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Abstract
The long-term intensive production system employed in the Sugar Estates in Ethiopia, characterized by monoculture, preharvest burning, and excessive tillage, has led to soil degradation with a concomitant decline in sugarcane yield. Therefore, a study was conducted at Wonji-Shoa Sugar Estate (WSSE) with the objective of evaluating the effectiveness of different green manuring plant species and cane stool destruction methods (SDMs) in improving cane yield. To that end, seven green manure plant species (sunn-hemp, lablab, cowpea, soybean, mungbean, dhaincha, and sugarcane trash) were evaluated under three SDMs (cultivating-out, spraying-out with herbicide, and maintaining the stool as it is) using a split-plot design. The experimental fields were established on two major soil types, with green manure crops incorporated into the soil before planting the sugarcane. The dry matter production and nitrogen contribution of the green manure plants, as well as the height, population, diameter, and yields of sugarcane, were determined and subsequently subjected to statistical and economic analysis. The results showed that cowpea, followed by lablab, dhaincha, and sunn hemp, were the most effective green manures in terms of improving cane performance, with up to 17–20 % yield advantage and 21–40 % net economic benefits over the control treatment. Additionally, the spraying-out MSD was as effective as the cultivating-out MSD, but both outperformed maintaining the stool. In conclusion, utilizing these green manuring crops in combination with the spraying-out SDM presents notable advantages for improved cane yield and enhanced economic benefits in a sustainable manner. Adoption of these practices, therefore, holds significant potential for reversing the constantly declining sugarcane yields at WSSE.
1. Introduction
The Ethiopian population has been growing very rapidly [1]. Rapid growth in the population significantly increases food consumption, putting further strain on vital natural resources such as water, soil, and energy [2, 3]. This situation exacerbates environmental issues such as agrochemical use, greenhouse gas emissions, and biodiversity loss which lead to declining crop productivity with a concomitant increase in food insecurity.
The impact of these challenges has manifested in the sugarcane plantations of Ethiopia. For example, the yield of sugarcane at the Wonji-Shoa Sugar Estate (WSSE) has significantly declined over the years. In 1972, the yield of sugarcane was 10.8 tons ha–1 month–1, which exceeds the global productivity by 2.3 tons ha–1 [4]. However, by 2022, the yield decreased to 4.3 tons ha–1 month–1 [5]. The same study indicated that, on average, sugarcane yields at WSSE decreased by 0.84 tons ha–1 year–1 [5]. A decreasing trend in sugarcane yields have also been observed in other estates nationwide.
The drastic decline in sugarcane yield has severely impacted Ethiopia’s sugar industry, jeopardizing its production capacity and long-term sustainability. Thus, the country’s progress toward achieving its ambitious goal of producing an adequate amount of sugar to fulfill its domestic demand and to export the surplus have been hindered [6]. Business Info Ethiopia [7] reported that local sugar demand was projected to reach 1.2 million tons between 2020 and 2021, while local production reach only 340,000 tons, leaving a significant gap of 860,000 tons. Therefore, Ethiopia has resorted to importing sugar by spending over US$150 million on sugar imports [7].
Prior studies indicate that long-term intensive production practices, including monoculture, can mainly lead to soil degradation, the accumulation of pests and pathogens, the presence of toxic compounds, and an imbalance in soil biodiversity [8,9]. Additionally, sugarcane is a heavy feeder of plant nutrients that can remove about 208 kg of nitrogen, 53 kg of phosphorus, 280 kg of potassium, 30 kg of sulfur, 3.4 kg of iron, 1.2 kg of manganese, and 0.6 kg of copper from 1 ha of land for every hundred tons of cane yield [10]. The combined impact of these factors eventually leads to soil fatigue, characterized by a gradual decrease in crop yields, even when fertilizers are applied and other soil management practices are implemented [9]. Likewise, at WSSE, significant declines in the major soil fertility indicators have been reported over the last seven decades [[11], [12], [13]], resulting in a negative impact on cane yield [14].
Several studies were conducted in Ethiopia in general and at WSSE in particular to improve cane yield through recommendations of optimum fertilizer rates, irrigation practices and mechanization technologies [15], improved varieties and agronomic practices [16] and crop protection practices [17]. However, neither of these endeavors have been effective in halting the decline in yields [18]. Therefore, practices that are crucial for mitigating the long-term decline in cane yield resulting from intensive production systems (monoculture, preharvest cane burning, excessive tillage etc) are imperative. In this regard, soil amendment via green manuring has proven to be an economically viable, environmentally sustainable, and socially acceptable practice that enhances soil health and crop productivity in a sustainable manner [19]. Other studies have also confirmed that green-manuring can be an effective way to manage yield decline and improve soil health [[20], [21], [22]]. For instance, Meyer [23] found that green manuring during a fallow period improved sugarcane yield by 45 % in the plant crop and 25 % in the first and second ratoon crops, making it 12.4 % more profitable than conventional cropping. When supplemented with artificial fertilizer such as urea, the yield improvement can reach up to 57 % [20].
The enhancement in cane yield due to green manuring is emanated from the improvements in the soil environment, including the changes in the physical, chemical, and microbial properties of soil [24]. Adekiya et al. [25], also reported that green manuring leads to improved crop performance by reducing soil bulk density and increasing the availability of soil organic matter, N, P, K, and Ca concentrations. Furthermore, green manures offer additional benefits, such as providing soil cover, suppressing weed growth [26] and various pests [27], and stabilizing the yields of subsequent crops [28].
Although, fallowing and planting Crotalaria juncea have been practiced in soils with high clay content at WSSE, it was not able to effectively reverse the decline in sugarcane yields. This might be partly due to the lack of evidence regarding the most suitable and effective green manure species and the optimal method for eradicating old sugarcane stool. Therefore, the objective of this study was to evaluate the effectiveness different green manuring species and methods for cane stool destruction in Wonji-Shoa Sugar Estate.
2. Materials and methods
2.1. Description of the study site
2.1.1. Location and Topography
The study was conducted in Wonji-Shoa Sugar Estate (WSSE), Upper Awash Valley, Central Ethiopia from 2021 to 2023. The sugar estate is located in Oromia Regional State in two districts of the East Shoa Zone (Adama and Bosat Districts) and in one district of the Arsi Zone (Dodota District). It is approximately 110 km from Addis Ababa in the southeasterly direction. The geographical location of the plantation site is within 8°19′54″–8°29′15″N and 39°13′34″–39°19′21″E at an altitude of 1540–1650 m above sea level (Fig. 1). The Awash River, the sole perennial river in the Awash Basin, crosses the plain, dividing the plantation into western and eastern banks.
Map of the study area (Adapted from Alemayehu et al., 2023).
2.1.2. Climate
According to Taye et al. [29], the study site exhibits a tropical savanna climate based on the Köppen climate classification. The long-term mean annual maximum and minimum temperatures from 1980 to 2020 [30] are 14.5 °C and 27.4 °C, respectively, while an average annual rainfall is 768 mm (Fig. 2). The primary rainy season occurs between June and September. Located in the transitional zone between semiarid and dry subhumid regimes, the climate at Wonji-Shoa Sugar Estate experiences distinct rainy and dry seasons during the summer and winter months, respectively.
Monthly mean rainfall and, maximum and minimum temperatures in the study area.
2.1.3. Geology
The Wonji Plain, which is situated in the main Ethiopian Rift Valley Region, is characterized by its geological formation resulting from volcanic activity and rift tectonics. The area comprises various geological units, including basalts (such as Alaji, Anchar, Arba-Gugguu, and Bofa), silicics (Arba Gurachaa), tuffs, ash flows, trachytes (Chilalo and Badda), and other volcanic rocks. Additionally, the region features lacustrine rift sediment that coexists with the Wonji volcanics. This sediment primarily consists of volcanoclastic materials, tuffs, silts, clays, and diatomites, with silts and clays being the predominant components. Alongside these formations, alluvial deposits are also prevalent in the area, occasionally intermixed with volcanic clastics [31].
2.1.4. Establishment
The establishment of the WSSE dates back to 1951 when an agreement was signed between the Dutch Company (H.V. Amsterdam) and the Ethiopian Government during the Regime of Emperor Hailesilassie. Subsequently, in 1954, the Wonji Sugar Factory commenced sugar production, marking a significant milestone in domestic sugar manufacturing within Ethiopia’s history. Presently, the sugarcane plantation spans approximately 11,000 ha [29]. The population of Wonji is estimated to be around 100,000, with 10,678 individuals employed by the Sugar Factory [32].
2.1.5. Soils
According to the soil map of the WSSE, two primary soil groups, vertisols and cambisols, comprise the majority (96 %) of the soils in the plantation [33]. Vertisols cover approximately 70 % of the land area and are widely distributed throughout the plantation, particularly in the lower parts of the alluvial landscape, where they from extensive plains and back swamps behind old levees. On the other hand, cambisols are typically associated with old channels, levees, sandbanks, and meander scars of the current and former courses of the Awash River. They exhibit a sinuous distribution across the Sugar Estate but occupy only 26 % of the total study area [33].
2.1.6. Land use/cover
Prior to the implementation of irrigated agriculture in the 1950s, the Wonji Plain and adjacent plateaus boasted abundant vegetation cover [31]. Various land use/cover (LUC) categories existed, supporting diverse animal and plant species. The site was predominantly recognized as grazing land for semipastoralists. However, over the past seventy years, the Wonji area has experienced substantial transformations in its land use/cover conditions.
2.2. Treatments and experimental design
In this study, two factors, namely, stool destruction method (SDM) and green manure plant species, were studied at two locations within the WSSE. The first location was area with vertisol soil type while the second location was area with cambisol soil types. The experimental design followed a split-plot design with three replications. The SDM, consisting of three treatments (spraying out with herbicide [SO], cultivating out with a disc plow [CO], and maintaining stools [MS]), was assigned to the main plot. The green manure plant species, which included Crotalaria juncea (sunn hemp), Glycine max (soybean), Lablab purpureus (lablab), Vigna unguiculata (cowpea), Sesbania aculeata (dhaincha), Vigna radiate (mungbean), sugarcane residue, and the control, were assigned to the subplots (Table 1). The selection of green manure species in this study was based on a literature review, specifically focusing on their successful performance in other sugarcane-growing countries [21,[34], [35], [36], [37], [38], [39]]. Sunn hemp was also used as a green manuring crop in a soil with a high clay content at WSSE.
Table 1
Treatments combination.
| Main Plots: Stool destruction methods | Sub plots: Green manuring species |
|---|---|
| 1. Spray out Sugarcane with herbicide (SO) 2. |
|
| 3. Cultivate out Sugarcane (CO) 4. |
|
| 5. Maintaining Sugarcane Stool (MS) |
|
2.3. Experimental management
The experiment was conducted in two phases. In the first phase, the green manure crops were grown, and the cane residue was mulched for about three months as per the indicated treatments (Table 1). In the second phase, sugarcane was planted following the incorporation of the green manures. Each experimental subplot consisted of 6 rows which have 1.45 m spacing and 6 m length, i.e., 52.2 m2 (Fig. 3). The data were collected from the central 4 rows of each plot. Adjacent plots were separated by 1.5 m, the distance between each replication was 2 furrows (2.9 m each), and the boundary distance in each replication was also 3 m (Fig. 3).
The field lay out of the experiment along with the dimensions of each plot and replicate. The serial number in each box indicates the plot number.
2.3.1. Main plot
The first treatment, “cultivating out the stool (stubble)”, is the conventional method of stool destruction at WSSE, where the process entails tilling the entire area, including both the rows and interspaces, to uproot and incorporate the stools into the soil using a disc plow. Subsequently, furrows with dimensions of 1.45 m in width and a depth of 25–30 cm were excavated using a tractor-mounted furrower [40].
The second treatment, known as “spraying out stool”, involved the destruction of the final ratoon crop using a nonresidual herbicide called glyphosate or roundup (C3H8NO5P) at a rate of 6.5 lit ha−1 [41]. The herbicide was applied when the ratoon crop grew sufficient foliage to ensure proper absorption and death of the stool.
The third treatment, “maintaining the stool”, consisted of sowing the green manures on the side of the furrow without destroying the stool, thereby allowing the green manure to climb, smother and kill the cane stool.
2.3.2. Sub plots
In the subplot treatments (green manures), the experimental fields were laid out while the geo-references of each plot were recorded using a Global Navigation Satellite System (GNSS) receptor. Additionally, the distances of selected plots from nearby fixed structures (benchmarks), such as trees and irrigation canals, were measured. Next, green manure legume seeds were sown by direct dibbling in holes spaced according to the seeding rate (Table 2), halfway between the bottom and top of the furrow side. The viability of the seeds was confirmed before sowing the green manures by conducting a germination test. Sowing was conducted on July 7 and 12, 2021, on the cambisol and vertisol, respectively. All cultural practices were performed following the production guidelines for each legume [42,43].
Table 2
Seeding rate and planting depths of the green manuring legumes.
| Green Manure Legume | Seed rate (kg ha−1) | Depth of planting (cm) |
|---|---|---|
| Cowpea (Vigna unguiculata) | 25.5 | 5 |
| Sunn hemp (Crotalaria juncea) | 35 | 5–10 |
| Lablab (Lablab purpureus) | 40 | 310 |
| Soybean (Glycine max) | 65 | ● 1–5 |
| Mungbean (Vigna radiata) | 25 | 4 |
| Sesban (Sesbania aculeata) | 35 | 5–10 |
At WSSE, sugarcane trash accounts for approximately 33 % of the total biomass of sugarcane which is about 15.6 t ha−1 [44]. Accordingly, for this specific treatment, the designated plot was mulched with dry trash at a rate of 15.6 t ha−1. To account for moisture content, the weight of the trash was adjusted during each application. This adjustment was made by collecting trash samples and determining the moisture content using the method described by Jagodziński et al. [45]. Cane residue (trash) blanketing was carried out from July 1–6, 2021.
The initial phase of the experiment lasted 3–4 months, and was terminated with the incorporation of each green manure crop. The optimal time for incorporating the green manure was determined to be the time when the pods of each green manure formed well but the seeds were still immature, at which stage the nitrogen and dry matter contents in a legume crop corresponded to the maximum [38]. In this study, green manure was incorporated into the soil from the end of September to the beginning of November 2021 based on the growth phase of each green manure. To perform the incorporation, the legume crop was mowed just above ground level and subsequently disked into the soil using a tractor-mounted disc harrow [40]. Rigorous supervision was implemented throughout the tillage operations to ensure strict adherence to the designated layout and prevent any mix-up of the treatments.
For the second phase experiment (cane planting), the land was prepared following the standard sugarcane production operation manual of the WSSE [40] after confirming that the soil was dried very well. Then, the fields were laid out in the same location as in the first phase of the experiment, following the registered GNSS coordinates and the recorded reference (benchmark) data. Then, two budded setts were prepared from the same portion of the seed cane, i.e., the middle of the stalk, from plants of similar age grown under similar environmental conditions. The sugarcane variety used in this study was NCo334, which is one of the major varieties planted in the sugar estate. About 42 setts per row were planted on 14 and 29 January 2022 on the cambisol (8.41852°, 39.24799°) and vertisol (8.39281°, 39.23630°), respectively, and covered with a thin layer of soil. Then, postplanting management practices, such as irrigation, fertilizer application, weeding, and molding, were executed as per the standard cultural practices of the Sugar Estate [40].
2.4. Data collection
2.4.1. Determination of total biomass yield and nitrogen contribution of green manure
The biomass of each plot of green manure was quantified by cutting and weighing the aboveground biomass from a randomly selected spot of one square meter (0.7 m × 1.45 m) from each test row (four rows per plot). The recorded value was then extrapolated to the total fresh biomass per hectare. Then, the moisture content of the fresh biomass was determined by randomly taking representative samples, weighing, and oven-drying them at 105 °C for 24 h [45]. Afterwards, the dried samples were weighed, and the moisture content (MC) was determined. Finally, the dry biomass (DB) per ha was determined using equation (1).
The nitrogen contribution was quantified by taking representative plant samples from the fresh biomass and subjecting them to tissue analysis in the laboratory to determine the concentration of nitrogen. The analysis of TN was performed by digesting the samples in a sulfuric-salicylic acid mixture [46]. Finally, the N contribution was calculated by multiplying the dry matter (ton ha−1) by the concentration of N (%) in the leaf tissue.
2.4.2. Data collection on sugarcane yield and yield components
The data collected from the planted sugarcane included the following parameters: the number of stalks per plot, stalk diameter, stalk height, and cane yield. The data for the stalk population (number of cane stalks per plot) and stalk height (from ground level up to the top visible dewlap) were recorded ten months after planting. Cane diameter data, on the other hand, were collected at harvest when the plants reached 16 months of age. For this measurement, ten random stalks were sampled from each harvestable plot. Additionally, these samples were sent to the laboratory for cane quality analysis. During harvest, the cane yield per plot was determined by weighing the cane. This value was then extrapolated to estimate the yield per hectare.
2.4.3. Sugar yield determination
To determine the sugar yield, the sampled sugarcane stalks were crushed using a mini-power crusher. Then, the percentages of pol, brix, and purity were determined, from which the estimated recoverable sugar percentage was calculated. The estimated recoverable sugar percentage (ERS) refers to the overall percentage of sugar that can be extracted from the cane. It was calculated using equation (2) [47].
where,
Brix refers to the concentration of dissolved solids in the sugar liquor or syrup. In this study, Brix measurements were obtained using a digital refractometer equipped with automatic temperature correction. The final brix value was expressed at a standardized temperature of 20 °C.
Pol% represents the apparent sucrose content in the sugarcane or juice, expressed as a mass percentage. To determine the Pol%, the sugarcane juice was mixed with lead acetate, filtered, and transferred into a tube within a polarimeter. The polarimeter measures the optical rotation of the sample, allowing for the determination of the Pol% value.
A nonsugar factor of 0.61 was applied in this study. This factor accounts for the amount of sucrose lost during the final processing stages.
Additionally, a cane factor of 0.75 was utilized. This factor represents the correlation between the theoretical yields of molasses mixed juice and primary juice for the same genotype and cut cane, as determined by the milling test [47].
Finally, the sugar yield was determined using equation (3).
2.5. Data analysis
To analyze the variances and make comparisons of means among treatments, the Genstat software statistical package (18th edition) provided by VSN International [48] was utilized. The analysis employed the Duncan multiple range test (DMRT) at a probability level of less than 5 % (P < 0.05) for conducting the mean comparisons.
2.6. Economic analysis
The economic analysis was conducted using partial budget analysis, as outlined by SHI [49]. The parameters utilized in the partial budget analysis included gross benefit (GB), variable cost (VC), net benefit (NB), and the marginal rate of return (MRR). To minimize any potential bias resulting from various factors, the sugar yield was corrected by adjusting it downward by 10 % while the variable costs were adjusted upward by the same percentage. Then, the gross benefit was determined by multiplying the corrected sugar yield per hectare by the farm gate price of sugar at Wonji/Shoa Sugar Estate (WSSE) at the time of harvest (May 2023), which was 44000 ETB/ton.
Variable costs encompassed the expenses incurred on inputs during the production process. These costs included herbicide-glyphosate, green manure seeds, labor (charged at 72 ETB per mandays), and tillage-related expenses. To calculate the net benefit (NB), the variable cost was subtracted from the gross benefit. It is worth noting that the costs associated with other inputs and production practices, such as labor, transportation, field preparation, weeding, fertilization, watering, and harvesting, remained consistent across all treatments. The marginal rate of return (MRR) was determined by dividing the difference between the net benefit of the treatment and the net benefit of the control (untreated) by the variable cost. For a treatment to be considered a viable option for sugarcane plantations, a minimum MRR of 1 was assumed.
3. Results and discussion
3.1. Dry matter production of green manures
In both vertisol and cambisol, the aboveground dry matter production, N concentration, and N contribution of green manures were significantly (P < 0.05) different among the species as well as between the stool destruction methods (SDM), whereas no interaction effect was detected (Table 3).
Table 3
Mean squares for above ground dry matter production (DMP) (ton ha−1), Nitrogen concentration (NCc) (%), and Nitrogen contribution (NCb) (ton ha−1) of the treatments on the two major soil types (vertisol and cambisol) of Wonji-Shoa Sugar Estate during 2021–2023 in central Ethiopia.
| Source of variation | df | Vertisol
|
Cambisol
|
||||
|---|---|---|---|---|---|---|---|
| DMP | NCc | NCb | DMP | NCc | NCb | ||
| Rep | 2 | 2.93 | 0.06 | 5024 | 1.6024 | 0.419 | 2211 |
| SDM | 2 | 14.03*** | 1.51*** | 24176*** | 2.8027*** | 0.881*** | 4207*** |
| GM | 5 | 23.12*** | 1.38*** | 18458*** | 15.9849*** | 0.816*** | 6343*** |
| SDM*GM | 10 | 1.25ns | 0.31ns | 1520ns | 0.0451 ns | 0.071 ns | 167 ns |
| Error | 30 | 1.02 | 0.16 | 1626 | 0.2106 | 0.056 | 117.4 |
| Total | 53 | ||||||
| CV (%) | 21.7 | 15.2 | 31.3 | 14.6 | 11.3 | 16.3 | |
ns, not significant;, “∗”,“∗∗”and“∗∗∗” Significant at P<0.05, P<0.01 and P<0.001, respectively; DF, degree of freedom; CV, coefficient of variation; SDM = Stool Destruction Method; GM, Green Manure.
Sunn hemp (Crotalaria juncea L.) resulted in the highest dry matter production, which was significantly greater (P <0.05) than that of mung bean (Vigna radiata L.), dhaincha (Sesbania aculeata L.), soybean (Glycine max L.), cowpea (Vigna unguiculata L.) and lablab (Lablab purpureus L.) by 70 %, 58 %, 33 %, 30 %, and 20 %, respectively (Table 4). However, mung bean produced the lowest dry matter in both soil groups, although it did not significantly differ from dhaincha in the cambisol. In vertisol, lablab performed similarly to cowpea and soybean but significantly outperformed dhaincha. In cambisol, cowpea was on par with dhaincha, soybean, cowpea, and mung bean, but all were significantly lower in dry matter production compared to lablab (Table 4).
Table 4
Above ground dry matter production, nitrogen concentration and nitrogen contribution of different green-manuring crops grown on lands prepared by cultivating out (CO), spraying out (SO) and maintaining the cane stool (MS) on the two major soil types (vertisol and cambisol) at Wonji-Shoa Sugar Estate during 2021–2023 in central Ethiopia.
| Treatment | Dry matter production (ton ha−1)
|
N content (%)
|
N contribution (ton ha−1)
|
|||
|---|---|---|---|---|---|---|
| Vertisol | Cambisol | Vertisol | Cambisol | Vertisol | Cambisol | |
| Green manuring crop | ||||||
| Sun hemp | 6.88a | 5.45a | 2.36bc | 1.96c | 166.9a | 107.72a |
| Lablab | 5.51b | 3.92b | 2.73 ab | 2.09bc | 149.9a | 81.50b |
| Cowpea | 4.82bc | 2.64cd | 2.85a | 2.26 ab | 139.3a | 60.41c |
| Soybean | 4.60bc | 2.79c | 3.01a | 2.35a | 140.0a | 65.64bc |
| Mung bean | 2.00d | 1.77d | 2.03c | 1.58d | 39.1b | 28.54d |
| Dhaincha | 4.37c | 2.31cd | 2.99a | 2.38a | 137.0a | 55.50c |
| Stool Destruction Method | ||||||
| Cultivating out (CO) | 4.81a | 3.46a | 2.67 ab | 2.12a | 130.3 ab | 73.73a |
| Spraying out (SO) | 5.52a | 3.28a | 2.95a | 2.31a | 164.6a | 76.93a |
| Maintaining (MS) | 3.76b | 2.71b | 2.37b | 1.87b | 91.3b | 49.0b |
The variations in biomass production among the legume plant species can be attributed to their unique genetic traits that impact their growth. Although legume plants share common physiological functions and essential resource needs such as carbon, nutrients, and water, there are significant differences in how efficiently they acquire resources, allocate them to different tissues, and undergo turnover [50]. For example, sunn hemp has fast initial development, reaching heights of up to 3.5 m [51] and hence producing substantial amount of biomass within a relatively short period of time [52]. On the other hand, mung bean is a dwarf-statured (0.15–1.25 m) plant with a short growth duration [53]. These results agree with the findings of Garside et al. [36] and Irin et al. [54], who reported the highest and lowest dry matter production for sunn hemp and mung bean, respectively.
Regarding the contribution of N to the soil in both vertisol and cambisol, sun hemp showed superior performance over all the studied green manuring plants (Table .4). However, except for mung bean, all the green manuring plants did not show significant (P < 0.05) variations in vertisol, whereas in cambisol, sunn hemp was significantly 277, 94, 78, 64, and 32 % greater than those of mung bean, dhaincha, cowpea, soybean, and lablab, respectively. This superior performance of sunn hemp may be because it produced the greatest amount of biomass (Table 4). Studies elsewhere also indicated that sunn hemp produced up to 5.9 ton ha−1 biomass and contributed 134–145 kg ha−1 N in a 9–12-week period [55].
In terms of the SDM, there were no significant (P < 0.05) differences observed between the SO and CO treatments for any of the studied parameters in either soil type (Table 4). However, when maintaining stools (MS) was practiced, dry matter production, N concentration and N contribution in both vertisol and cambisol were significantly lower than those in CO by 28 and 47 %, 28 and 21 %, and 13 and 25 %, respectively. However, the differences in N-concentration and N-contribution between the CO and MS treatments were not significant in vertisols. The SO was also significantly greater than MS in terms of dry matter production, N concentration and N contribution by 13 and 24 %, 43 and 80 %, and 50 and 57 % in the vertisol and cambisol, respectively.
The significantly (P < 0.05) low performances observed in MS could be attributed to competition between the regrowth of sugarcane and the green manure plants for resources. Moreover, the release of allelochemicals from the root system of the ratoon cane into the soil [56] could also contribute to the reduced performance of the green manures in the MS treatment. On the other hand, although tillage in the CO treatment was expected to enhance aeration and promote green manure growth, the statistically comparable performance of the SO treatment indicates that the application of glyphosate herbicide may compensate for the absence of tillage. According to Hove-Jensen et al. [57], phosphorus in a glyphosate herbicide undergoes transformation into a beneficial phosphorus source upon spraying. Additionally, the conservation of extra soil moisture due to leaving the land uncultivated and the improved microclimate during the growing season might also equally enhance growth under the spraying out method [58] Furthermore, the no-till method reduces mineralization and nitrification rates while promoting nitrogen immobilization [59]. This reduction in available nitrogen stimulates nitrogen fixation in legumes planted in no-till soil [60], leading to comparable or better nitrogen concentrations than those of the CO treatment. This finding corroborates the results of a study conducted on lentil (Lens culinaris), which demonstrated equal or greater yields under a no-till system [60].
For all the parameters examined, the performance of the vertisol was superior to that of the cambisol. This discrepancy may stem from the greater moisture retention capacity of the vertisol than the cambisol. Rainfall was the sole source of moisture for the green manures in the first phase of this study, i.e., growing the green manures. During this period, the rainfall was erratic both in quantity and distribution, and thus, the moisture retention capacity of the soils was the most limiting factor for growth. Additionally, the naturally greater soil fertility of vertisol compared to that of cambisol may also have contributed to the observed results. In line with these findings, Hamarashid et al. [61] and Tahir and Marschner [62] reported that soils with higher clay content typically have higher organic matter content and cation exchange capacity (CEC), resulting in improved nutrient and water retention capabilities, ultimately leading to increased biomass production.
3.2. Cane performance
In both the vertisol and cambisol, the analysis of variances in yield and yield components (cane population, height, diameter and yield, and sugar yield) indicated that the main effects of the SDM (the main plot), except for cane diameter in the vertisol, were significant (P < 0.05). The main effects of the green manuring treatments (the subplots), except for the cane diameter in the cambisol, were also significant (P < 0.05). However, no interaction effects were observed for any of the cane parameters considered in this study (Table 5).
Table 5
Mean squares for cane population, cane height (CH) (cm), cane yield (CY) (ton ha−1mo−1) and sugar yield (SY) (ton ha−1mo−1) of the treatments on the two major soil types (vertisol and cambisol) of Wonji-Shoa Sugar Estate during 2021–2023 in central Ethiopia.
| Source of variation | DF | Vertisol
|
Cambisol
|
||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| CP | CH | CD | CY | SY | CP | CH | CD | CY | SY | ||
| Rep | 2 | 16.7 | 33.00 | 0.034 | 6.38 | 0.23 | 199.46 | 2328.7 | 0.12 | 1.337 | 0.086 |
| SDM | 2 | 488.58*** | 1450.10*** | 0.019 ns | 6.14*** | 0.33*** | 1527.9*** | 3364.1*** | 0.30 ns | 24.404*** | 0.885*** |
| GM | 7 | 928.6*** | 367.30*** | 0.032 ns | 4.79*** | 0.17*** | 807.6*** | 300.6* | 0.09 ns | 5.028*** | 0.218*** |
| SDM*GM | 14 | 25.93 *** | 89.50*** | 0.013 ns | 0.43 ns | 0.02 ns | 51.11 ns | 165.2 ns | 0.10 ns | 0.338 ns | 0.008 ns |
| Error | 28 | 52.21 | 123.90 | 0.019 | 0.39 | 0.02 | 50.48 | 113.4 | 0.12 | 0.885 | 0.034 |
| Corrected total | 71 | ||||||||||
| CV (%) | 5.1 | 5.9 | 5.9 | 7 | 7.4 | 5.6 | 6.0 | 15 | 11.3 | 12 | |
ns, not significant; “∗”,“∗∗”and“∗∗∗” Significant at P<0.05, P<0.01 and P<0.001, respectively; DF, degree of freedom; CV, coefficient of variation; SDM, Stool Destruction Method; GM: Green Manure; CP, Cane Population; CH, Cane Height; CD, Cane Diameter; CY, Cane Yield; SY, Sugar Yield..
3.2.1. Cane yield components
In both vertisol and cambisol, the cane population in all the treated plots was significantly (P < 0.05) greater than that in the untreated plot (control). However, no significant differences were detected among plots that were green-manured with cowpea, lablab, sun hemp, or dhaincha (Table 6). The cowpea treatment resulted in the highest cane populations in both soil types. It showed significant increases of 26 %, 11.8 %, and 7.7 % over the control, mungbean, and cane trash treatments in vertisol and 27.1 %, 11.4 %, and 9 % over the control, soybean, and mungbean treatments in cambisol, respectively (Table 6). However, the cowpea treatment results were on par with the soybean treatment results for vertisol and the cane trash treatment results in cambisol. Furthermore, there were no significant differences between the sunn hemp, dhaincha, soybean, and cane trash treatments, as well as between the sunn hemp, mungbean and cane trash treatments in vertisol and cambisol, respectively.
Table 6
Yield components of sugarcane obtained from plots amended with different green manure crops and stool destruction methods at Wonji-Shoa Sugarcane Planation during 2021–2023 in central Ethiopia.
| Treatments | Population
|
Height (cm)
|
Diameter (cm)
|
|||
|---|---|---|---|---|---|---|
| Vertisol | Cambisol | Vertisol | Cambisol | Vertisol | Cambisol | |
| Green Manuring crops | ||||||
| Sunn hemp | 147.2 ab | 132.0 ab | 194.2a | 177.0a | 2.308 ab | 2.288 |
| Lablab | 150.4a | 133.5a | 190.5a | 181.9a | 2.406a | 2.251 |
| Cowpea | 151.2a | 136.0a | 191.2a | 183.7a | 2.398a | 2.504 |
| Soybean | 143.5 ab | 122.1c | 187.9a | 175.9 ab | 2.312 ab | 2.251 |
| Mungbean | 135.2c | 124.8bc | 185.6a | 173.0 ab | 2.291 ab | 2.366 |
| Dhaincha | 144.7 ab | 133.4a | 193.3a | 179.5a | 2.360 ab | 2.252 |
| Cane trash | 140.4bc | 130.0 ab | 184.4 ab | 176.1 ab | 2.322 ab | 2.401 |
| Control | 120.0d | 107.0d | 174.4 b | 165.0b | 2.223b | 2.196 |
| Stool Destruction Method | ||||||
| Cultivating out (CO) | 144.9a | 134.00a | 192.4a | 187.3a | 2.354 | 2.440a |
| Spraying out (SO) | 143.4a | 129.50a | 191.9a | 178.5 ab | 2.330 | 2.278 ab |
| Maintaining (MS) | 136.4b | 118.51b | 178.7b | 163.8b | 2.298 | 2.223b |
Regarding cane height, the plots treated with the green manures in both vertisol and cambisol were on par (P <0.05) with each other (Table 6). However, except for the cane trash treatment in vertisol and the soybean, mungbean, and cane trash treatments in cambisol, all treatments had significantly (P < 0.05) greater effects than did the control. Among the treatments, sunn hemp and cowpea resulted in the greatest cane heights, which were 11.4 and 11.33 % significantly greater than those of the cane plants obtained from the control in vertisol and cambisol, respectively.
Cane diameter was found to be greatest in response to green manuring with lablab (2.41 cm) and cowpea (2.40 cm) on vertisol, which resulted in significantly (P < 0.05) greater cane diameters than the cane diameter obtained from the control treatment (2.22 cm) by 8.3 %. However, the remaining treatments did not result in cane diameters that were significantly different from the control. In cambisol, green manure incorporation had no effect on cane diameter, where all the treated plots neither significantly differed from each other nor from the control.
Among the three parameters, cane population followed by cane height appeared to be more influenced by the green manure treatments, while cane diameter was less affected. This may be attributed to the fact that stalk diameter in sugarcane is primarily determined by genetic factors rather than by environmental factors [63]. However, for all the three parameters, sun hemp, lablab, cowpea, and dhaincha generally showed greater and more consistent performances than did the other treatments, indicating that green manuring of cane fields with these species may significantly enhance the yield components of sugarcane, which ultimately positivity influence the final yields of sugarcane. The superior performance of these species might be explained by their substantial contribution of nitrogen (146–166 kg ha−1) (Table 4) and other nutrients to the soil [24,25]. Studies elsewhere have also demonstrated that these particular green manure plants have superior effects on cane performance compared to other alternatives. For example, in Bangladesh, sun hemp and dhaincha enhanced the overall number of millable cane stalks by 17 % and 8 %, respectively [20]. In India, the application of these green manures increased the sugarcane population by 11–20 % [64]. Meena et al. [65] also reported that green manuring with sunn hemp significantly increased the number of tillers compared with that of the sole crop of sugarcane.
Regarding the methods of stool destruction, the cane population under the CO treatment was on par with the SO treatment, whereas both were significantly greater than that under the MS treatment by 6.23 % and 5.13 % in vertisol and by 13.07 % and 9.27 % in cambisol, respectively (Table 6). Similarly, in terms of cane height, CO and SO were 7.67 % and 7.39 %, respectively, significantly greater than MS. However, in the cambisol, the cane height in the MS treatment was on par with that in the SO treatment, but it was significantly lower (by 14.35 %) than that in the CO treatment. In terms of the cane diameter of the cambisol, the MS was on par with that of the SO but significantly lower than that of the CO by 9.76 %. In the vertisol, no significant differences were detected among all SDM treatments.
The absence of significant differences observed in cane populations, height, and diameter between the CO and SO methods indicates that both approaches can be used to destruct the cane stools. However, MS treatment is not feasible under WSSE conditions. These results could be linked to the corresponding findings in the biomass production of the green manures (Table 4), where the CO and SO treatments outperformed the MS treatment in both soil types. The lower biomass production of the green manure plant species in the MS might reduce the positive effects that would have been played in improving the physical, chemical and biological properties of the soil [24]. Moreover, the benefits of green manures in terms of providing soil cover, as well as suppressing weed growth and various pests [27], could not be realized in this scenario. This, in turn, limits the growth of the cane planted in such type of soils. Additionally, it is worth considering that the root system of ratoon cane releases allelochemicals into the soil [56], which may negatively affect the growth of the cane and contribute to the lower performance of the MS treatment.
3.2.2. Cane and sugar yields
Green manuring and stool destruction methods significantly (P < 0.05) influenced both cane and sugar yields. However, there were no interactions between the two factors in either of the variables (Table 5).
3.2.2.1. Effect of green manuring on cane and sugar yields
In the vertisol, green manuring with cowpea, sunn hemp, lablab, dhaincha, cane trash, and, to a lesser extent, soybean resulted in significantly (P < 0.05) greater cane yields than did the control treatment (Table 7). The cane yield obtained from green manuring with mung bean was on par with those of the soybean and untreated (control) plots but was significantly lower than those of all the other green manure-treated plots. For example, the cane yield obtained in response to green manuring with cowpea exceeded that in the control and mung bean green-manured plots by about 26 % and 20 %, respectively. Furthermore, the cane yields obtained in response to green manuring with cowpea, sunn hemp, lablab, dhaincha, and cane trash were in statistical parity.
Table 7
Cane and sugar yields of sugarcane obtained from plots amended with different green manure plant species at Wonji-Shoa Sugar Estate during 2021–2023 in central Ethiopia.
| Treatments | Cane yield (ton ha −1m−1)
|
Sugar yield (ton ha −1m−1)
|
||
|---|---|---|---|---|
| Vertisol | Cambisol | Vertisol | Cambisol | |
| Green Manuring crops | ||||
| Sunn hemp | 9.19 ab | 8.31 ab | 1.25 ab | 1.11b |
| Lablab | 9.65a | 9.07a | 1.31 ab | 1.25 ab |
| Cowpea | 9.67a | 9.13a | 1.34a | 1.28a |
| Soybean | 8.70bc | 7.87b | 1.20bc | 1.11b |
| Mungbean | 8.08cd | 7.82b | 1.11cd | 1.10b |
| Dhaincha | 9.34 ab | 8.51 ab | 1.27 ab | 1.19 ab |
| Cane trash | 9.17 ab | 8.78 ab | 1.24 ab | 1.23 ab |
| Control | 7.66d | 6.91c | 1.04d | 0.91c |
| Stool Destruction Method | ||||
| Cultivating out (CO) | 9.25a | 8.89a | 1.29a | 1.23a |
| Spraying out (SO) | 9.20a | 8.87a | 1.25a | 1.23a |
| Maintaining (MC) | 8.35b | 7.13b | 1.12b | 0.98b |
In the cambisol, green manuring with all plant species significantly (P < 0.05) increased cane yields compared with those in the control treatment (Table 7). For example, the cane yield obtained from plots green manured with cowpea exceeded the cane yield obtained from the control treatment by about 32 %. However, the cane yields obtained through green manuring with soybean and mung bean were significantly lower than those obtained through green manuring with cowpea and lablab, but both the former and the latter green manure plants showed statistically similar yields to those of sunn hemp, dhanicha and cane trash (Table 7).
The observed improvement in cane yield due to the green manuring treatments can be attributed to the corresponding increase in the stalk population, as shown in Table 6. This relationship is logical since the sugarcane population is crucial in determining sugarcane yield [66]. Bell and Garside [14] also noted that enhancements in sugarcane yield through soil amendment primarily occur due to the production of a greater number of stalks per unit area.
The current findings agree with those of Bokhtiar et al. [20], who reported yield improvements of 18 % and 3 % in plots treated with sunn hemp and dhaincha, respectively. Furthermore, Garside and Bell [67] reported a 21 % increase in subsequent sugarcane yields by incorporating legumes between sugarcane rotations. A study conducted in Bangladesh and Brazil [20] also revealed that cane yields significantly increased by 17–21 % when green manuring was employed compared to the yields obtained from the untreated control plot. These collective findings support the notion that green manuring, especially cowpea, lablab, sunn hemp and dhaincha, positively impacts sugarcane yield by promoting a greater stalk population, ultimately leading to improved productivity. For instance, the top-performing green manure crop in this study, cowpea, demonstrated a remarkable improvement in cane yield (9.4 ton ha−1 month−1), as it exhibited a 62 % increase compared to the average actual cane yield of WSSE obtained during the past 10 years (5.8 ton ha−1 month−1), but 9.4 % less than the highest yield achieved (10.3 ton ha−1month−1) during the second decade (1964–1973) of sugarcane production [3]. Therefore, these findings suggest that a strategic implementation of green manuring techniques in sugarcane cultivation at WSSE has the potential to increase cane productivity.
Like cane yields, in both vertisol and cambisol, green manuring application significantly (P <0.05) increased sugar yield compared with that in the control treatments. In the vertisol, green manuring with cowpea, lablab, dhaincha, sunn hemp, cane trash, and, to a lesser extent, soybean resulted in significantly (P < 0.05) greater sugar yields than the control treatment by 29 %, 26 %. 22 %, 20 %, 19 % and 15 %, respectively (Table 7). However, green manuring with mung bean resulted in a sugar yield that was in statistical parity with the sugar yield obtained from the control treatment. In the cambisol, except for the cowpea treatment which recorded the highest sugar yield, all the green manure treated plots showed no significant differences. Furthermore, green manuring with cowpea, did not significantly differ from that with lablab, dhaincha, or cane trash, while the values were 40 %, 14.9 % and 14.7 % significantly greater than those in the control, mung bean and soybean treatments, respectively (Table 7).
The sugar yield results observed in this study paralleled the trends observed in cane yield. This suggests that the variations in sugar yields were primarily influenced by differences in cane weight, which was consistent with the findings of Nixon and Simmonds [68]. This is mainly because the sucrose content is influenced more by soil moisture at maturity than by soil fertility, as highlighted by Zhou and Gwata [69]. These findings underline the significance of cane weight in determining sugar yield, emphasizing the need for optimizing management measures that enhance cane weight to improve sugar productivity. These results align with the findings of Ambrosano et al. [34], who observed that the use of green manures led to a sugar yield increase of 3 tons ha−1 compared to that of the control.
In general, the present study demonstrated that incorporating green manure crops during the fallow period can significantly enhance the yields and yield components of sugarcane. The positive impact of green manuring can be attributed to several factors. First, green manure enhances the effectiveness of nitrogen uptake while reducing nitrogen leaching loses [27]. For instance Sanaullah et al. [70] reported that green manure plants have the potential to fulfill 40–60 % of the total nitrogen requirement of subsequent crops. Additionally, they improve soil fertility by promoting nutrient retention, increasing soil organic matter (SOM), and enhancing microbial biomass [71]. An increase in soil organic matter content, in particular, influences soil moisture, temperature, and pH, leading to short- and medium-term changes in microbial populations [72,73]. Additionally, green manure plants have the ability to extract nutrients from deeper soil layers [24], which can then be utilized by plants. This sustained availability of macro- and micronutrients can enhance the activity of meristematic cells, cell elongation, metabolic processes, enzymatic activity, and growth hormones, ultimately promoting better vegetative growth, including cane population and height, which leads to increased cane and sugar yields. Furthermore, green manures play a significant role in suppressing weeds [26], controlling specific crop pests [27], reducing bulk density [25], and increasing water infiltration rates and water-holding capacity [74], which can create favorable conditions for cane growth.
The differences observed among the green manure plant species can be attributed to their varying quantities, chemical compositions, rates of decomposition, and release of nutrients to the soil, as well as their uptake by the planted crops [50]. As shown in Table 4, the cowpea, lablab, dhaincha, and sunhemp treatments returned greater amounts of biomass and nitrogen to the soil than did the other green manures. This could explain the consistently better performance of plots treated with these particular green manures (Table 7). Similar findings were reported by Manna et al. [75], who highlighted that sunn hemp, dhaincha, and cowpea were the most important green manure crops in India. Similarly, in southern Africa, lablab and sunn hemp are the most commonly used green manures [64]. In fact, these legume plant species have several characteristics that make them highly suitable for green manuring.
Cowpea, which exhibited the best performance in this study, is widely used as a soil renovator in the South [76], and is characterized by its ability to thrive in poor soils and provide dense ground cover that suppresses weed growth. It typically develops numerous tubercles on its roots, irrespective of soil inoculation. Additionally, cowpea is well suited as a warm season cover crop in subtropical environments, tolerating alkaline or acidic conditions, heat, and drought stress. These plants can produce substantial biomass with minimal or no nitrogen fertilizer application [50]. Lablab has unique characteristics in that it often produces more dry matter than cowpea, especially during drought conditions. Dhaincha can thrive under excessive moisture conditions in the field [23], which is a prevalent problem at the study site [31]. After decomposition, dhaincha enhances humus and available nitrogen content, reduces the carbon-to-nitrogen ratio, acts as a chelating compound, and increases the availability of micronutrients [77]. Sun hemp has rapid initial development, reaching heights of up to 3.5 m [51]. It can adapt to various soil types and produce substantial biomass while fixing nitrogen within a relatively short period [65].
3.2.2.2. Effect of stool destruction method on cane and sugar yields
Among the stool destruction methods, maintaining the stools (MS) treatment resulted in significantly (P < 0.05) lower sugar and cane yields compared to cultivating out (CO) and spraying out (SO), with both showing similar performance (Table 7). In vertisol, CO and SO achieved cane yields of 13.1 % and 12.5 %, and sugar yields that were 27.8 % and 26.5 % greater than those of the MS treatment, respectively. Similarly, in the cambisol, the CO and SO treatments resulted in 17.7 and 13.6 % greater cane yields and 26.4 and 29.5 % greater sugar yields than did the MS treatment, respectively.
The results obtained for both soil types can be attributed to the impact of the stool destruction method (SDM) on the biomass production of the green manures (Table 4). This, in turn, led to variations in the cane population and height (Table 6). Given that cane yield is predominantly influenced by the cane population and height [66], the result observed for these yield components might directly influence the sugar and cane yields (Table 6).
The statistically similar performances of SO and CO have practical implications for the study site. One of the challenges encountered during the implementation of green manuring at WSSE is the occurrence of rainfall at the beginning of the fallowing period, which makes the conventional stool destruction method (CO) difficult to execute. In such circumstances, the SO method has become the best alternative since it is not constrained by soil moisture levels.
3.3. Economic analysis
In both verisols (Table 8) and cambisosls (Table 9), the treatments involving cowpea yielded the greatest net benefits (NB) of ETB 844,540 ha−1 and 807,800 ha−1, which were 28.54 % and 39.9 % greater than those of the control, respectively. Following cowpea, lablab exhibited 25.6 % and 36.55 % higher NB than the control in vertisol and cambisosls, respectively. The control treatments resulted in the lowest field benefits of ETB 657,010 ha−1 in vertisol and 577,400 ha−1 in cambisosls. Similar trends were observed in the marginal rate of return (MRR) for both soil types. Consequently, the most economically viable option was the cowpea treatment, with marginal rate of return (MRR) of 110.9 and 151.2 in vertisol and cambisosls, respectively. This was followed by dhaincha, lablab, and sunn hemp (Table 8, Table 9). Overall, all the green manure treatments outperformed the untreated control, with MRR exceeding 1, surpassing the minimum threshold. This result is consistent with that of Meyer [23], who reported that, on average, green manuring was 12.4 % more profitable than conventional cropping methods.
Table 8
Partial budget analysis of green manure incorporation and method of stool destruction on Vertisol of Wonji-Shoa Sugar Estate during 2021–2023 in central Ethiopia.
| Treatment | SY (x*103 ton ha−1) | ASYb (x*103 ton ha−1) | GFB (x*103 ETB ha−1) | TVC (x*103 ETB ha−1) | NB (x*103 ETB ha−1) | PNB | MRR |
|---|---|---|---|---|---|---|---|
| Green manuring crops | |||||||
| Sun hemp | 20.05 | 18.05 | 794.1 | 1.55 | 792.53 | 20.63 | 79.75 |
| Lablab | 20.89 | 18.80 | 827.2 | 1.91 | 825.25 | 25.61 | 80.18 |
| Cowpea | 21.37 | 19.23 | 846.1 | 1.53 | 844.54 | 28.54 | 110.9 |
| Soybean | 19.22 | 17.30 | 761.0 | 2.53 | 758.47 | 15.44 | 37.06 |
| Mungbean | 17.78 | 16.01 | 704.3 | 1.61 | 702.66 | 6.95 | 26.37 |
| Dhaincha | 20.29 | 18.26 | 803.5 | 1.52 | 802.01 | 22.07 | 86.50 |
| Cane trash | 19.81 | 17.83 | 784.6 | 7.20 | 777.42 | 18.33 | 15.95 |
| Control | 16.59 | 14.93 | 657.0 | 0.00 | 657.01 | 0.00 | 0.00 |
| Stool destruction method | |||||||
| CO | 20.65 | 18.58 | 817.7 | 5.84 | 811.87 | 14.51 | 16.75 |
| SO | 19.93 | 17.94 | 789.4 | 5.34 | 784.01 | 10.58 | 13.53 |
| MC | 17.90 | 16.11 | 709.0 | 0.00 | 709.00 | 0.00 | 0.00 |
SY, sugar yield; ASY, Adjusted sugar yield; GFB, Gross field benefit; TVC, Total variable cost; NB, Net benefit; PNB, Percent net benefit relative to the control; MRR, Marginal rate of return; CO, Cultivating out; SO, Spraying out; MS, Maintaining the stool.
Table 9
Partial budget analysis of green manure incorporation and method of stool destruction on cambisol of Wonji-Shoa Sugar Estate in central Ethiopia.
| ′Treatment | SY (x*103 ton ha−1) | ASY (x*103 ton ha−1) | GFB (x*103 ETB ha−1) | TVC (x*103 ETB ha−1) | NB (x*103 ETB ha−1) | PNB | MRR | |
|---|---|---|---|---|---|---|---|---|
| Green Manuring crops | ||||||||
| Sun hemp | 17.63 | 16.03 | 705.2 | 1.55 | 703. 7 | 21.86 | 82.61 | |
| Lablab | 19.76 | 17.96 | 790.4 | 1.91 | 788.5 | 36.55 | 111.51 | |
| Cowpea | 20.23 | 18.39 | 809.3 | 1.53 | 807.8 | 39.90 | 151.20 | |
| Soybean | 17.63 | 16.03 | 705.2 | 2.53 | 702.7 | 21.69 | 50.61 | |
| Mungbean | 17.39 | 15.81 | 695.7 | 1.61 | 694.1 | 20.21 | 73.36 | |
| Dhaincha | 18.81 | 17.10 | 752.5 | 1.52 | 751.0 | 30.06 | 114.86 | |
| Cane trash | 19.52 | 17.75 | 780.9 | 7.20 | 773.7 | 34.00 | 28.27 | |
| Control | 14.44 | 13.12 | 577.4 | 0.00 | 577.4 | 0.00 | 0.00 | |
| Stool Destruction Method | ||||||||
| CO | 19.52 | 17.75 | 780.9 | 5.84 | 775.1 | 25.01 | 27.55 | |
| SO | 19.41 | 17.64 | 776.2 | 5.34 | 770.9 | 24.33 | 29.23 | |
| MC | 15.50 | 14.09 | 620.0 | 0.00 | 620.0 | 0.00 | 0.00 | |
SY, sugar yield; ASY, Adjusted sugar yield; GFB, Gross field benefit; TVC, Total variable cost; NB, Net benefit; PNB, Percent net benefit relative to the control; MRR, Marginal rate of return; CO, Cultivating out; SO, Spraying out; MS, Maintaining the stool.
Regarding the methods used for stool destruction, CO in vertisol demonstrated the highest MRR at 16.75, followed by SO at 13.53. This resulted in NBs that were 14.51 % and 10.58 % greater than those the control, respectively. In the cambisol, SO exhibited a slightly greater MRR (29.23) than did CO (27.55), and both methods yielded NB that were 25.01 % and 24.33 % greater than those of the control, respectively. Consequently, the most economically advantageous stool destruction methods were CO in vertisol and SO in cambisol. However, given the negligible differences between the two methods, the preference for the use of SO is justified due to its additional advantages in terms of improving soil health and its ease of application.
The prominent strength of this study is its comprehensive analysis, considering the yield components, cane yield and sugar yield as well as the economic benefit of the studied factors on the two main soil types present in the WSSE. The other notable strength of this study was its integration of three out of the four sustainable pillars for cane production (Franco et al., 2018), which included crop rotation, minimum tillage, and residue retention. Hence, this study represents a pioneering endeavor to comprehensively address yield decline management in the Ethiopian Sugar Estates. Its versatile approach sets it apart as the first of its kind in tackling this crucial aspect of sugar production in Ethiopia. However, there are some limitations of the study and areas that require further investigation, particularly regarding the interaction between green manures and artificial fertilizers, as well as the selection of suitable varieties of the identified green manuring crops. Additionally, it is advantageous to validate the findings of the present study and to conduct similar investigations on other sugar estates across the country.
3.4. Conclusions
The results of this study have demonstrated that green manuring significantly increased cane and sugar yields. Cowpea was found to be the most effective green manuring plant for improving sugarcane yields, followed by lablab, sunn hemp, and dhaincha. In vertisol, these green manures led to yield improvements of 20–26 %, with net economic benefits of 21–29 %, respectively, while also contributing up to 167 kg nitrogen ha−1. In cambisol, similar positive effects were observed, with cane yield improvements of 20–32 % and net economic benefits of 22–40 %, respectively, along with a contribution of up to 108 kg nitrogen ha−1. In terms of stool destruction methods, spraying out was found to be as effective as cultivating out. However, considering the additional benefits it provides for soil health and its ease of application, spraying out is the most preferable method. Therefore, regular green manuring during the fallow period with cowpea, lablab, dhaincha, or sunn hemp should be implemented in all fields of the plantation in the indicated order using the spraying out method of stool destruction. Future research should prioritize investigating the combined effect of green manuring and the application of mineral fertilizers using the spraying out method of stool destruction, which can further maximize cane and sugar yields while simultaneously enhancing the economic and environmental sustainability of sugarcane production in the country. In the face of continuously declining cane yields across the country, it is crucial for researchers, policymakers, and cane managers to consider incorporating green manuring and effective stool management methods into the standard operating procedures of sugar estates. This integration can prove to be beneficial for addressing these challenges and improving overall cane productivity.
CRediT authorship contribution statement
Alemayehu Dengia: Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Nigussae Dechassa: Writing – review & editing, Resources, Project administration. Lemma Wogi: Writing – review & editing, Validation, Supervision, Conceptualization. Berhanu Amsalu: Writing – review & editing, Supervision, Resources, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the Ethiopian Sugar Industry Group for supporting the research. Thanks are also due to Wonji-Shoa Sugar Factory and Wonji Research and Extension Services for providing the experimental site and the necessary resources, and collecting field data, respectively.
References
