The experiment was conducted with the objective to evaluate the effects of fertilizer level and row spacing on soybean morphological characteristics, forage yield, and nutritive value in the highlands of Ethiopia. A factorial arrangement of randomized complete block design (RCBD) using two factors (row spacing and four fertilizer levels) containing three row spacings of 50, 60, and 70 cm were combined with four 0, 100, 120, and 140 kg ha⁻¹ NPS fertilizer rates. Growth parameters and forage yield data were gathered at the 50% flowering stage, weighed, dried, and ground. Subsamples were then taken for analysis of the following parameters: ash, dry matter (DM), organic matter (OM), in vitro DM digestibility (IVDMD), fiber contents (neutral detergent fiber [NDF], acid detergent fiber [ADF], and acid detergent lignin [ADL]), and metabolizable energy (ME). The collected data were subjected to a general linear model (GLM) for statistical analysis system (SAS) version 9.0. There was significant interaction (p < 0.05) effect in the fertilizer level and spacing only in the number of nodules among growth parameters, but for nutritional value of forage soybean DM, crude protein (CP), CP yield (CPY), and ME. Both spacing and fertilizer application showed significant effects (p < 0.001) on the majority of growth characteristics like plant height (PH), number of branches (NBs), number of leaf per plant (NLPP), number of nodule (N), DM yield (DMY), and leaf-to-stem ratio (LSR). The growth parameters and nutritional quality had better performance in 140 kg ha⁻¹ NPS application and 70 cm row spacing than in lower fertilizer levels. Moreover, significantly (p < 0.001), maximum DMY (12.2 t ha⁻¹) was recorded from 50 cm row spacing. Regarding fertilizer levels, maximum DMY was recorded from high-level NPS fertilizer application (10.9 t ha⁻¹). The row spacing of 70 cm had the highest CP content (16%). The best marginal rate of return (MRR; 859.85%) and largest net profit (40212 birr) were obtained from 140 and 100 kg NPS fertilizer treatments with 50 cm row spacing, respectively. In the current result, forage soybean with a rate of 100 kg ha⁻¹ NPS fertilizer and 50 cm row spacing was chosen among the treatments in the current study and is advised for wider exploitation through irrigation application.

1. Introduction

Livestock production in Ethiopia faces several constraints, one of the most critical being a shortage of feed and poor feeding practices [1, 2]. The primary feed resources for ruminant animals in the country are green fodder from natural pastures (grazing) and crop residues, both of which are low in nutritional value [3]. A major limitation of grazing is its low dry matter yield (DMY) [4]. In particular, feed scarcity during the dry season poses a significant barrier to optimal livestock productivity in Ethiopia [5]. Improving both the quantity and quality of forage through the inclusion of forage legumes is essential for enhancing animal performance [6]. However, only a very small proportion (0.32%) of improved feed is currently used in Ethiopian livestock systems [7]. Therefore, integrating legume forages into farming systems presents a viable solution to address these feed-related challenges.

Soybean is one promising forage legume that can enhance the nutrient content of natural pastures and agricultural residues. It has been cultivated for its nutritious hay and silage, and historically, soybean was primarily used as fodder prior to World War II [8]. Moreover, soybean holds great potential as a high-protein, nutrient-rich forage suitable for pasture, hay, or silage, comparable in quality to alfalfa [9].

Like other forage species, the growth and yield of forage soybeans are influenced by row spacing. For instance, the highest biomass yields (5673 and 4792 kg ha⁻¹) for grain-type soybean were obtained with narrower (40 cm) inter-row spacing, while the lowest yields (2930 and 3522 kg ha⁻¹) were recorded at wider (100 cm) spacings [10]. However, the forage quality of total herbage, as well as the leaf and pod fractions, was not significantly affected by row spacing [11]. Similarly, [12] reported no significant differences in forage yield among row spacings of 20, 40, or 60 cm, although 80 cm resulted in the lowest yield.

To the best of the authors’ knowledge, no studies have yet investigated the nutritional quality and economic return (including marginal rate of return [MRR]) of forage soybean across specific combinations of fertilizer application rates and row spacing, which could provide valuable insights for farmers. Our hypothesis was that the combined effects of different levels of row spacing and fertilizer levels would bring a different result on forage biomass yield, nutritional quality and economic return. Therefore, the purpose of this research was to assess the combination of effects of two factors, that is, row spacing and fertilizer levels, on forage biomass yield, nutritional quality, and economic feasibility of forage soybean in the highlands of Ethiopia.

2. Materials and Methods

2.1. Description of the Study Area

The study was conducted at the Koga irrigation scheme of Amhara region, in Kudmi kebele (local administration system in Ethiopia), North Mecha district of the west Gojam zone, Ethiopia. The study area is situated at an altitude that ranges from 1800 to 2500 m.a.s.l. geographically the study area is located between 11°20′20″–11°24′20″ N latitude and 37°4′20″–37°10′20″ E longitude. During the experimentation period, the amount of rainfall was 152.5 mm, and the average temperature was 19.36°C (Table 1).

Table 1. Monthly rainfall, minimum, mean, and maximum temperature of the study area during the growing season.

Climate data	Months
Climate data	December	January	February	March	April	May
Rainfall	0	0	0	0	32.8	119.7
Minimum temperature	9.1	9.9	9.8	9.4	9.4	9.4
Average temperature	19	19.65	19.55	19.35	19.2	19.4
Maximum temperature	28.9	29.4	29.3	29.3	29	29.4

Note: Source: Bahir Dar meteorological station 2021/2022.

2.2. Land Preparation

The experimental land was cleared, plowed by oxen, and harrowed to fine tilth before laying out plots and planting. The land was leveled out to maintain a well-prepared seedbed. Soil samples were taken using the diagonal soil sampling method in the experimental area at 0–20 cm depth using an auger. The soil sample analysis is presented in Table 2.

Table 2. Soil sample analysis.

Soil samples	Result
TN	0.155
AP	4.72
Soil organic carbon	1.56
pH	4.98
Organic matter	2.69%

2.3. Experimental Design and Management

The experimental material was a newly released variety called soybean accession name (TG X1990_114FN). A factorial arrangement with a randomized complete block design (RCBD) having two factors (fertilizer rate and row spacing) with three replications was applied in the experiment. The experiment had 12 treatment combinations that were three-row spacing (50, 60, and 70 cm) with four fertilizer rates (0, 100, 120, and 140 kg ha⁻¹ NPS).

NPS fertilizer ratio (by weight): 19% nitrogen (N); 38% phosphorus pentoxide (P₂O₅) → which is equivalent to ~16.6% elemental phosphorus (P); 7% sulfur (S). This is often referred to as 19-38-7 NPS (Table 3).

Table 3. Fertilizer composition.

NPS rate (kg/ha)	N (kg/ha)	P₂O₅ (kg/ha)	S (kg/ha)
0	0	0	0
100	19	38	7
120	22.8	45.6	8.4
140	26.6	53.2	9.8

The total number of plots was 36, the plot size was 3 × 3 m, the spacing between each replication was 1.5 m, and the distance between plots was 1 m. The total experimental area was 48 × 14 m (672 m²) with an individual plot size of 9 m². The furrow irrigation system was applied.

2.4. Data Collection

All morphological characteristics data were collected from the randomly selected sample of 10 plants of each plot from the middle of two rows at 50% flowering from the net plot area, and the mean was computed [13]. To determine root length and the number of roots in plants, by manual, roots are carefully excavated or washed and counted by eye [14].

2.4.1. DMY

A 500 g of fresh sub-samples were taken from the total fresh biomass [15]. Then manually chopped into small pieces, and it was dried in an air draft oven at 65°C for 72 h to determine DM content (DMC; %). This formula calculates the DMY from the green forage yield (GFY) and the DMC of the forage. Here’s how each component contributes:

(1)

where:

•
DMY = dry matter yield in quintals per hectare (q/ha)
•
GFY = green forage yield in quintals per hectare (q/ha)
•
DMC = dry matter content as a percentage (%)

The crude protein yield (CPY) was calculated after determining the DMY of the harvested forage and then crude protein (CP; %) of the forage multiplied by the DMY of the harvested forage.

2.4.2. Leaf-to-Stem Ratio (LSR)

The LSR was estimated using the following formula after drying and weighing of leaves and stems.

(2)

2.4.3. Forage Chemical Composition

To determine the chemical constituents, 500 g of fresh samples were taken, stored in an airtight bag, and subjected to a forced air draft oven at 65°C for 72 h. Then, the dried samples were ground and allowed to pass through a 1 mm width sieve. Determining dry organic matter digestibility (DOMD) is crucial for evaluating the nutritional quality of forages and feeds, using in vitro methods using the procedure of Tilley and Terry’s [16]. The determination of organic matter (OM) following the methods of [17]. Ash was determined by igniting at 550°C overnight. CP was determined by Kjeldahl method. Acid detergent fiber (ADF), neutral detergent fiber (NDF), and acid detergent lignin (ADL) were analyzed by using the procedure of Van Soest et al.’s [18] method. Tilley and Terry’s [16] procedure was used to determine in vitro DM digestibility (IVDMD), and then metabolizable energy (ME) was determined by using the procedure of [19].

2.5. Data Analysis

Data on morphological characteristics, total DM yields, and nutritive values were subjected to analysis of variance (ANOVA). Using the general linear model (GLM) procedure of statistical analysis system (SAS) statistical computer package version 9.0). Duncan’s Multiple Range Test (DMRT) was used at the probability (p) of 95% to test the significant differences between treatment means.

The statistical model for the analysis of data was:

(3)

where

Y_ijk = all dependent variables (morphological data, forage yield, chemical compositions, and partial budget analysis);

μ = overall mean; β_j = effect of replication; τ_i = effect of fertilizer; γ_k = effect of row spacing; (τγ) _jk = interaction effect (fertilizer and row spacing); and E_ijk = residual error.

2.6. Partial Budget Analysis

The economic analysis was performed using partial budget analysis following the procedure described by the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) [20]. The partial budget analysis was based only on the variable cost of fertilizer and benefits from the DMY. In cooperatives, the average cost to purchase NPS per quintal during the times we conducted our experiments was 3776 birr. Regarding fresh forage soybean cultivars, there was no market price or field price. Because of this, we used the related work of other authors and projected that, using the local market price, the cost of cultivating Lablab purpureus would be roughly four birr per kg [21]. As a result, the price for forage soybean was (p) = 4 birr per kg was used. This price was assumed to market price estimation of forage soybean in the current study. The selling price of dry forage soybean was taken as total return (TR) in the analysis.

3. Results

3.1. Effects of NPS Fertilizer Levels and Row Spacing on Growth Parameters and DMY

3.1.1. Plant Height (PH)

In the current result, PH was highly significantly (p < 0.001) influenced by NPS fertilizer and row spacing variation, but no significant difference in their interaction effect. The highest PH (175.4 cm) was recorded from plants that grew on the application of 140 kg ha⁻¹ NPS fertilizer (167.7 cm) from 70 cm inter-row spacing, and the shortest (168.4 cm) was from no NPS application, and PH (159.2 cm) was observed from 50 cm row spacing.

3.1.2. Number of Branches Per Plant (NBPPs)

NBPPs of forage soybean was significantly (p < 0.001) impacted by both of the primary parameters in the current study: NPS fertilizer and row spacing. On the number of major branches, their interaction effects, however, do not demonstrate any discernible impact. The highest number of branches (NBs; 19) per plant was observed when 140 kg ha-1 of fertilizer was applied, and the lowest NBs (11) was observed when no fertilizer was applied. Furthermore, the width of the rows (70 cm) produced the greatest NBs (16), whilst the narrowest row spacing (50 cm) produced the fewest (14).

3.1.3. Number of Leaf Per Plant (NLPP)

The NLPPs of forage soybean was significantly impacted (p < 0.001) by the interaction between NPS fertilizer and row spacing. In the current study, however, there was no interaction between the NPS fertilizer and row spacing and the NLPP of forage soybean. The highest NLPPs (100.6) was seen when 140 kg ha⁻¹ of NPS fertilizer was applied, followed by 81.5 from 120 kg ha⁻¹ of NPS fertilizer, and the lowest NLPPs (55.7) from 0 kg ha⁻¹ of NPS fertilizer. In a similar vein, the lowest (70.0) and maximum (82.8) NLPPs were reported from (50 cm) and (70 cm) row spacing, respectively.

3.1.4. Number of Nodules Per Plant (NNPPs)

The inter-row spacing variation, NPS fertilizer, and their interaction effect all had a highly significant (p < 0.001) impact on the NNPPs. The resultant range of nodule counts per plant was therefore 7–19. The application of 140 kg ha⁻¹ NPS resulted in the greatest number of nodules (19), whereas the application of no NPS resulted in the lowest number of nodules (7). Nodules were counted with a maximum of 16 at a row spacing of 70 cm and a minimum of 11 at a row spacing of 50 cm.

3.1.5. Number of Roots Per Plant

There was no discernible difference in the quantity of roots per plant between NPS application and row spacing. According to the current results, the 100 kg ha⁻¹ NPS application produced the greatest number of roots (31.9), while the 120 kg ha⁻¹ NPS application produced the fewest roots (28.9). The greatest number of roots (31.65) was found at a row spacing of 50 cm, while the lowest number (29.3) was found at a row spacing of 70 cm.

3.1.6. Root Length

The current finding showed that NPS application, row spacing, and their interaction did not significantly affect root length. Although the differences in root length were not statistically significant (p > 0.05), the data still suggest a trend worth noting. The average root length in the treatment group was 26.9 cm, compared to 25.01 cm in the control group. While variability within groups likely contributed to the lack of significance, the observed trend may indicate a biological response that warrants further investigation under controlled or larger sample size conditions.

3.1.7. DMY

The current finding showed that NPS fertilizer and row spacing had a significant (p < 0.001) effect on the DMY of forage soybean. The highest DMY was 10.9 and 10.5 t ha⁻¹ were recorded from 120 and 140 kg ha⁻¹ NPS rate of fertilizer, respectively, followed by 9.9 t ha⁻¹ was recorded from 100 kg ha⁻¹ NPS, while the lowest DMY of 8.7 t ha⁻¹ was recorded from no fertilizer application, respectively. Similarly, the highest DMY (12.2 t ha⁻¹) was recorded from 50 cm row spacing, followed by (9.2 t ha⁻¹) recorded from 60 cm row spacing, while the lowest DMY (8.5) was recorded from 70 cm row spacing.

3.1.8. LSR

LSR was significantly (p < 0.05) affected by the NPS application, but not significantly affected by row spacing and their interaction of NPS and row spacing. The highest LSR (0.8) was recorded from the application of 100 kg ha⁻¹ NPS, followed by (0.7) recorded from the 120 kg ha⁻¹ NPS application. The lowest LSR (0.6) was recorded from no NPS application. The application of NPS fertilizer increases soil fertility appropriate to produce more leaves and make the plant grow vigorously.

3.2. Effects of NPS Fertilizer Levels, Row Spacing, and Their Interaction on Nutritive Value

The chemical composition of forage soybean is presented in Table 4. The highest CP content (17.33%) was recorded from 140 kg NPS fertilizer application, while the lowest (13.24%) was from no NPS fertilizer application. The 50 cm row spacing produced the lowest CP (14.7%), whereas 70 cm row spacing produced the greatest CP content (16%). As per the current findings, the ME of forage soybean was significantly impacted (p < 0.001) by the use of NPS fertilizer, row spacing, and their combination. The application of 140 kg ha⁻¹ NPS fertilizer produced the highest ME (11.7), whereas the application of no NPS fertilizer produced the lowest ME (9.0). From 70 cm row spacing, the highest ME (11.8) was reported, and from 50 cm row spacing, the lowest (10.2).

Table 4. Effect of NPS fertilizer, row spacing, and their interaction on agronomic characteristics and dry matter yield.

Factors	Parameters
NPS	PH (cm)	NPBPP (pcs)	NLPP (pcs)	NNPP (pcs)	NRPP (pcs)	RL (cm)	DMY (t ha⁻¹)	LSR
140	175.45^a	19.43^a	100.64^a	18.80^a	31.79^a	26.02	10.49^a	0.68^ab
120	166.03^b	16.32^b	81.48^b	17.59^a	28.9^a	26.9	10.86^a	0.72^a
100	164.46^b	14.14^c	66.05^c	13.85^b	31.94^a	25.16	9.86a^b	0.76^a
0	168.64^c	11.08^d	55.73^d	7.10^c	29.36^a	25.01	8.67^b	0.57^b
SEM	1.6	0.4	1.7	0.6	1.2	0.5	0.5	0.04
Row spacing
70	167.76^a	16.1^a	82.79^a	16.5^a	29.3^a	26.1a	8.49^b	0.66^a
60	163.44^b	15.2^ab	75.15^b	15.15^a	30.5^a	25.27a	9.2^b	0.69^a
50	159.23^b	14.39^b	69.99^c	11.35^b	31.65^a	25.45a	12.2^a	0.7^a
SEM	1.39	0.35	1.48	0.5	1.05	0.44	0.44	0.04
Mean	163.65	15.24	75.98	14.33	30.49	25.77	9.97	0.68
CV	2.94	8.17	6.54	13.25	11.14	6.1	15.56	19.7
p-Value
NPS	<0.0001	<0.0001	<0.001	<0.001	0.14	0.06	0.03	0.04
Row	0.0019	0.01	<0.001	<0.001	0.26	0.59	<0.001	0.72
NPS ^∗row	0.96	0.87	0.37	<0.001	0.52	0.72	0.06.	0.92

Note: Means in the same column at each factors followed by different letters differ significantly at ^∗p < 0.05, SL is level of significance.
Abbreviations: CV, coefficient of variation; DMY, dry matter yield; LSR, leaf-to-steam ratio; NLPP, number of leaf per plant; NNPP, number of nodule per plant; NPBPP, number of primary branch per plant; NRPP, number of root per plant; NS, non-significant; PH, plant height; RL, root length.
^∗∗p < 0.01.
^∗∗∗p < 0.001.

3.3. Interaction Effect of NPS Fertilizer and Row Spacing on Nutritional Quality

The interaction effect of NPS fertilizer and row spacing was statistically significant on DM percentage, CP, CPY, and ME (Table 5). However, ash, OM, ADF, NDF, lignin (ADL), and digestibility did not show an interaction effect.

Table 5. Interaction effects of NPS fertilizer and row spacing on nutritional quality.

Fertilizer rate	Row spacing	DM (%)	Ash (%)	OM (%)	CP (%)	CPY (%)	NDF (%)	ADF (%)	ADL (%)	IVDMD (%)	ME (MJ kg⁻¹)
0	50	93.5^ab	6.69	93.3	10.99^e	1.13^c	40.85	21.6	6.28	73.57	7.1^d
0	60	91.36^cb	6.34	93.7	13.7^d	1.09^c	42.46	22.67	5.6	72.99	9.88^c
0	70	90.47^cd	7.22	92.8	15.04^bcd	1.13^c	42.07	19.71	5.44	73.86	10^c
100	50	94.39^a	6.58	93.4	16.43a^bcd	1.82^ab	40.6	21.37	5.47	74	10.7bc
100	60	90.27^cd	6.4	93.6	13.83^d	1.28^bc	43.08	21.4	5.8	72.98	11.2^bc
100	70	90.07^cd	6.53	93.4	15.5^bcd	1.42^bc	42.34	21.3	5.96	73.47	10.95^bc
120	50	92.98^ab	7.01	93	15.66^bcd	2.07^a	40.63	20.8	5.6	74.08	12.13^ab
120	60	90.2^cd	6.84	93.2	17.67^ab	2.03^a	41.67	20.2	6.13	73.75	11^bc
120	70	90.2^cd	6.7	93.3	14.47^cd	1.14^c	42.76	20.78	5.81	73.5	12.63^a
140	50	90.29^cd	7.06	92.9	15.77^bcd	2.25^a	42.24	21.1	5.79	73.54	10.96^bc
140	60	90.28^cd	7.38	92.6	17.14^abc	1.38^bc	41.75	20.55	6.07	73.9	11^bc
140	70	89.4^d	7.83	92.2	19.08^a	1.78^ab	41.7	20.6	5.67	74.8	13.14^a
Overall mean	—	91.12	6.88	93.1	15.44	1.54	41.8	21.01	5.8	73.7	10.89
CV	—	1.07	12.6	10.1	10.09	19.5	3.17	6.4	6.35	1.4	7.22
SE	—	0.51	0.51	0.92	0.75	0.44	0.77	0.21	0.21	0.62	0.46
p-Value	—	<0.0001	0.66	0.66	0.002	0.002	0.39	0.53	0.17	0.73	<0.0001

Note: Means in the same column at each factors followed by different letters differ significantly at ^∗p < 0.05, SL is Level of significance.
Abbreviations: ADF, acid detergent fiber; ADL, acid detergent lignin; CP, crude protein; CPY, crude protein yield; DM, dry matter; DOMD, dry organic matter digestibility; IVDMD, in vitro dry matter digestibility; ME, metabolizable energy; NDF, neutral detergent fiber; NS, non-significant; OM, organic matter.
^∗∗p < 0.01.
^∗∗∗p < 0.001.

3.4. Partial Budget Analysis

The current study’s total partial budget analysis was shown in Table 6. The best MRR (859.85%) and largest net profit (40212 birr) were obtained from 140 and 100 kg NPS fertilizer treatments with 50 cm row spacing, respectively. (Table 7)

Table 6. Effect of fertilizer rate, row spacing, and their interaction effect on chemical composition.

Factors	DM (%)	Ash (%)	OM (%)	CP (%)	CPY (kg ha⁻¹)	NDF (%)	ADF (%)	ADL (%)	IVDMD (%)	DOMD (%)	ME (MJ kg⁻¹)
Fertilizer rate	DM (%)	Ash (%)	OM (%)	CP (%)	CPY (kg ha⁻¹)	NDF (%)	ADF (%)	ADL (%)	IVDMD (%)	DOMD (%)	ME (MJ kg⁻¹)
0	90.04^b	6.75	93.24	13.24^c	1.12	41.79	21.33	5.77	73.47	68.47	8.99^c
100	90.55^b	6.51	93.49	15.26^b	1.51	42.01	21.36	5.75	73.49	68.49	10.95^b
120	91.26^b	6.86	93.14	15.93^b	1.75	41.69	20.6	5.85	73.78	68.78	11.90^a
140	92.60^a	7.43	92.57	17.33^a	1.81	41.90	20.76	5.80	74.10	69.10	11.70^ab
SEM	0.38	0.28	0.289	0.52	0.1	0.44	0.45	0.12	0.35	0.35	0.26
Row spacing
50	91.49	6.84	93.16	14.71	1.80	41.10	21.23	5.79	73.8	68.8	10.22
60	90.53	6.74	93.26	15.59	1.45	42.24	21.20	5.9	73.4	68.4	10.78
70	91.34	7.07	92.9	16.02	1.37	42.22	20.60	5.72	73.9	68.9	11.68
SEM	0.33	0.25	0.25	0.45	0.08	0.38	0.39	0.1	0.3	0.3	0.23
Overall mean	91.12	6.88	93.11	15.44	1.54	41.85	21.02	5.8	73.72	68.7	10.89
CV	1.3	13	0.96	10.33	19.50	3.13	6.40	6.34	1.46	1.56	7.34
p-Value
NPS	0.001	0.20	0.20	0.001	0.96	0.54	0.92	0.58	0.57	0.001	0.001
Row	0.12	0.65	0.65	0.15	0.07	0.45	0.48	0.49	0.49	0.001	0.12
NPS X row	<0.0001	0.66	0.66	0.002	0.002	0.39	0.53	0.17	0.73	0.51	<0.0001

Note: Means in the same column at each factors followed by different letters differ significantly at ^∗p < 0.05, SL is Level of significance.
Abbreviations: ADF, acid detergent fiber; ADL, acid detergent lignin; CP, crude protein; CPY, crude protein yield; DM, dry matter; DOMD, dry organic matter digestibility; IVDMD, in vitro dry matter digestibility; ME, metabolizable energy; NDF, neutral detergent fiber; NS, non-significant; OM, organic matter.
^∗∗p < 0.01.
^∗∗∗p < 0.001.

Table 7. Partial budget analysis.

Description	Treatments
Description	Trt1	Trt2	Trt3	Trt4	Trt5	Trt6	Trt7	Trt8	Trt9	Trt10	Trt11	Trt12
DMY (kg ha⁻¹)	10,423	8010	7583	11,117	9287	9193	13,207	11,500	7877	14,107	8037	9333
ADMY (kg ha⁻¹) (A)	9381	7209	6825	10005	8358	8274	11,886	10,350	7089	12,696	7233	8,400
Cost of NPS	3776	3776	3776	3776	3776	3776	3776	3776	3776	3776	3776	3,776
TVC (B)	0	0	0	3776	3776	3776	4531	4531	4531	5286	5286	5,286
Price birr kg⁻¹ (C)	4	4	4	4	4	4	4	4	4	4	4	4
Gross benefit (A × C) (D)	37,524	28,836	27,300	40,020	33,432	33,096	47,544	41,400	28,356	50,784	28,932	33,600
Net benefit (D–B) (birr)	37,524	28,836	27,300	36,244	29,656	29,320	43,013	36,869	23,825	45,498	23,646	28,314
Change in TVC (∆TVC)	0	0	0	3776	3776	3776	4531.2	4531	4531.2	5286.4	5286.4	5286.4
Change in net benefit (∆NR)	37,524	28,836	27,300	32,468	25,880	25,544	38481.8	32,338	19,294	40,212	18,360	23027.6
MRR (%) = (∆NR/∆TVC cost) × 100	—	—	—	859.8517	685.381	676.483	849.263	713.7	425.8	760.66	347.3	435.601

Abbreviations: ADMY, adjusted dry matter yield; DMY, dry matter yield; MRR, marginal rate of return; NPS, nitrogen, phosphorous, and sulfur; TVC, total variable cost.

4. Discussion

4.1. Effects of NPS Fertilizer Levels and Row Spacing on Agronomic Parameters and DMY

4.1.1. PH

Because nitrogen fertilizer directly contributes to photosynthesis, is a fundamental component of protein in plant parts, and aids in the activation of enzymes, it may be particularly important for plant growth and development. This may explain why plants receiving the highest NPS fertilizer had the highest PH and wider interrow spacing. The maximum vegetative growth of the plants under higher N and S availability is indicated by the increase in PH with increased NPS application rate. P is crucial for early root development, which may increase the plant’s nutrient uptake and ultimately lead to increased vegetative growth. More efficient use of light is achieved by wider row spacing, which speeds up canopy development.

The present results corroborated to Agegn’s [22] report, which found that at blended fertilizer rates, soybean PH varied extremely highly considerably. According to the author, the highest PH of 63.4 cm was achieved by applying 200 kg of NPS ha⁻¹, but the lowest PH of 48.2 cm was achieved in other regions of the nation when no NPS fertilizer was applied to soybeans. When bioorganic and inorganic fertilizers are applied alone to faba beans, the PH at 90% maturity shows a significant difference (p < 0.01) [23]. Similarly, the findings of Adem et al. [24] reported that NPS fertilizer level had a significant effect on lablab accessions. The longest PH (176.79 cm) was recorded from the higher NPS fertilizer group (150 kg ha⁻¹).

The current findings were consistent with those of [25], who reported that the plants growing on the application of 200 kg ha⁻¹ blended NPS fertilizer with 70 cm inter-row spacing had the highest PH (107 cm), and the plants growing on the control treatment of soybeans had the lowest PH (60 cm). Conversely, [10] found plants with higher plant densities, that is, plants with closer spacing between each otherhad noticeably taller plants. Furthermore, at Kersa Woreda in the Jimma Zone of South Western Ethiopia, the study by Rogers et al. [15] showed that when PH grew in response to increases in plant population and NP fertilizer, lodging issues, low branching, and low pod per plant resulted. The discrepancy between earlier reports with the current result might be due to the environment, soil type, agroecology, fertilizer, harvesting stage, and management.

4.1.2. NBPPs

Because nitrogen, phosphorus, and sulfur are nutrients that are essential to plant health and promote vegetative development, it is possible that the NBPP increased as NPS fertilizer rates were increased. The lack of nutritional competition was the cause of the increase in branch count with increasing row space. The results of the current study were consistent with [26], who found that the application of 200 kg ha⁻¹ NPS on 70 cm row spacing produced the greatest NBs (21.1). According to Agegn’s [22] report, the application of 200 kg ha⁻¹ NPS fertilizer resulted in the highest number of primary branches (6.6), while the was recorded with the application of 200 kg ha⁻¹ NPS fertilizer whereas, the minimum number of primary branches (4.1) was obtained with no application of NPS fertilizer on soybean in Ethiopia.

Additionally, Rogers et al. [15] demonstrated that the number of primary branches increases with decreasing planting density. Moreover, the findings of Xu’s [27] corroborated the current results, which reported that the number of primary branches increases with decreasing planting density. Similarly, Acikgoz [28] observed that when row spacing grew, so did the NBPP. The observed discrepancy in the number of primary branches of soybean between the current study and the previous authors’ study could potentially be attributed to differences in soil type, agroecological conditions, and management practices.

4.1.3. NLPP

According to Adem et al. [24], who investigated L. purpureus L, the maximum NLPP (178.3) was found at the highest fertilizer level (150 kg ha⁻¹). This investigation is consistent with their findings. This may be the case because using a lot of fertilizer raised the nitrogen levels, which in turn encouraged plant growth and increased the number of leaves. In Ethiopia, the application of 75 kg ha⁻¹ NPS + 2.5 t ha⁻¹ vermicompost enhances the amount of common bean leaves [29]. The current study’s findings were inferior to that of [30] investigation, which found that the maximum mean number of leaves seen for intra- and inter-row spacing was 112 and 127, respectively. The variations in the NLPPs of soybean (Glycine max [L.] Merrill) between this study and the previous authors’ may have resulted from genotype, agro-ecological variables, management variations, and differences in soil types.

4.1.4. NNPPs

The importance of the P and S elements in NPS, which are employed to enhance nodulation and subsequently improve N2 fixation, may be the cause of the higher number of nodules that developed on the soybean root following NPS administration. The current results are consistent with those of [31], who found that at the highest amount of 75 kg ha⁻¹ NPS mixed with 2 t ha⁻¹ vermicompost, the greatest NNPPs (32) was seen. The treatment that was administered on soybean in Western Ethiopia with the lowest NPS (50 kg ha⁻¹) along with 0 t ha⁻¹ vermicompost produced the fewest nodules per plant while also yielding the lowest number of nodules overall. The findings of Rogers et al. [15] provided support for the current finding’s outcome. At the lowest level of NP, there were exactly three nodules, the minimum number. In Central Mozambique, soybeans at a rate of 60 kg ha⁻¹ P₂O₅ had the greatest number of nodules (41) [32]. On the other hand, Rasul et al. [33] found that mungbean plants’ interrow spacing had no discernible impact on the nodulation process. In southwest Ethiopia, there was no discernible change in inter-row spacing, variety, or their interaction with soybean [34]. On common beans in other regions of Ethiopia, the major impacts of interspacing and the interaction were not significant [35]. The causes of the variation could be attributed to management, fertilizer, genotype, and agroecology.

4.1.5. Number of Roots Per Plant

However, under no nitrate conditions compared to high nitrate supply conditions [28, 36], discovered that the total number of roots and the number of roots per plant, respectively, were significantly significant for forage soybean in Poland. Conversely, Ramos-Ulate et al. [37] demonstrated that, on Alfalfa fodder, the total number of roots was considerably higher under K₂SO₄ NPS at the 1/8 level. The variance in the number of roots per plant in the soybean studied here compared to the previous authors might be attributed to genotype, agro-ecological variations, soil type differences, and management changes.

4.1.6. Root Length

In contrast to our findings [27, 36], showed that the total root length was highly significant under no nitrate conditions than high nitrate supply conditions for forage soybean. The variance in the values of the number of roots per plant (G. max (L). Merrill) between the current study and the previous authors’ may be caused by genotype, agro-ecological variations, soil type differences, management changes, and fertilizer types.

4.1.7. DMY

The highest DMY was found in the areas with the highest NPS fertilizer application. This could be due to the supply of significant nutrients to the soil by the NPS fertilizer application, which were necessary for longer PH, more leaves, and more branches per plant. The current results corroborated those of [12], who found that growing row spacing considerably decreased the amount of DM produced by forage soybeans. According to [10, 38], biomass yield of forage soybean increased dramatically when inter-row spacing was reduced from 100 to 40 cm and intra-row spacing was reduced from 15 to 5 cm in New Hampshire and northwest Ethiopia, respectively. When compared to broader row spacing, narrower row spacing enhanced DM yield [39]. Variations in soil type, genotype, harvesting stage, fertilizer, and possibly management could be the cause of the variance in DMY values seen in the current study compared to the prior study.

4.1.8. LSR

In these parameters, reports of [40] found nonsignificant results for Narbon Vetch (Vicia narbonensis) by the main effect of row spacing. The present results were greater than those of the study by Darmosarkoro [41], which found that 102 days after planting, the forage soybean’s LSR was 0.5. This varied because of the impact of fertilizer application and environmental modifications. The current study’s result is not as high as that of [42] research on alfalfa (Medicago sativa L.) in Turkey, where Cultivar Magnum 5 had the greatest LSR (1.1). The environment, fertilizer, and genotype could all be contributing factors to the discrepancy of the results.

4.2. Effects of NPS Fertilizer Levels, Row Spacing, and Their Interaction on Nutritive Value of Forage Soybean

4.2.1. DMC

The present discovery aligns with according to Hasan et al. [43], the yield of cowpea fodder in Bangladesh increased gradually in both DM and OM as nitrogen fertilizer dosages increased. According to Abuye et al. [44], the sub-humid climate of Ethiopia allowed for the growth of 92.6% of the DM in the lablab. The current result was less than that of [23], who found that lab–lab forage from the dry regions of Ethiopia produced the greatest DM (93.7). This variation may be due to the fact that genetic variations and environmental differences, and management.

4.2.2. Ash

On the other hand, Ebbisa and Amdemariam’s [23] findings demonstrated that the amount of ash content had a significant impact on the amount of NPS fertilizer applied to lablab (L. purpureus L) grown under irrigation in dry lands of Afar, Ethiopia. They found that the lablab accessions with the highest ash content (12.8% and 12.3%) were Tulu and 147, respectively, and these were grown using the highest level of NPS application. The differences between the current findings and the preceding results could be attributed to differences in genotype, temperature, agroecology, soil type, and management.

4.2.3. OM

The OM content finding was greater than that of the study by Sruamsiri and Silman [45], which found that the OM content of soybean pod husk was 59.7. In a similar vein, the present results exceeded those of [43], whose investigation on cowpea fodder in Bangladesh revealed the highest amount of OM (90.2%). On the other hand, when compared to Indonesian stylosanthes forages without NPK fertilization, the production of OM rose by 16.1%–115.2% [46]. Genetic variation, environmental conditions, fertilizer, soil type, and management approaches could all be contributing causes to these divergent results.

4.2.4. CP

Amino acids, the building blocks of proteins, and nucleic acids, the building blocks of genetic material (RNA and DNA), both require nitrogen as a fundamental component. Since nitrogen is vital to the metabolism of energy and the synthesis of proteins, it is necessary for the growth of plants. Nitrogen is absorbed by the plant as nitrate. Nitrogen is used by plants to create the protoplasm and amino acids needed to develop their tissues and proteins. Additionally, the production of the enzymes needed to support different biochemical activities requires nitrogen. The current result was consistent with the findings of [47], who found that the amount of CP in the feed was impacted by nitrogen application and that CP levels might rise with increasing application of fertilizer. The results of [23] demonstrated that the overall mean value of lablab (L. purpureus L) CP was the highest (21.6% and 20.9%) from 11,640 and 147 lablab accessions, respectively. In a similar vein, several writers likewise report no appreciable variation in CP with respect to row spacing. For example, row spacing has no effect on the CP [12]. Similarly, row spacing had no effect on CP, as demonstrated by Albayrak and Yüksel [39]. Perhaps the soil or the harvesting stage causes a difference in CP.

4.2.5. CPY

The current results were in line with those of [43], who observed that the CP production in Bangladesh rose noticeably when N fertilizer dosages for cowpea fodder increased. According to Lissbrant et al. [48], there was a considerable rise in CPY when P fertilizer treatments increased for alfalfa forage located south of West Lafayette. Acikgoz et al. [12] stated that the maximum CPY was produced with 25-cm row spacing of forage soybean in Turkey, corroborate our findings.

4.2.6. NDF

The finding of [49] showed that the NDF contents of vetiver were unaffected by phosphorus rates. In a similar vein, [23] found that applying NPS fertilizer to dry areas in the Afar region had no effect on the NDF content of lablab. The present results were consistent with the study by Acikgoz et al. [12], which found that row spacing had no effect on the NDF concentration of soybean fodder. Seiter et al. [38], in contrast, demonstrated that soybeans grown in rows separated by 18 cm provided greater quality fodder than those planted in rows separated by 36 cm. Row spacing, however, had no effect on NDF [39]. The present results surpass those of [50], who reported that lablab’s NDF concentration was 39.9% when it was subjected to small-scale irrigation. Environmental factors, including soil type, temperature, moisture content, and fertilizer, may be to blame for this. Similar to [44], who reported that the NDF concentration of lablab was cultivated under sub-humid climatic conditions in western Oromia, Ethiopia, the NDF content of the current result was lower. In comparison to the findings of [44, 50], the current result (NDF content) was lower. The differences between the current and prior results could be caused by genetic variation, soil type, temperature, moisture content, etc.

4.2.7. ADF

The present results were consistent with the research by Acikgoz et al. [12], which found that row spacing had no effect on the ADF concentration of soybean feed in Pakistan. Furthermore, the present discovery aligns with the research conducted by Ebbisa and Amdemariam [23], which revealed that the application of NPS fertilizer to arid areas in the Afar region has no effect on the ADF content of lablab. The present investigation yielded a lower result than the one by Zeleke et al. [50], which reported that lablab’s ADF content was 29.1% when grown under small-scale irrigation. Similar to [44], who reported that the ADF content of the current result from all treatment groups was lower. These findings are superior to the existing ones. Fertilizer, management practices, and environmental factors could all be at blame for this.

4.2.8. ADL

In contrast [44], who said that the ADL content of lablab was (3.5%) grown under the sub-humid climatic conditions of western Oromia, Ethiopia, the ADL content of the current result from all treatment groups was greater. The current findings’ result was less than that of a study conducted in the Amibara area of Afar region, Ethiopia’s dry lands, by Ebbisa and Amdemariam [23], which found that the lablab’s ADL content was (6.0) under irrigation. The environment, genetic variety, soil, fertilizer, and agroecology may all be contributing factors to the current results’ discrepancy with previous findings.

4.2.9. The IVDMD

Compared to Adem et al. [24], who reported that the IVDMD content of lablab (L. purpureus L) under irrigation in Ethiopia’s drylands was 65.7%, the IVDMD content of the current finding was greater. The current IVDMD content finding was more than that of [43], who found that 30 kg N ha-1 applied from cowpea fodder in Bangladesh produced the maximum OMD (59.2%). Agroecological factors may be the cause of the discrepancy in soybean IVDMD values between the current study and the previous authors.

4.2.10. DOMD

The new discovery outperformed the research of [44], which stated that 45.7% of the lablab’s IVOMD was grown in Ethiopia. In a similar vein, the present results exceeded those of [51], who reported that the IVOMD of cowpea, produced under rainfed conditions in Bako, eastern Ethiopia, was 61.1%. Genetic diversity, soil type, agroecology, fertilizer, and management can all contribute to variations in IVOMD concentration.

4.2.11. DMC

The interaction impact of fertilizer and row spacing was statistically very significant when it came to DM. The interaction effects of row spacing and NPS fertilizer application on the DMC in the various nations were not well documented. The current study’s results, however, were not as strong as those of [23], who found that there was a 93.56% interaction effect between lablab accession and fertilizer and DMC. In a similar vein, [44] stated that the lablab’s DMC was 92.6%. This difference may be attributed to management, genotype, and agroecology.

4.2.12. CP

On the CP content of forage soybean, there was a substantial interaction effect between the NPS fertilizer rate and row spacing. The findings of [38] reported no interaction effect between plant density and row distance of forage soybean, which is contrary to the current findings. According to Abuye et al. [44], the CP content does not affect the interaction between NPS fertilizer rate and accession. Row spacing, fertilizer, agroecology, and harvesting stage could all vary.

4.2.13. CPY

The CPY of forage soybean was significantly impacted by the relationship between the NPS fertilizer rate and row spacing. As PH, leaf count, branch count, and DM output increased, so did the current findings, which show that fertilizer use increased along with row spacing decreased.

4.2.14. ME

This research yielded a greater result than the one obtained by Hasan et al. [43] observed that there was no discernible variation in Bangladesh’s cowpea forage’s ME content. Variations in habitat, genetics, and fertilizer application could be the cause of this contradictory result. On the ME of forage soybean, there was a substantial interaction effect between the NPS fertilizer rate and row spacing. In comparison to [43], who discovered that the ME ranged between 7.34 and 7.86 MJ/kg DM, the current result was higher. This diversity resulted from variations in soil type, agroecology, management, and fertilizer. Even then, there was little data available in the nation about the connection between NPS fertilizer and forage legume row spacing.

4.2.15. Partial Budget Analysis

Fertilizer and row spacing had quite different net advantages. This is caused by variations in the DMY production potential of soybean fodder between trials for both row spacing and fertilizer. The current finding led to the following results: 859.85%, 849.26%, and 760.66% MRR for 50 cm row spacing with 100, 120, and 140 kg NPS fertilizer, respectively. This suggests that the producer can obtain 8.6, 8.4, and 7.6 birr, respectively, for every birr invested in forage soybean production. The current result disagrees with [10], who reported that the economic analysis showed that the maximum net benefit was obtained in soybeans planted with 60 cm inter row spacing for grain yield. The result of MRR in the present study was found to be higher than the lowest allowable rate of return [20]. In the current study, fertilizer responded more strongly to high net benefits and MRR with 50 cm row spacing, without taking into account other benefits and costs that were constant for all treatment groups. As a result, only expenses that change at experimental sites over the course of the trial were taken into account in this partial budget analysis.

5. Conclusion

The results of this study demonstrated that, with the exception of DMY and CPY, morphological metrics performed better at 140 kg ha⁻¹ NPS fertilizer rate and 70 cm row spacing. There was no difference in terms of spacing or nutritional quality at 140 kg ha⁻¹. Greater DMY, CPY, and a superior net benefit were the outcomes of the interaction between the 50 cm row spacing and 140 kg ha⁻¹ NPS fertilizer. On the other hand, a high rate of marginal return (MRR) was obtained by using 50 cm row spacing with 100 kg ha⁻¹ NPS fertilizer, owing to the low cost of fertilizer and high DM output. The research’s conclusion was that it was preferable to sow forage soybeans with a 50 cm row spacing and 100 kg ha⁻¹ of NPS fertilizer.

Disclosure

The authors kindly express that the current MS submitted to the Journal of Advances in Agriculture is drafted from the thesis title “Effects of Fertilizer Rate and Row Spacing on Forage Soybean (Glycine Max [L.] Merrill) Biomass Yield and Nutritional Quality Under Irrigation at Koga Site, West Gojam Zone, Amhara Region,” by Hilena Yifred submitted to the School of Graduate Studies of Bahir Dar University and deposited in the thesis hub of the university at the URI: http://ir.bdu.edu.et/handle/123456789/15455.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

The authors received no specific funding for this work.

Acknowledgments

The first author acknowledged the Ethiopian Institute of Agricultural Research (EIAR) for providing her study leave and necessary research inputs.

Open Research

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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All articles

Effects of Fertilizer Rate and Row Spacing on Forage Soybean (Glycine max [L.] Merrill) Biomass Yield and Quality in the Highlands of Ethiopia

Abstract

1. Introduction

2. Materials and Methods

2.1. Description of the Study Area

2.2. Land Preparation

2.3. Experimental Design and Management

2.4. Data Collection

2.4.1. DMY

2.4.2. Leaf-to-Stem Ratio (LSR)

2.4.3. Forage Chemical Composition

2.5. Data Analysis

2.6. Partial Budget Analysis

3. Results

3.1. Effects of NPS Fertilizer Levels and Row Spacing on Growth Parameters and DMY

3.1.1. Plant Height (PH)

3.1.2. Number of Branches Per Plant (NBPPs)

3.1.3. Number of Leaf Per Plant (NLPP)

3.1.4. Number of Nodules Per Plant (NNPPs)

3.1.5. Number of Roots Per Plant

3.1.6. Root Length

3.1.7. DMY

3.1.8. LSR

3.2. Effects of NPS Fertilizer Levels, Row Spacing, and Their Interaction on Nutritive Value

3.3. Interaction Effect of NPS Fertilizer and Row Spacing on Nutritional Quality

3.4. Partial Budget Analysis

4. Discussion

4.1. Effects of NPS Fertilizer Levels and Row Spacing on Agronomic Parameters and DMY

4.1.1. PH

4.1.2. NBPPs

4.1.3. NLPP

4.1.4. NNPPs

4.1.5. Number of Roots Per Plant

4.1.6. Root Length

4.1.7. DMY

4.1.8. LSR

4.2. Effects of NPS Fertilizer Levels, Row Spacing, and Their Interaction on Nutritive Value of Forage Soybean

4.2.1. DMC

4.2.2. Ash

4.2.3. OM

4.2.4. CP

4.2.5. CPY

4.2.6. NDF

4.2.7. ADF

4.2.8. ADL

4.2.9. The IVDMD

4.2.10. DOMD

4.2.11. DMC

4.2.12. CP

4.2.13. CPY

4.2.14. ME

4.2.15. Partial Budget Analysis

5. Conclusion

Disclosure

Conflicts of Interest

Funding

Acknowledgments

Open Research

Data Availability Statement

References

References

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