Agronomic and Utilization Potential of Three Elephant Grass Cultivars for Energy, Forage, and Soil Improvement in Vietnam

Abstract

Elephant grass (Pennisetum purpureum Schumach, EG) is a promising biomass energy crop due to its high productivity and adaptability to harsh environments. In the transition to renewable energy, varietal evaluation is essential to identify cultivars that maximize biomass and energy yield. This study assessed three varieties (VS-19, VA-06, and VDP as control) across three harvest cycles (new planting, first regrowth, and second regrowth) between 2022 and 2024 at the Cotton and Agricultural Development Research Institute, Ninh Thuan Province, Vietnam. The site was characterized by mean temperatures of 25–36 °C, relative humidity of 65–82%, and average precipitation of 75.7 mm per month. Agronomic traits, energy potential (heating oil equivalent per hectare, HOE/ha), forage quality, and soil amendment value of the EG were examined to address the research question whether EG can be integrated into a three-cycle utilization model (energy, forage, soil amendment) to support a circular bioeconomy in Vietnam. All cultivars showed good growth, strong drought tolerance, and resistance to pests and diseases. VS-19 showed superior tillering, strong lodging resistance, and the highest biomass yield (63.8 t/ha) with an energy output of 32,636 HOE/ha, while VA-06 (56.1 t/ha; 28,699 HOE/ha) and VDP (54.7 t/ha; 27,952 HOE/ha) produced slightly lower but comparable outputs. Forage evaluation indicated moderate nutritional quality, while residues from the third cycle showed favorable carbon and nutrients content, making EG suitable as a soil amendment. EG thus demonstrates high biomass and energy yields, forage potential, and soil improvement capacity, reinforcing its role in integrated bioenergy and agricultural systems.

1. Introduction

The global transition away from fossil fuels towards renewable, sustainable energy systems is critical for addressing climate change and achieving the targets set by the Paris Agreement. Biomass energy, derived from plant material and organic residues, has emerged as a key component in decarbonization strategies due to its carbon neutrality and compatibility with existing combustion-based infrastructure. Unlike wind or solar, biomass offers the advantage of dispatchable, storable energy, making it essential in countries with growing energy demand and agricultural potential. Vietnam, as a signatory to the Paris Agreement and a nation committed to reaching net-zero carbon emissions by 2050, is actively exploring biomass as a strategic resource within its National Power Development Plan VIII. Agricultural residues such as rice husk and straw dominate the domestic biomass landscape, with rice straw alone estimated at over 97 million tons annually and an energy potential of more than 380 TWh [1,2,3]. However, challenges such as low bulk density, high ash and silica content, seasonality, and collection inefficiencies limit their scalability for industrial fuel substitution.

Biomass energy has gained increasing global attention as a sustainable alternative to fossil fuels, particularly in the context of climate change mitigation and rural energy security. Among second-generation energy crops, elephant grass (Pennisetum purpureum Schumach.; hereafter referred to as EG) also known as Napier grass stands out for its fast growth, high biomass yield, and adaptability to marginal lands [4,5]. It is a perennial C4 grass native to Africa but widely cultivated in tropical and subtropical regions for forage, soil conservation, and more recently, renewable energy purposes. EG comprises up to 80% of the forage ingested by cows in many tropical and subtropical countries [6]. It is widely used as cattle feed both in cut-and-carry systems and as pasture, with silage being the most common form. It is also considered a valuable feed source for pigs due to its high content of soluble carbohydrates that support effective fermentation [7]. EG can reach up to 6 m in height and allows multiple harvests per year under favorable conditions [8].

Biomass from EG can be converted into energy through various pathways, including direct combustion for heat and power, biochemical conversion into bioethanol and biogas, and thermochemical methods such as pyrolysis and gasification. These technologies offer flexible options for integrating biomass into local and national energy systems [4,5,8]. Given its high productivity and stress tolerance, EG is increasingly recognized as a promising feedstock for bioenergy development, particularly in developing countries with abundant agricultural land but limited access to modern energy. EG varieties demonstrated a higher average lower heating value (LHV) of 16.7 MJ/kg compared to rice husk, sugarcane bagasse, and sorghum, and showed comparable calorific content to sugarcane straw, coconut fiber, and corn stover. In addition, EG had lower ash content (4.7%) than rice husk, sugarcane bagasse, coconut fiber, and sorghum, indicating better combustion efficiency. These traits, along with high dry matter (DM) productivity, support EG as a high-quality, energy-efficient lignocellulosic biomass for direct combustion [9].

In China, trials of Pennisetum hybridum under marginal land conditions have yielded 74 t DM/ha/year, with favorable combustion and carbon content properties compared to crop residues [10]. Other researchers emphasized the energy aspects of this grass as a biomass for energy production including heating value, ash content, and volatiles [11,12,13]. In Indonesia, which has expanded pellet exports to East Asian markets, faces sustainability questions from overexploitation of forest residues [14], hence, transition toward dedicated crops like EG is increasingly emphasized. In the United States, EG, marketed as Giant King grass, has shown impressive yields exceeding 100 t DM/ha/year in southern climates [15], outperforming switchgrass (11.6–39.4 t/ha) in the Midwest and Southern US [16]. EG exhibits an exceptionally high growth rate, with average annual yields ranging from 30 to 40 metric tons per hectare (MT/ha). Globally, its dry matter yield varies considerably, from as low as 14 MT/ha in Malawi to as high as 85 MT/ha in El Salvador [17]. In Brazil, EG cultivar BRS Capiaçu shows strong potential as a sustainable biomass alternative to native Amazon wood in Mato Grosso. Its dry mass yield reached 44.1 t/ha in 180 days, with higher heating values (HHV) ranging between 17.9 and 18.9 MJ/kg. Proximate analysis and density varied with plant parts and age, but HHV remained consistent. The grass adapts well to local climate and soils with short cutting intervals (every 3 months). These traits support its use in reducing deforestation and promoting regional energy generation [18].

These comparisons showcase the agronomic advantages of EG over conventional biomass sources, particularly in terms of yield stability, energy density, and multi-cycle harvests per year. Moreover, as a sterile, clonal crop, EG minimizes ecological risks related to seed dispersal. Some cultivars, such as EG hybrids are sterile or have very low seed viability, which reduces the risk of seed dispersal and invasiveness [19]. Unlike annual crops, its perennial nature eliminates land preparation and input requirements, improving long-term sustainability and carbon sequestration benefits [5,20].

The cropping system in the South-central coast region of Vietnam, in order of cultivated area, includes rice, maize, forage grasses, and several other vegetable crops. Among them, the area of EG cultivation is relatively large, and it is mainly grown for ruminant livestock feeding. Despite its promising potential as a high-yielding energy crop, the large-scale deployment of EG requires localized agronomic validation. Key factors such as plant height, tillering ability, lodging resistance, pest tolerance, and dry matter content must be assessed under diverse climatic and edaphic conditions. Identifying cultivars with superior biomass yield and energy output has become essential for ensuring sustainable bioenergy systems. Variations in growth potential, regrowth ability, and energy output among different cultivars can significantly influence their suitability for large-scale bioenergy production [21]. Therefore, assessing the performance of improved cultivars against traditional controls provides valuable insights into their potential role in advancing bioenergy sustainability. This study aims to evaluate the agronomic performance, energy, forage and soil amendment potential of two elite cultivars (VS-19 and VA-06) and a local control (VDP) across three successive harvest cycles in the arid region of Ninh Thuan, South-central Vietnam. Growth dynamics, biomass yield, dry matter quality, suitability for energy applications and forage quality are analyzed and compared to conventional feedstocks. The findings contribute to Vietnam’s efforts to diversify its biomass supply chain, reduce reliance on fossil fuels, and achieve national carbon neutrality goals. Building on global experiences in China, Japan, Indonesia, the United States, Europe, and South America.

2. Materials and Methods

2.1. Experimental Site and Duration

The field trials were conducted from 2022 to 2024 at a lowland area in the Experimental Station of the Cotton and Agricultural Development Research Institute Nha Ho, Hamlet, Nhon Son Commune, Ninh Son District, Ninh Thuan Province, geographical coordinates; 11°38′05.9″ N 108°53′00.9″ E. The site experiences a tropical monsoon climate with distinct rainy and dry seasons and the average rainfall per month during the growth period was 75.7 mm.

2.2. Plant Materials and Experimental Design

Three cultivars of EG were evaluated, namely, VS-19, VA-06, and VDP (control). VS-19 is the EG variety selected by Viet Seed Co., Ltd. (Hanoi, Vietnam), one of the pioneers in the studies of energy crops in Vietnam and was officially released in 2019. VA-06 is the EG variety that was introduced from Taiwan and was officially released in 2007. VDP variety is the local traditional EG variety that has long been cultivated in Vietnam, with an unknown origin. The experimental design followed a randomized complete block layout with three replicates. Each variety was planted on an area of 180 m² (5 m × 10 m per plot × 3 replicates), totaling 540 m². Plant spacing was 80 cm × 50 cm. Fertilization, irrigation, and other agronomic practices followed conventional methods.

2.3. Harvest Cycles and Data Collection

The EG cultivars were harvested in three consecutive cycles: (i) new planting cycle at 189 days after planting, (ii) first regrowth cycle at 120 days after the first cut, and (iii) second regrowth cycle at 120 days after the second cut. The harvest frequency was three times in 14 months. At each harvest, all above-ground biomass within each plot was weighed. Sub-samples were oven-dried at 70 °C until constant weight to determine dry-matter percentage. Dry biomass yield in tons per hectare (t/ha) was then calculated.

The agronomic parameters assessed were drought tolerance, tillering, lodging resistance, and pest and disease resistance. Drought tolerance is the ability of plants to maintain growth under limited soil moisture conditions. Even in environments with heavy rainfall or irrigation it remains an important trait because temporary dry spells, irregular rainfall distribution, or poor water retention in soils can still stress crops. Drought stress can reduce crop yields by 15–50% [22]. Drought tolerance was evaluated through visual scoring of survival and plant strength, with a lower score of 1 indicating stronger tolerance and a higher score of 5 indicating weaker drought tolerance. Tillering, the production of side shoots (tillers) from the base of the main stem and a key determinant of biomass yield [23], was scored in the same way, where values closer to 1 reflected better tillering ability. Lodging, the tendency of crops to fall over due to weak stems or environmental stress, which reduces photosynthesis, yield, and harvest efficiency [24], was assessed by visual scoring, with 1 denoting stronger resistance and a score of 5 denoting very weak lodging resistance. Pest and disease resistance, defined as the ability of plants to withstand infections or infestations that reduce yield or quality, was evaluated through visual observation of symptoms with a score of 1 indicating excellent resistance.

The forage quality of the EG variety VS-19 was evaluated to determine its suitability as animal feed in comparison to Mombasa guinea grass, a common forage grass whose data was collected from TH milk farm, one of the biggest milk farms in Vietnam.

2.4. Potential Energy Yield Calculation

The energy output of all the EG varieties was calculated based on the HHV of VS-19. The dry biomass yields reported for the EG varieties represent the individual yields obtained from each harvest cycle of the respective varieties. The energy output (MJ/ha) was first estimated using Equation (1):

Energy output (MJ/ha) = Dry biomass yield (t/ha) × Higher heating value (MJ/t)

(1)

The HHV of VS-19 (18.4 MJ/kg) was converted from kcal/kg to MJ/kg using the conversion; 1 kcal = 0.004184 MJ [25]. The results in MJ/ha were then expressed as heating oil equivalent per hectare (HOE/ha), whereby 1 L HOE = 36 MJ (HHV).

2.5. Climatic Conditions

The climatic conditions during the experimental trials were generally warm and humid conditions typical of central Vietnam. Average daytime temperatures rose slightly across cycles, from 25 to 34 °C in the new planting cycle to 30–36 °C in the second regrowth cycle, while nighttime temperatures ranged from 21 to 28 °C, 20–28 °C, and 25–29 °C for the respective cycles. Relative humidity showed a modest decline from 65 to 82% in the first cycle to 67–75% by the second regrowth. The experiments were conducted entirely under rainfed conditions, and no other form of irrigation was used. Seasonal rainfall remained moderate, with heavy rain days decreasing from 15 days during the new planting cycle, with a mean precipitation of 126.8 mm to 11 days in the first regrowth with a mean precipitation of 17.6 mm, before slightly increasing to 13 days in the second regrowth (49.7 mm mean precipitation). Concurrently, the number of sunny days declined progressively from 116 to 94 and 72 days, reflecting a gradual shift toward more overcast conditions.

3. Results

3.1. Agronomic Characteristics and Resistance

The agronomic characteristics and resistance of the tested cultivars are shown in

Table 1. Agronomic characteristics and resistance of tested cultivars.

Harvest Cycle	Indicator	Unit	VS-19	VA-06	VDP (Control)
Harvest cycle 1 (new planting)	Planting date		13 June 2022	13 June 2022	13 June 2022
	First harvest date		19 December 2022	19 December 2022	19 December 2022
	Growth duration	Days	189	189	189
	Growth ability	Score	1	1	1
	Tillering ability	Score	2	2	1
	Drought tolerance	Score	1	1	1
	Lodging resistance	Score	1	1	1
	Pest and disease resistance (noted)	Score	1	1	1
Harvest cycle 2 (first regrowth)	First harvest date		19 December 2022	19 December 2022	19 December 2022
	Second Harvest date		18 April 2023	18 April 2023	18 April 2023
	Growth duration (2nd cycle)	Days	120	120	120
	Growth ability	Score	1	1	1
	Tillering ability	Score	1	2	1
	Drought tolerance	Score	1	1	1
	Lodging resistance	Score	3	5	1
	Pest and disease resistance (noted)	Score	1	1	1
Harvest cycle 3 (second regrowth)	Second harvest date		18 April 2023	18 April 2023	18 April 2023
	Third harvest date		16 August 2023	16 August 2023	16 August 2023
	Growth duration (3rd cycle)	Days	120	120	120
	Growth ability	Score	1	1	1
	Tillering ability	Score	1	2	2
	Drought tolerance	Score	1	1	1
	Lodging resistance	Score	1	1	1
	Pest and disease resistance (noted)	Score	1	1	1

. All varieties exhibited very good growth ability and drought tolerance (score 1). None of the three tested varieties showed any signs of pest or disease infection (score 1). VS-19 had the best tillering ability across the cycles with scores of (2, 1, 1), while VA-06 showed the weakest performance (2, 2, 2), and the local control variety VDP scored (1, 1, 2), indicating moderate performance. Lodging resistance over the three cycles for VS-19 was (2, 3, 1), indicating moderate to good stability; VA-06 scored (1, 5, 1), the weakest stability in the 2nd cycle, while VDP (control) had the best performance with consistent lodging resistance of (1, 1, 1). At the time of harvest for the newly planted cycle, all varieties had relatively tall plant height.

3.2. Morphological Description of Varieties Before Harvest

The morphological assessment of the three EG varieties shows consistent differences in growth performance across the harvest cycles as displayed in Figure 1. In the first harvest, VS-19 attained the greatest plant height (4.2 m), surpassing VA-06 (3.9 m) and the control, VDP (3.8 m). VS-19 also produced more visible internodes (17) than VA-06 (14) and VDP (16), suggesting better elongation potential, while the number of dried basal leaves was slightly lower than VA-06 and VDP. By the second harvest, plant height declined for all varieties but remained highest in VS-19 (3.8 m), with similar trends observed in internode numbers. In the third harvest, the decline in growth was more pronounced for all EG varieties, yet VS-19 still maintained superior height (2.9 m) compared to VA-06 (2.7 m) and VDP (2.5 m). The number of dried basal leaves and visible internodes also followed this pattern, with VS-19 generally showing balanced leaf retention and internode development. VS-19 exhibited stronger and more sustained morphological performance than VA-06 and VDP, particularly in plant height and internode formation, making it the most promising variety for high biomass production across the successive harvest cycles.

EG primarily reproduces vegetatively (it produces sterile flowers) and exhibits semi-indeterminate growth, making the determination of the optimal harvest time critically important. Based on the research results and data collected, the recommended criteria for harvesting VS-19 to achieve optimal biomass yield are as follows:

• Morphological indicators: Plant height should exceed 3 m; number of visible internodes should be more than 12; number of senescent (dried basal leaves) should be 8–9.
• Growth duration: At least 120 days.
• Physiological stage: Harvest should occur before the onset of synchronized flowering.
• Weather and climate: Harvest should be carried out before the dry season begins.

3.3. Biomass Yield

The fresh biomass yield and dry biomass yield of the tested cultivars are displayed in

Table 2. Biomass yield, dry matter content, and statistical comparison.

Harvest Cycle	Indicator	Unit	VS-19	VA-06	VDP (Control)
Harvest cycle 1	Fresh biomass yield	t/ha	78.9 ± 2.1	72.3 ± 1.9	75.5 ± 1.9
	Dry matter content (*)	%	26.9 ± 0.5	28.9 ± 0.5	25.0 ± 0.6
	Dry biomass yield	t/ha	21.2 ± 0.6	20.9 ± 0.6	18.9 ± 0.6
Harvest cycle 2	Fresh biomass yield	t/ha	80.6 ± 2.1	72.6 ± 1.8	80.0 ± 2.0
	Dry matter content	%	30.0 ± 0.5	27.6 ± 0.4	28.1 ± 0.5
	Dry biomass yield	t/ha	24.2 ± 0.7	20.1 ± 0.6	22.5 ± 0.6
Harvest cycle 3	Fresh biomass yield	t/ha	62.8 ± 1.9	59.0 ± 1.9	50.4 ± 1.7
	Dry matter content	%	29.2 ± 0.5	25.7 ± 0.4	26.4 ± 0.9
	Dry biomass yield	t/ha	18.3 ± 0.5	15.2 ± 0.5	13.3 ± 0.5
Total	Total annual biomass yield (+)	t/ha	222.3 ± 3.6	204.0 ± 3.1	205.9 ± 3.2
	Average dry matter content	%	28.7 ± 0.3	27.5 ± 0.3	26.6 ± 0.4
	Total annual dry biomass yield (+)	t/ha	63.8 ± 1.1	56.1 ± 1.0	54.7 ± 1.1

Values are expressed as mean ± SE (standard errors), p < 0.05 (ANOVA followed by Tukey’s HSD); (*) Dry matter content = percentage of biomass weight that remains after removing moisture.

. Across the three harvest cycles within approximately one year (14 months to be exact), the VS-19 variety produced the highest dry biomass yield (63.8 t/ha), 16.7% higher than the control. Following that, variety VA-06 yielded 56.1 t/ha, which is 2.7% higher than the control VDP (54.7 t/ha). The VS-19 and VDP varieties exhibited stable yields across all three harvests, whereas VA-06 showed lower stability, possibly due to its poorer lodging resistance and tillering ability. The average dry matter content for VS-19, VA-06, and VDP were 28.7%, 27.5%, and 26.6%, respectively. The biomass yield in the third harvest cycle was noticeably lower than in the previous two cycles, mainly due to high and continuous rainfall causing localized waterlogging, and fewer days with strong sunlight intensity, which inhibited plant growth.

3.4. Biomass Quality

3.4.1. Biomass Quality for Animal Feed of VS-19 Cultivar

The results for cultivar VS-19, evaluated to determine EG quality as a forage crop are presented in

Table 3. Biomass quality for use as animal feed (harvest cycle 2) of VS-19 cultivar.

	Unit	EG VS-19	Mombasa Guinea
Growth duration	days	120.00 ± 6.00	60.0 ± 3.0
Sample type		Fresh biomass	Fresh biomass
Certification body		IASVN	Dairy One
Certification date		May-23	May-18
Dry Matter (DM)	%	33.7 ± 1.7	94.2 ± 4.7
Crude Protein (DM)	%	3.5 ± 0.2	8.4 ± 0.4
Crude Fiber (DM)	%	42.6 ± 2.1	-
Crude Ash (DM)	%	8.0 ± 0.4	9.2 ± 0.5
NDF (DM)	%	75.4 ± 3.8	56.4 ± 2.8
ADF (DM)	%	40.6 ± 2.0	37.3 ± 1.9
TDN (DM)	%	58.7 ± 2.9	60.0 ± 3.0
Calcium (DM)	%	0.4 ± 0.0	0.8 ± 0.0
Phosphorus (DM)	%	0.2 ± 0.0	0.3 ± 0.0
Magnesium (DM)	ppm	0.2 ± 0.0	0.5 ± 0.0
Potassium (DM)	ppm	1.6 ± 0.1	1.0 ± 0.1
Total sugars (DM)	%	5.1 ± 0.3	-

Source: Nutrition and Livestock Feed Analysis Laboratory—Southern Institute of Animal Husbandry.

. Crude protein (3.5%) for VS-19 is lower, limiting its ability to meet livestock protein requirements without supplementation compared to Mombasa (8.4%). The high fiber content, 42.6% crude fiber and 75.4% neutral detergent fiber (NDF), result in lower digestibility; however, its total digestible nutrients (TDN) at 58.7% remain comparable to Mombasa. Mineral levels are generally lower, except for potassium, which is higher in VS-19. The elephant grass shows less nutrient density per unit weight compared to Mombasa possibly due to the harvest age of the elephant grass, being more mature after 120 days. In theory, the quality could be adjusted by harvesting the grass at a younger stage, but this follows the typical rule that “quality is inversely proportional to yield.” There is no absolute standard for evaluating the quality of fresh forage, assessments depend entirely on the purpose, target livestock, and stage of use. Overall analysis results indicate that elephant grass VS-19 is suitable for use as feed for ruminant livestock.

3.4.2. Biomass Quality for Use as Organic Fertilizer of VS-19 Cultivar

The biomass quality of VS-19 for use as organic fertilizer from the third harvest cycle was evaluated, and the results are presented in

Table 4. Biomass quality for organic fertilizer application of VS-19 cultivar.

Harvest Cycle	Indicator	Unit	Value
Third harvest cycle (Regrowth)	Soil acidity (pH KCl)		6.7
Third harvest cycle (Regrowth)	Moisture content	%	5.7
Third harvest cycle (Regrowth)	Total organic carbon (TOC)	%	65.9
Third harvest cycle (Regrowth)	Total nitrogen (Nts)	% N	1.1
Third harvest cycle (Regrowth)	Total phosphorus (P2O5 total)	% P2O5	0.2
Third harvest cycle (Regrowth)	Available phosphorus (P2O5 avail)	% P2O5	0.03
Third harvest cycle (Regrowth)	Total potassium (K2O total)	% K2O	0.58
Third harvest cycle (Regrowth)	Available potassium (K2O avail)	% K2O	0.55

. Analysis of pelletized samples revealed a very low moisture content of 5.7%, making it suitable for storage and use as raw material for organic fertilizer production. The slightly neutral pH (6.7) supports nutrient availability, while high organic carbon (65.9%) improves soil structure, though largely in undecomposed form. Moderate nitrogen (1.1%) benefits crop growth, phosphorus is low (0.2% total, 0.03% available), and potassium is high (0.58% total, 0.55% available), enhancing soil fertility. The biomass quality of EG at the third harvest cycle suggests good potential as an organic soil amendment.

3.4.3. Biomass Energy Quality

Different biomass quality characteristics were assessed based on the variety with the highest dry biomass yield, which was VS-19. The results, obtained from the Analysis and Testing Center 1–VINACONTROL, showed that the biomass energy indicators of the VS-19 EG variety were all superior in quality compared to the typical international organization for standardization (ISO) [26] standard values for similar types of biomass material as seen in

Table 5. Biomass energy quality for VS-19 (harvest cycle 1).

Indicator	Unit	VS-19	ISO17225-1 (*)
Total carbon (on dry basis)	%	48.0 ± 2.4	46.0 ± 2.3
HHV (dry basis)	MJ/kg	18.4	18.0
LHV (dry basis)	MJ/kg	17.1	17.2
Ash content (dry basis)	%	6.0 ± 0.0	7.0 ± 0.4
Nitrogen (dry basis)	%	0.4 ± 0.0	1.3 ± 0.1
Chlorine (dry basis)	%	0.4 ± 0.0	0.7 ± 0.0
Sulfur (dry basis)	%	0.1 ± 0.0	0.2 ± 0.0
Potassium (dry basis)	mg/kg	12.2 ± 0.6	15.0 ± 0.8
Sodium (dry basis)	mg/kg	129.0 ± 6.5	3.0 ± 0.2
Calcium (dry basis)	mg/kg	2.7 ± 0.1	3.5 ± 0.3
Silica (SiO2) (dry basis)	mg/kg	26.3 ± 1.3	-

Source: Test report by Analysis and Testing Center 1—VINACONTROL. (*) Typical values cited from the international organization for standardization (ISO) [26].

.

3.5. Potential Energy Output

Results of the potential energy output of the 3 cultivars across the 3 harvest cycles are shown in Figure 2. The potential energy output results show differences amongst the EG varieties. In the first harvest, VS-19 recorded the highest output of 10,871 HOE/ha, slightly exceeding VA-06 (10,692.9 HOE/ha) and clearly outperforming the control VDP (9659.9 HOE/ha). During the second harvest, energy output increased for VS-19 with an output of 12,385.6 HOE/ha, followed by VDP whose energy output decreased slightly from the first harvest cycle (10,253.1 HOE/ha) and VA-06 increased to 11,490.7 HOE/ha. By the third harvest, energy output declined across all varieties, though VS-19 maintained the highest energy output (9378.7 HOE/ha), compared to VA-06 (7752.5 HOE/ha) and VDP the lowest at 6801.3 HOE/ha. When considering the cumulative totals for annual energy output, VS-19 delivered the greatest total energy output (32,636.1 HOE/ha), outperforming both VA-06 (28,698.5 HOE/ha) and VDP (27,951.9 HOE/ha). VS-19 exhibits superior and more consistent energy potential across successive harvest cycles, with its advantage most evident in the second harvest. Overall, VS-19 proves most suitable for sustained bioenergy production, whereas VA-06 and VDP deliver relatively similar low energy output. VS-19 not only produces more biomass but also provides greater energy returns, making it the most energy-efficient cultivar. Performance rankings further reinforce these trends, with VS-19 rated high, VA-06 medium, and VDP low.

3.6. Comparative Characteristics of Various Biomass Feedstocks

The comparative biomass characteristics of VS-19 were compared in relation to other potential energy feedstocks, including switchgrass, rice husk, and other elephant grass cultivars from previous studies, with a focus on key parameters relevant to its evaluation as a bioenergy resource. The results are summarized in

Table 6. Comparative evaluation of various biomass feedstocks.

Criteria	Elephant Grass VS-19	Elephant Grass	Switch Grass	Rice Husk
Reference	Current study	[9,17,18,27,28]	[29]	[30,31]
Biomass yield DM (t/ha)	21.3 ± 0.6	46.2–85.0	16.2	5.2–7.0
Carbon (%)	48.0	43.4–46.5	47.3	14.0
Ash %	6.0 ± 0.3	3.8–8.9	4.2	15.6
Silica %	Low	0.04–0.07	Low	High
HHV (MJ/kg)	18.4	17.9–18.2	-	13.3–14.4
LHV (MJ/kg)	17.2	14.9–16.7	17.4	13.8
Harvest frequency per year	2 to 4	3 to 4	2	once
Suitability for pelletization	Suitable	Excellent	Suitable	Low (brittle, abrasive)
Suitability for combustion	Excellent (low ash, high yield)	Excellent	Excellent (low ash, high yield)	Poor (slagging risk, silica)
Value added products potential	Biochar, heat and power, biogas, bioethanol, silage,	Biofuels	Biochar, heat and power, biogas, bioethanol, silage,	Ash for construction,
Land-use competition	Grown on marginal land	Grown on marginal land	Grown on marginal land	None (crop residue)

, showing some of the important characteristics for assessing energy potential. These comparisons position elephant grass relative to other energy crops and residues, emphasizing its competitive advantages in biomass yield and energy output.

4. Discussion

The performance of the evaluated EG varieties demonstrates a strong potential for bioenergy applications when compared with other perennial grasses and agricultural residues. The EG varieties in this study consistently produced higher biomass yields. In contrast, Miscanthus × giganteus, has shown limited adaptability under harsh conditions, resulting in reduced yields of only 13.5–14.7 t/ha annually in Czech Republic [32]. The annual dry matter yield of all 3 grown cultivars in the current study (54.7–63.8 t/ha) exceeded that of switchgrass, (6.9–16.2 t/ha) [29,33], and was far superior to rice husk residues, which typically average 7.00 t/ha [30] and rye grass (2.0–3.7 t/ha) [34]. When compared with other elephant grass studies, Dresch et al. [27] in Brazil reported much higher dry biomass yields of 64.7 t/ha at 60 days, 145.5 t/ha at 90 days, and 291.3 t/ha at 120 days, identifying 120 days as the optimum harvest interval which is the optimum harvest interval in the current study. Similarly, ref. [28] in Thailand reported 50.8 t/ha at 60 days. While yields as high as 145 and 291 t/ha [27], exceed the commonly reported range for EG worldwide (14–85 t/ha) [9,17,18,28], such outcomes may reflect favorable conditions including genotype, cultivation practices, soil fertility, climate, and intensive management [5,27]. A key difference was precipitation: the current study’s monthly average precipitation was 75.7 mm, compared with an average of 294.5 mm over the four months in the study by Dresch et al. [27]. Overestimations may also occur when results are extrapolated from small plots to per-hectare values or when fresh biomass is mistakenly reported as dry matter yield amongst many other reasons. These factors can help explain yield figures that are not representative of typical field conditions or global benchmarks. It is however interesting that such high yields were reported, and the cultivation conditions that produced them warrant further investigation.

Ash content is another critical quality parameter influencing suitability for thermochemical conversion. The ash content of EG varieties in this study (6.0 ± 0.3%) aligns well with previous findings, including 3.5–7.3% (mean 5.4%) reported by da Silva et al. [8], and 3.21–6.1% (mean 4.7%) reported by Marafon et al. [9]. Such values fall within the desirable range for combustion, whereas levels above this range may increase slagging and fouling risks [8]. In comparison, sugarcane bagasse was reported to exhibit similar ash content with an average ash content of 6.2% [35], while rice husk shows significantly higher levels (15.3%), which greatly limit its suitability for direct combustion. The lower silica content of EG further enhances its potential by reducing slagging risks relative to silica-rich feedstocks like rice husk. Calorific values further underline the energy potential of EG. VS-19 recorded a HHV of 4400 kcal/kg (18.4 MJ/kg) and a LHV of 4100 kcal/kg (17.2 MJ/kg), which are consistent with the LHV reported for other EG genotypes; 4041–4304 kcal/kg [8] and 4209–4400 kcal/kg [9]. These values compare favorably with sugarcane bagasse (3803–4152 kcal/kg) [35], suggesting that EG is equally competitive as an energy feedstock. VS-19 produced the greatest annual net energy output (32,636 HOE/ha), followed by VA-06 (28,699 HOE/ha) and VDP (27,952 HOE/ha). Using the standard HOE definition (1 L HOE = 36 MJ) and representative HHV values for herbaceous biomass (typically 17 MJ/kg); typical Miscanthus which stands at approximately 13–15 t DM/ha per year would yield about 6.0–7.2 × 10³ HOE/ha, switchgrass (3.4–8.1× 10³ HOE/ha), and rice husk residues would yield approximately about 2.9 × 10³ HOE/ha, based on the reported dry biomass yields [24,25,28]. These comparisons show that the higher dry matter yield of EG translates into substantially greater potential energy output per-hectare compared with other perennial grasses and agricultural residues. Moreover, expressing the results in terms of heating oil equivalents makes it possible to visualize the potential substitution of fossil fuels, for instance, the annual output of VS-19 corresponds to roughly 32,600 L of heating oil per hectare, demonstrating the scale of replacement achievable through elephant grass-based bioenergy pathways. Nevertheless, site conditions, genotype, and management practices remain critical determinants of overall productivity. From a techno-economic perspective, the combined advantages of high yield, moderate ash, low silica, and stable calorific value make EG highly suitable for pelletization and combustion for heat and power generation. Its versatility also extends to the production of biochar, silage, animal feed, and biogas, while being cultivable on marginal lands, thereby minimizing land-use competition with food crops. This positions EG as a superior biomass resource compared to other perennial grasses and residues such as rice husk, which, despite having value-added applications in ash utilization, suffers from brittle structure, high silica, and poor combustion performance.

Based on the experimental results, a three-cycle utilization model of EG demonstrates a highly promising approach for integrated and sustainable agricultural systems. In the first and second harvest cycle, EG exhibited the highest biomass yield and a favorable lignocellulosic composition, making it an efficient candidate for use as solid biofuel in combustion systems, especially for rural energy needs. During the second regrowth cycle, the nutritional composition is acceptable, characterized by moderate digestible fiber and total digestible nutrients, indicating its suitability for ruminant feed. Elephant grass harvested at 120 days has been reported to exhibit high NDF content (75.7%) for the whole plant [26], which is comparable to the NDF observed for VS-19 in the current study (75.4%). While NDF is an important parameter for assessing forage quality, higher fiber content can reduce digestibility and limit feed intake in animals. Therefore, for optimal forage production, EG is best harvested at 35–45 days of growth [27]. However, if the primary purpose is energy production, a longer harvest interval of at least 90 days is recommended to maximize biomass yield [28]. The residues or suboptimal parts from the third cycle can be composted and applied as organic fertilizer. This closed-loop strategy not only maximizes the economic value of EG at each stage of its growth but also reduces waste and external input requirements such as synthetic fertilizers. Ultimately, it supports a circular bioeconomy model tailored for tropical agriculture, enhancing both productivity and environmental sustainability.

Beyond its energy potential, grass-based biomass offers important environmental benefits such as carbon sequestration through photosynthesis, rehabilitation of degraded lands, and erosion control via ground cover. Miscanthus was reported to exhibit higher photosynthetic efficiency than maize and requires fewer external inputs [36]. Similarly, switchgrass provides a positive net energy balance, contributing to greenhouse gas mitigation when used as a substitute for fossil fuels [37].

A potential limitation in this study is the assumption that the HHVs of EG varieties VA-06 and VDP are comparable to that of VS-19. Using the HHV of VS-19 across all cultivars may introduce some inaccuracy in the calculated energy outputs. To improve accuracy and strengthen the reliability of comparative energy assessments, future studies should directly measure the HHV of each variety.

5. Conclusions

The agronomic performance of three elephant grass (EG) cultivars (VS-19, VA-06, and VDP) across three successive harvest cycles was assessed. Energy characteristics, forage quality, and potential for soil improvement were assessed primarily based on VS-19 cultivar, the high-biomass-yielding cultivar. A harvesting frequency of three times within 14 months was achieved. In practice, the decision to harvest depends not only on plant age but also on factors such as internode number, plant height, and expected biomass yield. Therefore, the actual number of harvests per unit of time may vary significantly. VS-19 demonstrated the highest and most stable biomass yield (63.81 t/ha annually), translating into the greatest annual potential energy output (32,636 HOE/ha). While VA-06 (28,699 HOE/ha) and VDP (27,952 HOE/ha) also showed potential, VS-19 consistently outperformed them. VS-19 displayed moderate nutritional quality. Its crude protein content was below recommended levels for ruminants, though its total digestible nutrients were acceptable. However, the high fiber fraction reduces digestibility; therefore, earlier harvesting within 60 days is recommended to improve feed value. Despite these limitations, EG remains a viable forage source, particularly for ruminants, when harvest timing and management are optimized. Biomass residues from the third harvest cycle demonstrated potential for soil improvement. Low moisture content, near-neutral pH, high organic carbon, and a balanced macronutrient profile make EG residues suitable as organic soil amendments, enhancing soil fertility and nutrient cycling and reducing reliance on synthetic fertilizers. Overall, EG emerges as a versatile, multipurpose crop with strong potential in integrated agricultural systems. A three-cycle utilization model can maximize its value: the harvest from the first and second cycles prioritized for bioenergy and forage, and from the third cycle as material for organic fertilizer. This approach supports a circular bioeconomy by reducing waste, diversifying applications, and minimizing land-use competition with food crops.