each month during the growing season for each research year can be found at the provided link—DOI: 10.5281/zenodo.13908645.

The experimental area experiences a Baltic climate characterized by high variability in weather conditions, with long and cold winters and short and comparatively warm summers. Temperature and rainfall average data for the research period (2019–2023) and a long-term period (1983–2023) collected near the experimental area (Rezekne hydrometeorological stations) show this pattern, as seen in Figure 2a,b.

2.2. Yield of Hemp Plant’s Biomass, Stem, Fibers, Seed, and Height of Hemp Plants

Seven weeks after sowing during the vegetation period, every three weeks, hemp plant height was measured on 10 marked plants in each plot, with four replicates for each variety. In the middle of September, 19 weeks after sowing, the hemp plants from one square meter of each plot were harvested manually using a hand trimmer, and the plant density was tallied by calculating the number of plants per square meter. To determine the biomass yield, the weight of the harvested hemp plants was measured right after harvesting. Then, hemp plants were dried in a hangar using a warm-air generator before further processing. Once the green plants were dry, hemp stems were harvested and weighed, and their average height was recorded. The dried samples were manually beaten to extract the seeds, which were then cleaned and weighed using the sample cleaner “MLN” (PFEUFFER, Kitzingen, Germany). The seeds’ mass and oil content were measured using an Infratec 1241 Detector Unit (FOSS, Hilleroed, Denmark). The oil content of the seeds was determined over a four-year period (2020–2023). The hemp stems were processed with a decorticator to separate the fibers and shives, and the fiber content was determined for each sample by weighing the fiber mass.

2.3. Calorific Value and Ash Melting Point of Hemp Shives

After obtaining the hemp fibers, the hemp shives were collected and ground. The calorific value of the samples was determined according to ISO 21404 (2020) [28] using a calorimetric calorific value-determination device (6772 Calorimetric Thermometer, Parr Instrument Company, Frankfurt, Germany). The heat capacity was determined for dry samples of hemp stems in constant volume. To obtain ash, samples of hemp shives were burned in a muffle furnace, heating them for 120 min at 550 °C. About 1 g of ash was used to form the cones, and the melting point of the ash was determined using the Ash Fusibility Test Furnace—CAF G5 (Carbolite Gero, Hope Valley, UK)—according to the ISO 18125:2017 standard [29]. A supplementary approach to enhance the melting point of the ash was employed by introducing calcium-containing minerals. The addition of calcium-based compounds is known to elevate the melting point of ash, and calcium-rich rock sources, such as dolomite and limestone, are available in Latvia. Calcium carbonate or calcium hydroxide were incorporated in concentrations ranging from 1% to 10% by weight. The mineral additives were introduced to the crushed hemp shives prior to combustion in a muffle furnace.

2.4. Statistical Analyses

The statistical analysis used IBM SPSS Statistics Version 20.0 (Armonk, NY, USA: IBM Corp.). Mean values (MV) and standard error (SE) were determined from 5 parallel measurements (mean value from each year). The data were analyzed using one-way ANOVA with subsequent post hoc Tukey tests. Data were reported in the format of MV ± SE. The significance level for all statistical tests was set at α = 0.05. The correlation of hemp fiber and oil content with other hemp parameters was determined using EXCEL (Office 2021).

3. Results and Discussion

3.1. Growth Condition

The years 2019 and 2020 are noted as the warmest years on record since data collection began in 1924. Meanwhile, the summers of 2022 and 2023 were the third and fourth warmest. The average temperature of the five-year study (2019–2023) was 1.7 °C above (Figure 2a) the long-term average (1983–2023). During the five-year study, the average temperatures in June (+14.8 °C) and August (15.5 °C) were 3.2 degrees and 2.3 degrees higher, respectively, compared to long-term observations from 1983 to 2023. During the five-year study period (2019–2023), the average annual precipitation was 2.1 mm lower than the long-term observations from 1983 to 2023. Notably, at the beginning (May) and end (August) of the growing season, precipitation levels during these five years were significantly higher than the long-term averages, exceeding 5.8 mm. Conversely, in the mid-vegetation months of June and July, precipitation was markedly below the long-term average, falling short by 7 mm.

3.2. Hemp Growth Dynamics and Growing Density

Hemp growth dynamics were determined during the vegetation period from the end of June to the end of August. When evaluating the plant heights and growth of hemp varieties, summarized in Figure 3A, it was observed that the most intense growth for all varieties was observed at the beginning of the vegetation.

In the initial seven weeks following sowing, all hemp varieties exhibited comparable growth rates that were not statistically significant. The height ranged from 55 cm for the ‘Loja’ variety to 77 cm for both ‘Futura 75’ and ‘Futura 83’. The greatest growth in the period at 7–10 weeks after sowing was observed in ‘KA-2-2011’ (47 cm) among the hemp seed varieties and in ‘Futura 83’ (73 cm) among the fiber varieties. The smallest increase for seed and fiber variety was for ‘Loja’ 17 cm and ‘Bialobrzeskie’ 39 cm, respectively. The growth of seed hemp from mid-July to the beginning of August was about a third as much as in the early stages of the vegetation, except for the variety ‘Loja’, the growth of which was not observed. All fiber hemp varieties experienced an increase of approximately 30 to 40 cm during this period. ‘Bialobrzeskie’ exhibited growth comparable to that of the prior period, while the other varieties showed an increase of only about one-third to one-half. Our data revealed that seed hemp varieties exhibited minimal to no elongation during the late growth period (1–31 August).

In contrast, fiber hemp varieties demonstrated a slightly reduced growth rate compared to the mid-growing season. In the final growing season of hemp seeds, energy is devoted to the maturation of the seeds, so there is no growth. The seed hemp varieties exhibited varying heights, with ‘Loja’ measuring 74 cm and ‘KA-2-2011’ reaching 124 cm, revealing a statistically significant difference. In contrast, the other seed cultivars demonstrated similar plant heights, ranging from 95 cm to 115 cm. The height of fiber hemp plants reached 182 cm for ‘USO-31’ and ‘Austa’ and 220–230 cm for Futura 75 and Futura 83. ‘Bialobrzeskie’ showed a slightly smaller plant height.

Other authors’ studies describe ‘Finola’ as a short variety of seed hemp [30,31]. ‘Purini’, in a study by other authors, shows a similar plant height under equivalent climatic conditions [32]. Simultaneously, research conducted by other authors indicates that ‘Henola’ exhibits a plant height double that in our study [33]. In studies by other authors, ‘USO-31’ and ‘Futura 75’ show similar plant heights [32,34,35], while ‘Bialobrzeskie’ hemp has variable plant heights in different studies [32,34,35,36]. The widespread use of the ‘Bialobrzeskie’ variety across Europe can account for the varying plant heights observed in different studies, as these disparities may be attributed to the diverse climatic conditions present in various regions. Futura varieties are known for their impressive plant height, characterized by other authors [37,38]. To our knowledge, no peer-reviewed scientific studies are available on the height and density of the ‘Adzelviesi’, ‘KA-2_2011’, ‘Loja’, ‘Estica’, and ‘Austa’ hemp varieties.

Plant density for hemp varieties before harvest was 188–259 plants per m², which is twice as high as in other studies [35,38]. An increased plant density can potentially enhance yields, such as biomass, fibers, or seeds. However, this can also impact plant thickness and, in turn, may lead to a decrease in biomass or fiber production. By analyzing our findings with the research of other scholars, we can assess how the density of plants per square meter influences overall yield, which could result in greater production on a reduced land area.

3.3. Hemp Total Biomass, Stem, and Fiber Yield

The total hemp biomass varied statistically significantly between fiber and seed hemp, with values ranging from 27 to 47 t ha⁻¹ and 15 to 28 t ha⁻¹, respectively (Figure 4A). The variety ‘Bialobrzeskie’ produced the highest amount of fiber for varieties of hemp biomass, with a yield of 47.2 t ha⁻¹, closely followed by ‘Futura 75’ (40.9 t ha⁻¹) and ‘Futura 83’ (41.4 t ha⁻¹), which did not differ significantly from ‘Bialobrzeskie’ in terms of total biomass. Regarding biomass yield, ‘USO-31’ appeared as the lowest-performing fiber hemp variety, producing 27.2 t ha⁻¹. This yield was not statistically significantly different from those of ‘Austa’ and seed hemp varieties. The biomass yield of the fiber hemp variety ‘Austa’ was comparable to that of the fiber hemp varieties ‘Futura’ and ‘USO-31’, showing no statistically significant difference. Additionally, ‘Austa’ exhibited similar biomass yields to most seed hemp varieties, except for ‘Adzelviesi’ and ‘Finola’, which differed significantly. The biomass yield of all seed hemp varieties was not statistically significantly different from each other, nor was it significantly different from fiber hemp varieties ‘Austa’ and ‘USO-31’.

It was found that stem yield varied significantly between the two groups (Figure 4B), except for the seed hemp variety ‘KA-2-2011’, which does not differ significantly from the fiber hemp varieties ‘USO-31’, ‘Austa’, and ‘Bialobrzeskie’.

The ‘Futura 75’ variety achieved the highest stem yield of 31.2 t ha⁻¹, and the ‘Futura 83’ variety yielded 28.4 t ha⁻¹, statistically not significantly different from each other and also from ‘Austa’ and ‘Bialobrzeskie’, while the other three fiber hemp varieties produced yields ranging from 19 to 22 t ha⁻¹, not significantly different from each other. The seed hemp varieties exhibited a diverse range of stem yields, from 6 to 13 tons per hectare, with no statistically significant differences between them, except for KA-2-2011, which achieved the highest yield (13.1 t ha⁻¹), and Loja, which recorded the lowest yield (5.6 t ha⁻¹).

Figure 4 illustrates that the stem yield of fiber hemp varieties is 1.5 times lower than that of biomass, except for the ‘Bialobrzeskie’ variety, where the stem yield is 2.2 times less than the biomass yield. This indicates that ‘Bialobrzeskie’ is particularly advantageous for biomass production. For seed hemp varieties, the situation varies: the stem yield for ‘Estica’ and ‘Loja’ is 3 to 3.3 times lower than the biomass yield. At the same time, ‘KA-2-2011’ shows a 1.7-fold reduction, comparable to fiber hemp varieties. In contrast, other seed hemp varieties display a stem yield 2 to 2.4 times lower than their biomass yield. Thus, ‘KA-2-2011’ is a promising candidate for dual-use potential.

Both fiber hemp and seed hemp varieties exhibited a similar fiber content range of 26% to 39% of hemp stem, with no statistically significant difference between varieties, except for ‘Austa’ and ‘Bialobrzeskie’, which demonstrated the highest fiber content (Figure 5A). The fiber yield of fiber hemp varieties is significantly higher than that of seed hemp varieties, with a notable increase of 2–5.5 times (Figure 5B). The fiber yield of seed hemp varieties varies from 1.7 t ha⁻¹ for the ‘Finola’ and ‘Loja’ varieties to 3.35 t ha⁻¹ for the ‘KA-2-2011’ variety, with no statistically significant differences observed. Notably, the highest fiber yield among the fiber hemp varieties was achieved by ‘Futura 75’ at 10.75 t ha⁻¹, followed closely by ‘Futura 83’, ‘Bialobrzeskie’, and ‘Austa’ at 9.8, 8,7, and 8,0 t ha⁻¹, without statistically significant differences. On the other hand, ‘USO-31’ showed the lowest fiber yield among the fiber hemp varieties at 6.3 t ha⁻¹, not statistically different from ‘Bialobrzeskie’, ‘Austa’, and seed hemp ‘KA-2-2021’.

To the best of our knowledge, there are no peer-reviewed articles available on the biomass, fiber, and seed yields of the seed hemps ‘Adzelviesi’ and ‘KA-2-2011’ (excluding our previous publication of the study [26]), ‘Estica’, ‘Loja’, and fiber hemp ‘Austa’ varieties. Regarding the remaining hemp varieties—‘Purini’, ‘Finola’, ‘Henola’, ‘USO-31’, ‘Futura’, and ‘Bialobrzeskie’—similar results on biomass, stem, and fiber yield have been obtained in the research of other authors in the Baltic Sea region [32,34,39,40,41]. Likewise, in a similar study under Mediterranean conditions, equivalent average biomass, stem, and fiber results were obtained [35]. This indicates that plant density, measured as the number of plants per square meter, has little to no impact on the yield of biomass and fiber.

Figure 4 and Figure 5 illustrate that the fiber varieties ‘Futura 75’ and ‘Futura 83’, originally developed for central and southern European climates, demonstrate height yields of stems and fibers in the Baltic Sea region. It is also worth noting that the fiber content and yield of the varieties ‘Austa’ and ‘Bialobrzeskie’ are statistically comparable to those of the two ‘Futura’ varieties. This suggests that any examined fiber hemp varieties are viable options for fiber production and cultivation in the Baltic Sea region.

Fiber yield correlates with hemp biomass yield and hemp plant height (Figure 6a,b). Specifically, the data show that fiber yield increases with increasing biomass yield and hemp plant height. The data presented in Figure 6 and the regression coefficient reveal that the correlation between fiber yield and hemp plant height is stronger than the correlation between fiber yield and biomass yield.

In a study conducted under Mediterranean conditions, Tsalaki et al. reported a similar correlation between fiber yield and biomass yield [35]. In contrast, our research demonstrates a significant correlation (R² = 0.9123) between fiber yield and the height of hemp, whereas their study reported a considerably weaker correlation (R² = 0.298). The seed variety ‘Estica’ and the fiber variety ‘Bialobrzeskie’ stand out in Figure 6a due to their higher biomass outcomes. In contrast, the varieties ‘Loja’ and ‘Bialobrzeskie’, which exhibited the shortest plant heights, are outliers to the correlation line in Figure 6b.

3.4. Seed Yield, 1000-Seed Weight, Oil Content, and Oil Yield

Seed yield varied significantly between seed hemp and fiber hemp varieties, except ‘Estica’ and ‘USO-31’, which demonstrated similar results (Figure 7A). The ‘Henola’ variety produced an exceptionally high seed yield of 3.4 t ha⁻¹, which was 1.4–2.2 times higher than the seed yields of other seed hemp varieties. In contrast, the ‘Estica’ variety had a significantly lower seed yield of 1.6 t ha⁻¹ than the other seed hemp varieties, except ‘Adzelviesi’ and ‘Loja’. The remaining seed hemp varieties, ranging from 1.9 to 2.4 t ha⁻¹, showed no statistically significant differences in their yields. Among the fiber hemp varieties tested, a statistically significantly higher seed yield was achieved by ‘USO-31’, showing 1.3 t ha⁻¹, comparable to the performance of the seed hemp variety ‘Estica’. The other fiber hemp varieties yielded seeds in the 0.5 to 0.8 t ha⁻¹ range, with no significant differences. An analysis of the mass of 1000 seeds across hemp varieties revealed significant size disparities (Figure 7B). The ‘Estica’ variety produced the largest seeds, with a mass of 16.4g per 1000 seeds, significantly greater than those of other varieties, except ‘Adzelviesi’ and ‘Purini’. The ‘Bialobrzeskie’ variety had the second-largest seeds, weighing 14.7 g per 1000 seeds, statistically equivalent to all seed hemp varieties except ‘KA-2-2011’. At the same time, ‘Futura 83’ exhibited the lightest seeds, weighing 8.4 g per 1000 seeds, which shows a statistically significant difference compared to other hemp varieties. The other varieties fell between 11.2 g and 12.9 g per 1000 seeds, with no statistically significant differences.

The seed yield of ‘Henola’ is almost double that reported in another study [33]. The USO-31 hemp variety also showed almost twice the seed yield of the other study [42]. The potential for increased seed yield can be attributed to a greater plant density per square meter compared to studies conducted by other researchers. This supports the observation of higher seed yields and suggests that hemp can be cultivated at elevated densities within smaller land plots, resulting in proportionally enhanced yields. The seed yield of ‘Bialobrzeskie’ and ‘Futura75’ varies significantly across different studies, ranging from 0.6 to 2.5 t ha⁻¹ [33,38,42]. The differences in productivity among various hemp varieties under different environmental conditions and climatic factors highlight the value of site-specific cultivation approaches. This indicates the importance of the study for developing hemp cultivation in the Baltic Sea region. The findings illustrated in Figure 7 indicate that the ‘Henola’ variety is well-suited for cultivation in the Baltic Sea region’s climate, even though it was originally developed for the Central European climate. The seed yield of ‘KA-2-2011’ matches that of other seed hemp varieties, but it exhibits a significantly higher fiber yield, highlighting its promising potential for dual-purpose growing. This variety, developed in Latvia, has not yet been examined or reported in the peer-reviewed literature. Similarly, ‘USO-31’ demonstrates potential for dual use due to its relatively high seed yield, a subject addressed in studies conducted by other researchers [42].

A statistical analysis revealed that the oil content in hemp seeds does not differ significantly among seed hemp varieties, with a mean value of 36.6% (range: 33–39%) (Figure 8A). ‘Futura 83’ (32.8%) exhibited a significantly lower oil content from ‘Henola’ (38.9%), which is associated with its smaller and bigger seed mass, respectively. This may be attributed to the increased proportion of polysaccharide-rich seed coats in smaller seeds, reducing the available space for oil.

In other studies, the content of oil in the hemp seeds was also determined to be around 30%; accordingly, the yield of oil is proportional to the yield of the seeds [43,44].

Similar to seed yield, oil yield varies significantly between seed hemp and fiber hemp varieties, except for ‘USO-31’, which exhibits a similar performance to ‘Adzelviesi’, ‘Estica’, and ‘Loja’ (Figure 8B). The ‘Henola’ variety stands out for its high oil yield (1.4 t ha⁻¹), with a significant difference from other seed hemp varieties. Its oil yield is 1.5 to 2.1 times higher than other seed hemp varieties and 8 times higher than fiber hemp varieties. The oil yields of the other seed hemp varieties are not statistically significantly different from one another, ranging from 0.6 to 0.9 t ha⁻¹.

The correlation analysis revealed a significant inverse relationship between seed yield and total biomass yield, stem yield, and plant height (Figure 9). Specifically, seed yield decreased as biomass yield (Figure 9a), stem yield (Figure 9b), and hemp plant height (Figure 9d) increased. No correlation was observed between seed yield and 1000-seed weight (Figure 9c). This observation is interesting and draws attention when assessing the choice of hemp varieties for dual cultivation. The relationship between hemp fiber yield, biomass yield, and plant height is positively correlated, whereas seed yield is negatively correlated. This indicates that the most promising dual-purpose hemp varieties, specifically ‘USO’ and ‘KA-2-2011’, are at the hypothetical intersection of these correlation trends. In a similar study conducted under Mediterranean conditions, Tsalaki et al.’s results showed that the correlation between seed yield and 1000-seed weight, stem yield, stem fiber content, fiber flexibility, plan density, height, stem diameter, and fiber strength was not significant [35].

3.5. The Potential of Hemp Shives as a Solid Fuel

Table 1. Calorific values and ash melting point of hemp shives from various hemp varieties.

Hemp Varieties	Calorific Value, MJ kg−1	Ash Melting Point, °C
Atdzelviesi	16.75 ± 0.02	1100 ± 7
Purini	16.44 ± 0.01	1180 ± 8
Finola	17.00 ± 0.04	1120 ± 3
Estica	16.50 ± 0.03	1100 ± 7
USO 32	16.38 ± 0.04	1000 ± 7
Futura 75	17.46 ± 0.03	1120 ± 7
Futura 83	17.34 ± 0.03	1110 ± 5
Austa	16.84 ± 0.03	1080 ± 4
Bialobrzeskie	17.25 ± 0.02	1050 ± 7

shows the results of the calorific value of the shives and their ash melting temperature (deformation start temperature, IT) for the investigated hemp varieties.

The calorific values of the hemp shives ranged from 16.38 to 17.46 MJ kg⁻¹, with no significant impact from the hemp variety. In the studies of other authors, the calorific value of hemp shives was in the range of 15.5–18.9 MJ kg⁻¹ [45,46,47]. The calorific value of hemp shives is comparable to that of wood pellets, which averages around 17 to 20 MJ kg⁻¹ [48]. This suggests that, regardless of variety, hemp shives can be used as a viable energy source, similar to wood. Regarding ash melting point, our findings indicate that it falls between 1000 and 1180 °C, equivalent to the ash melting point of other agricultural pellets [49]. However, since soil properties and fertilizer application influence the chemical composition of ash, the hemp variety has no significant effect on the melting point. Notably, the melting point of hemp shive ash is generally lower than wood ash but higher than straw ash. If the ash melting temperature is below 1100 °C, it could pose issues for heating systems [49]. Fortunately, hemp flakes can be used as an additive to wood pellets or briquettes, which could help mitigate this problem. Increasing the ash melting temperature may be necessary if hemp shives are used as solid fuel alone.

The primary challenge in using hemp shives as an energy source is their ash’s relatively low melting point. To address this issue, we propose adding minerals to hemp-containing fuel [50]. Our experiments used shives from the ‘Bialobrezskie’ hemp variety, a high-biomass fiber hemp with a high shive yield.

Table 2. Effect of mineral addition on ash melting point of solid fuel from hemp shives.

Added Mineral	Mass Fraction of Mineral, %	Ash Melting Point, °C
	0	1050 ± 7
CaCO3	10	1345 ± 6
CaCO3	5	1340 ± 4
CaCO3	3	1320 ± 5
CaCO3	1	1250 ± 6
Ca(OH)2	10	1390 ± 7
Ca(OH)2	5	1380 ± 4
Ca(OH)2	3	1385 ± 3
Ca(OH)2	1	1250 ± 4

shows the added minerals, their mass fraction in the hemp shive fuel, and the ash melting temperature.

A small amount (1%) of calcium-containing compounds (such as Ca(OH)₂ and CaCO₃) is sufficient to achieve the desired ash melting temperature, which would not pose any issues in heating systems. Exceeding 3% calcium content does not significantly improve ash melting temperature further, but it increases the amount of ash and unnecessary mineral costs. Calcium hydroxide is more effective due to its higher calcium mass fraction (56%) than calcium carbonate (40%). Additionally, calcium hydroxide helps mitigate corrosion risk and acid precipitation by neutralizing sulfur and chlorine compounds produced during combustion.

4. Conclusions

Evaluating the cultivation potential of fiber and seed hemp varieties in the Baltic Sea climate, it can be concluded that the most suitable variety for obtaining high seed and oil yield is ‘Henola’, which showed a 3.5 t ha⁻¹ seed yield and 1.3 t ha⁻¹ oil yield, exceeding the results of other hemp seed varieties by 1.5–2.1 times. ‘Futura 75’, with a fiber yield of 10.8 t ha⁻¹, was the most productive variety for obtaining fibers. Evaluating both fiber and oil yield, ‘USO-31’, which showed the highest seed yield (1.3 t ha⁻¹) among fiber hemp varieties, as well as ‘KA-2-2011’, which showed the third highest seed yield (2.2 t ha⁻¹) and the highest fiber yield (3.4 t ha⁻¹) of the seed hemp varieties, can be recommended as the most suitable varieties for dual-purpose cultivation in the Baltic Sea climate. The ‘Bialobrzeskie’ variety demonstrated the highest biomass yield, making it a candidate for cultivation as a renewable energy source or to enhance CO₂ uptake. The viability of hemp shives as a fuel source is evident, given their similar calorific value to wood fuel. It is recommended to incorporate 1–3% calcium hydroxide or calcium carbonate into the hemp shives mixture to elevate the ash melting point of hemp shive-based fuel.