Effects of planting density and variety on productivity of maize-faba bean intercropping system

Abstract

Maize (Zea mays)-faba bean (Vicia faba) intercropping is a recent practice in eastern Ethiopia and there is limited information on optimum planting density and suitable varieties of faba bean for productive intercropping with maize. Therefore, a field experiment was conducted in eastern Ethiopia during the 2018 and 2019 main cropping seasons to determine the effects of variety and density of faba bean on the yield components of the crops and the productivity of the intercropping. Treatments consisted of three farmers’ varieties (Yeferenji Baqela, Yehabesha Baqela, Batte) and one improved variety (Gachena)intercropped with maize in an additive design at three plant densities (25%, 50%, and 75% of their recommended sole crop density of 250,000 plants ha⁻¹) and the recommended 100% plant density of sole faba bean and sole maize. A randomized complete block design in a factorial arrangement of treatments replicated three times per treatment was used. Analysis of data revealed that sole maize produced a significantly higher number of ears per plant (1.70), thousand kernel weight (287.9 g), aboveground dry biomass (19.11 t ha⁻¹) and grain yield (6.16 t ha⁻¹) than intercropped maize. Among the intercropping systems, maize intercropped with 25% of the sole faba bean produced a significantly higher grain yield (5.56 t ha⁻¹) than 50% and 75% plant density. Similarly, the 75% plant density of the sole faba bean intercropped with maize produced the highest grain yield (0.96 t ha⁻¹) compared to the 25% and 50% plant densities. Faba bean planted in the sole system produced a significantly higher seed yield (2.08 t ha⁻¹) than the intercropped faba bean with maize. Faba bean variety Gachena was found to be superior than the other varieties in number of pods per plant (10.23), aboveground dry biomass (3.73 t ha⁻¹), grain yield (1.96 t ha⁻¹) and harvest index (26.75%). Land equivalent ratio showed 18.4% higher yield advantage when maize was intercropped with faba bean than when it was sole-cropped. The highest gross monetary value (99318 Ethiopian Birr ha⁻¹) and maize equivalent yield (6621.2 kg ha⁻¹) were obtained from maize intercropped with Gachena faba bean variety with no significant differences among the plant densities. It is concluded that intercropping of maize with 75% plant density of improved Gachena variety resulted in superior performance of the system in terms of productivity and economic return.

1. Introduction

Crop production in modern agriculture that relies on conventional farming and monocropping systems of genetically identical plants threatens the sustainability of agricultural production systems [1]. Such systems depend mostly on the use of chemical fertilizers and pesticides for higher crop yields and crop protection against pests and diseases [2]. However, over-reliance on the use of agrochemicals in conventional production systems has caused serious on-site and off-site environmental problems [3]. Therefore, establishing sustainable agricultural production systems through agricultural intensification and crop diversification with fewer chemicals remain an important option for sustainable food production [4]. Intercropping of legumes in cereal-based cropping systems represents a key technology in the drive towards the sustainable intensification and diversification of agriculture [5].

Maize and faba bean are crops used as major food sources in Ethiopia. Maize is mainly grown as sole and sometimes intercropped with legume crops. Although, productivity of maize in Ethiopia has been showing improvement in recent years maize yield in the Eastern Hararghe Zone of Ethiopia is still low with an average yield of 2.87 t ha⁻¹compared to the national average maize yield of 4.18 t ha⁻¹ [6]. Furthermore, the productivity of faba bean in Eastern Hararghe Zone is very low (1.42 t ha⁻¹) compared to the national average yield of 2.12 t ha⁻¹ [6]. Faba bean is considered as a valuable crop for intercropping with maize and has several good features such as shade tolerance [7] and the ability to fix atmospheric nitrogen and adding nitrogen to the soil [8]. Eastern Hararghe Zone is among a number of administrative Zones in Ethiopia that are frequently affected by recurrent drought, erratic rainfall, poor crop management practices, and severe land degradation [9]. At the same time, the majority of smallholder farmers in the Zone have a very small land size of about 0.65 ha per household [10] and use sole cropping system not capable of producing enough food to sustain their livelihoods. As a result, the vast majority of the smallholder farmers suffer from chronic food shortages and malnutrition [11]. In this regard, integrating legumes in a cropping system is considered to make an important contribution to smallholder farmers’ capacity to improve yields and attain household food security [12].

The productivity and efficiency of a cereal-legume intercropping system can be affected by component crops densities often reflected in crop yields. Increasing planting density of maize from 44,440 to 53,330 plants ha⁻¹ reduced soybean seed yield by 21 and 23%, respectively, compared with intercropping at 38,000 maize plants per hectare [13]. The intercrops of maize and common bean in a 100% sole maize population (44,444 plants ha⁻¹) and 50% of the sole bean population (125,000 plants ha⁻¹) resulted in an overall high yield [14]. When the component crops are present in approximately equal numbers, productivity and efficiency are determined by the component crops with a more aggressive competitive capacity for resource, which is usually the cereal crop. Planting at full seeding rate of each crops resulted in poor performance of both crops yielding not well because of heavy competition resulting from intense overcrowding [15]. Increasing density of one species in an intercropping system might increase its productivity while decreasing the productivity of the associated species [16]. As a result, Land Equivalent Ratio (LER) as well as the contribution of individual species to LER might change by planting density of component species in intercropping systems. According to Getachew et al. [17], all intercropped proportions of wheat-faba bean had a greater land equivalent ratio than sole crops, with the total land equivalent ratios for intercrops ranging from 1.03 to 1.22. Similar result was reported by Li et al. [18], in maize and faba bean intercropping system.

Studies done in Ethiopia with regard to faba bean crop focussed mainly on improved varieties released from research centre and with less emphasis on farmers' landraces or varieties [19]. Mersha [20] reported that among improved faba bean varieties intercropped with maize the variety Welki gave the highest seed yield (1202 kg ha⁻¹) than other varieties. However, the majority of farmers in Ethiopia plant local varieties of faba bean. The study on socio economic data indicated that 3826 ha of land is under faba bean production and only 83 ha of this is with improved varieties of faba bean in Eastern Hararghe Zone [21]. Besides, farmers also harvest reasonable amounts of yield using local varieties. The study done by Wondimu [22] showed that local variety of faba bean yielded significantly higher than improved variety, signifying that the local varieties/landraces can perform better in their natural habitat unlike the common notion that improved varieties always yield better than local ones. Similarly, Tekle et al. [23] reported that a local faba bean cultivar produced a higher number of pods (21.6) than an improved genotype. However, no research has been conducted in the Eastern Hararghe Zone of Ethiopia to elucidate the influence of varying planting density of farmers’ faba bean varieties in an intercropping system with maize. Thus, this study was undertaken to determine the effect of faba bean varieties and population density of faba bean in intercropping with maize on yield components and yield of the component crops and on the productivity of the system.

2. Materials and methods

2.1. Description of the study site

The study was conducted in Haramaya district of Eastern Hararghe Zone, Ethiopia, from June to December 2018 and May to December 2019 main cropping seasons. The research site is located at a latitude of 9°24′58″ N, a longitude of 42°02′18″ E and an altitude of 2005 m above sea level. The total rainfall amounts in 2018 and 2019 cropping season from the month of May to December were 642.6 mm and 911.91 mm, respectively, with mean temperatures of 17.63 °C and 19.11 °C, respectively (Fig. 1). The soil of the experimental site is classified as Vertisol having a sandy clay loam texture. Its soil pH is moderately alkaline, available P is medium, total N and organic carbon is low with high CEC.

Fig. 1.

Monthly total rainfall (mm) and monthly mean average (T_avg) temperatures (^oC) of the study site during the cropping seasons.

2.2. Planting material

One improved maize variety known as Baate, which is an open-pollinated variety released by Haramaya University in 2017 was used in the experiment. Variety requires 145 days to reach maturity and can grow to a height of about 180 cm and produce a grain yield ranging between 4.5 and 6.0 t ha⁻¹on research fields and 3.5 to 4.5 t ha⁻¹ on farmers' field [24]. Threefarmers'faba bean varieties (Yeferenji Baqela, Yehabesha Baqela, and Batte) and one improved variety of faba bean known as Gachena which was released by Haramaya University in 2008 were used. Gachena variety is adapted well to the altitude ranging between 2000 and 3000 m above sea level and the average annual rainfall of 700–1000 mm. It yielded 1.7 to 3.0 t ha⁻¹ on research station and 1.1 to 2.8 t ha⁻¹ on farmers' field [25]. Yeferenji Baqela is characterized by big grain size and cream colour. Yehabesha Baqela is a small seed size with a light brown seed colour while Batte variety is flat in shape and a very big seed size.

2.3. Treatments and experimental design

The treatments consisted of a factorial combination of four varieties of faba bean (Yeferenji Baqela, Yehabesha Baqela, Batte, and Gachena) and three population densities (25%, 50%, and 75% of the recommended sole crop density of 250,000 plants ha⁻¹) intercropped with maize, and the respective sole faba bean varieties and sole maize resulting in 12 intercropping treatments. The experiments were laid out as a randomized complete block design with three replications.

The size of each plot was 3.75 m × 3.0 m = 11.25 m². Maize was used as a main crop and spaced at the distance of 0.75 m × 0.25 m for both intercropping and sole cropping system. The inter- and intra-row spacing between faba bean plants for sole cropping was 0.4 m × 0.1 m, respectively. The intra-row spacing between faba bean plants for 25, 50, and 75% population densities were 16.7, 8.6, and 5.7 cm, respectively. However, the inter-row spacing between faba bean plants to maize plants was 37.5 cm. Faba bean was planted manually in a 1:1 arrangement with maize when the maize plant reached knee-high. All growth and yield data of component crops were recorded from the plot area of 2.625 m × 2.5 m for intercropped maize and faba bean, sole-cropped maize, and sole-cropped faba bean treatments. Half of the recommended rate of 87 kg N ha⁻¹ [24] in the form of urea (46% N) was applied manually in rows for maize during sowing at the distance of 5 cm away from the seeds, and the remaining half of the fertilizer was applied when the plant reached knee-high. All phosphorus fertilizer at the recommended rate of 46 kg P₂O₅ ha⁻¹ in the form of triple super phosphate (46% P₂O₅) was applied during sowing of maize.

2.4. Data collection and measurement

2.4.1. Yield and yield components data collection

2.4.1.1. Maize

The number of ears per plant and kernels per ear were recorded from ten randomly selected plants per plot at harvest. Number of kernels per ear refers to the number of rows per ear multiplied by the number of kernels per row. Thousands kernel weight (g) was determined from the bulk seed of each plot using a sensitive balance and the weight was adjusted to 12.5% seed moisture level. All maize stalks samples from each plot were harvested at ground level and sun-dried until a constant weight was achieved and used for total aboveground dry biomass (t ha⁻¹) determination. Grain yield (t ha⁻¹) was recorded for each net plot and adjusted to 12.5% seed moisture content as:

$$adjustedgrainyield=Actualgrainyield\times \frac{100-M}{100-D}$$ (1)

where, M is the measured moisture content in the grain and D is the designated moisture content (12.5%).

Harvest index (%) was determined as the ratio of grain yield to the aboveground dry biomass at harvest per net plot area multiplied by 100.

2.4.1.2. Faba bean

The number of pods per plant at harvest was determined by counting the pods from five randomly selected plants from each plot and expressed per plant basis. Then ten randomly selected mature pods from the five randomly selected plants were threshed using stick beating and seeds counted for determining seeds per pod. Hundred grain weight from bulked grain of each net plot of five sets were used to determine hundred seed weight per plot. The hundred seed weight was adjusted to 10% moisture content. The aboveground dry biomass was measured at crop physiological maturity from ten randomly selected plants each plot after sun drying the whole plant samples the grains to a constant weight. The mean aboveground dry biomass per plant was multiplied by the total population of maize per hectare to obtain the total aboveground dry biomass per hectare. The weight of harvested grain yield from each net plot was determined and the moisture content of the grain was measured using a moisture tester meter and the yield was adjusted to 10% grain moisture content as:

$$Adjustedgrainyield=Actualgrainyield\times \frac{100-M}{100-D}$$ (2)

where, M is measured moisture in grain and D is the designated moisture content (10%).

Harvest index (%) was calculated as the ratio of grain yield to total aboveground dry biomass yield multiplied by 100.

2.4.2. Productivity of the intercropping system

Land equivalent ratio (LER): The intercropping system productivity was evaluated based on land equivalent ratio (LER). LER was calculated as stated by Willey [26].

$$LER=\frac{Y_{MF}}{Y_{MM}}+\frac{Y_{FM}}{Y_{FF}}$$ (3)

where, _YMF = yield of maize in intercropping, _YFM = yield of faba bean in intercropping, _YMM = yield of maize in pure stand, and _YFF = yield of faba bean in pure stand.

Gross monetary value (GMV): Gross monetary value (GMV) was calculated as the product of yields of the component crops (kg ha⁻¹) multiplied by their respective unit prices during harvesting time. An average of 2018–2019 market price of Ethiopian Birr (ETB) 15.00 kg⁻¹ for maize and ETB 35.00 kg⁻¹ for faba bean grains were taken for the economic analysis. One USD was 32.2 ETB in the first quarter year of 2020.

Maize equivalent yield (MEY): Maize was the main crop; therefore, yield of faba bean in the intercropping system was converted to maize equivalent yield as stated by Choudhary et al. [27].

$$M_{EY}=Y_{MF}+\left(\frac{Y_{FM}\times P_F}{P_M}\right)$$ (4)

where; _MEY = Maize equivalent yield; _YMF = intercrop maize grain yield (kg ha⁻¹); _YFM = intercrop faba bean grain yield (kg ha⁻¹); _PM = price of maize grain kg⁻¹; _PF = price of faba bean grain kg⁻¹.

Monetary advantage index (MAI): The yields of the component crops in the cropping system were measured in terms of monetary value of return to evaluate its economic benefit. The MAI was calculated as:

$$MAI=\left[\left(P_M\times Y_{MF}\right)+\left(P_F\times Y_{FM}\right)\right]\times \frac{LER-1}{LER}$$ (5)

where: P_M = Price of maize; Y_MF = yield of maize in intercropping; P_F = Price of faba bean; Y_FM = yield of faba bean in intercropping; LER = land equivalent ratio.

2.5. Data analysis

Homogeneity tests were made for error variances for the parameters across the years using the F_max test before proceeding to the analysis of variance.

$$F_{\mathit{max}}=\frac{Largermeansquareerror}{Smallermeansquareerror}$$ (6)

As the F_max ≤ 3.00, the parameters were considered homogeneous so that combined analysis of variance (ANOVA) for the two year's data was done using SAS (statistical analysis system) version 9.4 program [28]. Differences among treatment means were delineated using LSD (least significance difference) test at 5% level of significance when the ANOVA showed significant differences.

3. Results and discussion

3.1. Maize yield and yield components

3.1.1. Number of ears per plant

The main effects of variety and planting density of the intercropped faba bean on the number of ears per plant were significant (p < 0.01). However, the interaction effects of year, cropping system, variety and plant density were not significant (Table 1). The number of ears per plant of maize recorded in 2019 cropping year (1.63) surpassed the number of ear per plant in 2018 cropping year by 23.9% (Table 1). The presence of sufficient moisture in 2019 cropping year might have caused vigorous growth of maize, thereby resulting in the production of greater number of ears per plant. The highest number of ears per plant (1.48) was recorded for maize intercropped with faba bean variety Gachena which was in statistical parity with the number of ears per plant produced by maize with Yehabesha Baqela, and exceeded the minimum number of ears per plant recorded in intercropping with the Batte maize variety by about 4.7% (Table 1). This implies Gachena and Yehabesha Baqela varieties might inherently exert low competition on to the intercropped maize for growth resources. In agreement with this result, Jibril et al. [29] reported that maize intercropped with common bean variety Nasir produced a higher number of cobs per plant (1.95) than maize intercropped with a local common bean variety (1.42). This may be due to the more nitrogen fixation capacity of the improved common bean variety Nasir.

Table 1.

Main effects of variety and plant density of the intercropped faba bean on yield and yield components of maize under intercropping and sole cropping at Haramaya during the 2018 and 22019 main cropping season.

Factor	NEPP	NKPE	TKW (g)	AGDB (t ha−1)	GY (t ha−1)	HI (%)
Cropping year
2018	1.24b	496.14b	209.32b	13.42b	4.56b	33.94a
2019	1.63a	602.51a	327.97a	19.04a	5.52a	29.05b
F-test	***	***	***	***	***	***
LSD (5%)	0.03	20.35	8.23	0.66	0.33	1.84
Faba bean varieties
M + Yehabesha Baqela	1.44ab	548.48	265.18	16.81	5.09	30.36
M + Yeferenji Baqela	1.42b	547.42	270.51	15.93	4.89	31.51
M + Batte	1.41b	553.29	272.73	16.48	5.08	31.40
M + Gachena	1.48a	548.11	286.16	15.70	5.11	32.72
F-test	**	Ns	Ns	Ns	Ns	Ns
LSD (5%)	0.04	Ns	Ns	Ns	Ns	Ns
Faba bean population
M + Faba bean (25%)	1.54a	543.22	283.16a	17.58a	5.56a	32.54
M + Faba bean (50%)	1.44b	551.59	267.39b	16.21b	5.04b	31.44
M + Faba bean (75%)	1.33c	553.16	255.38c	14.90c	4.53c	30.51
F-test	***	Ns	***	***	***	Ns
LSD (5%)	0.04	Ns	10.08	0.81	0.4	Ns
CV (%)	4.42	7.56	6.23	8.24	13.02	11.98
Cropping systems
Intercropped maize	1.44b	549.3	268.6b	16.23b	5.04b	31.50
Sole maize	1.70a	561.2	287.9a	19.11a	6.16a	33.02
F-test	***	Ns	**	***	***	Ns
LSD (5%)	0.19	Ns	54.21	2.92	0.76	Ns
CV (%)	15.6	12.0	23.7	21.0	17.5	15.1

Means in the same column followed by the same letter(s) are not significantly different at 5% level of significance according to LSD test; M = maize; NEPP = Number of ears per plant; NKPE = Number of kernels per ear; TKW = Thousand kernel weight; AGDB = Aboveground dry biomass; GY = Grain yield; HI = Harvest index; CV = coefficient of variation; Ns = Not significant at p < 0.05; ** and *** indicates significant difference at probability level of 0.01 and 0.001, respectively.

The number of ears per plant of maize intercropped with 25% of the sole faba bean density (1.54) was significantly higher than the number of ears per plant of maize intercropped with 50% and 75% of the sole faba bean plant densities by 7.6% and 13.6%, respectively (Table 1). Lower inter-species competition for growth resources at 25% of the sole-cropped faba bean density might be the reason for a higher number of ears per plant. Consistent to this result, Teshome et al. [30] reported the highest number of ear per plant (2.30) from maize intercropped with 25% of soybean population but the lowest number of ears per plant (1.95) from the 75% of soybean population. Sole-cropped maize produced significantly higher number of ears per plant (1.7) than intercropped maize (Table 1). Sole-cropped maize may have utilized all growth resources due to no competition and could have led to the production of increased number of ears per plant. Consistent with this result, Thobatsi [31] also reported a significantly higher number of ears per plant of maize (1.71) from sole-cropped maize than from intercropped maize with cowpea (1.51).

3.1.2. Number of kernels per ear

The two seasons combined analysis of variance showed significant (p < 0.001) differences in the number of kernels per ear (NKPE) due to cropping year. A significantly higher number of kernels per ear (602.51) were obtained in the 2019 cropping year than in 2018 cropping year possibly due to higher rainfall received during the growing season, which may have prompted increased numbers of ears. However, there was no significant difference in this variable due to faba bean variety, plant density, cropping system, and their interactions (Table 1).

3.1.3. Thousands kernel weight

There was no significant difference in thousands kernels weight (TKW) due to faba bean variety. However, this variable was significantly (p˂0.001) affected by the associated faba bean plant density, cropping year, and cropping system (Table 1). The highest thousands kernel weight (283.16 g) was obtained from maize intercropped at the density of 25% faba bean, which exceeded the thousands kernels weight obtained from the maize intercropped with 50% and 75% of the sole-cropped faba bean plant density by 5.6% and 9.8%, respectively (Table 1). At higher planting densities, yield may be restricted by limitations in the capacity for endosperm growth either by number, size or activity of endosperm cells [32]. Similarly, Teshome et al. [30] reported the highest TKW (243.88 g) from maize intercropped with 25% of soybean population while intercropping maize with 75% soybean population resulted in the lowest TKW (229.11 g). Sole-cropped maize produced significantly higher TKW (287.9 g) than the intercropped maize (268.6 g) (Table 1). Sole-cropped maize may not have faced stiff intercrop competition for growth limiting resources so that grains may have accumulated enough starch to achieve increased kernel weights. In agreement with this result, Alemayehu et al. [33] reported a significantly higher thousands kernel weight of maize (318 g) in sole-cropped cropping maize than in maize intercropped with common bean (291 g).

3.1.4. Aboveground dry biomass

Cropping year, faba bean planting density, and cropping system had significant effects on aboveground dry biomass (Table 1). Main effect of faba bean variety and the interaction effects of cropping year, faba bean planting density, and cropping system were not significant on AGDB of maize. The AGDB of maize obtained in the 2019 cropping year (19.04 t ha⁻¹) surpassed the AGDB of maize obtained in cropping year of 2018 by about 29.5%. The highest AGDB (17.58 t ha⁻¹) was obtained from the maize intercropped with 25% of faba bean plant densities, which exceeded the AGDB of maize intercropped with 50% and 75% of the sole-cropped faba bean plant densities by 7.8% and 15.2%, respectively. The lowest AGDB of maize in response to intercropping with 75% of faba bean density may be due to less space between plants for growth factors such that the individual maize plants may have been prone to stiff competition possibly leading to the growth of thinner-stemmed maize. In agreement with this result, Teshome et al. [30] reported a decrease in dry matter yield of individual maize plants with increase in the intercropped soybean plant population density. On the other hand, sole-cropped maize had a significantly higher AGDB (19.11 t ha⁻¹) than the intercropped maize (16.23 t ha⁻¹) (Table 1), which might be attributed to the absence of inter-species competition in sole-cropped maize. This result is in agreement with the finding of Stoltz et al. [34] who reported that sole-cropped maize had a higher dry matter yield (by 44–57%) than maize intercropped with faba bean. However, the range of variation in present study was for about 15.1%.

3.1.5. Grain yield of maize

The interaction effects did not significantly affect the grain yield of the maize (Table 1). The analysis of variance showed that grain yield (GY) was significantly (p < 0.001) affected by faba bean population, cropping year, and cropping system. Grain yield of maize obtained in 2019 cropping year (5.52 t ha⁻¹) exceeded the grain yield of maize recorded in the 2018 cropping year by about 17.4% (Table 1). The presence of higher rainfall during 2019 cropping year may have been conducive to achieve improved grain yield. Consistent with this result, Cudjoe et al. [35] reported a positive relationship between annual rainfall and maize yield. The results also revealed that grain yield of maize intercropped with 25% of faba bean density (5.56 t ha⁻¹) surpassed the grain yield of maize intercropped with 75% faba bean density by about 18.5% (Table 1). This could be attributed to the fact that maize intercropped with 25% density faba bean had the highest number of ears per plant and thousands kernel weights due to less inter-species competition of faba bean for growth resources. Similar to the current result, Ashenafi et al. [36] reported a superior maize grain yield (4527.00 kg ha⁻¹) from intercropping with 50% population density of common bean, but less yield (4166.11 kg ha⁻¹) from maize intercropped with 100% common bean planting density. Corroborating the results of this study, Negasa et al. [37] also reported the highest maize yield (7.19 t ha⁻¹) from 100% maize population density with 25% plant density of soybean while the lowest grain yield (7.04 t ha⁻¹) was obtained from a combination of 100% maize with 75% plant population density of soybean.

The intercropped faba bean varieties did not significantly affect grain yield of the associated maize. On the other hand, sole-cropped maize had significantly higher grain yield (6.16 t ha⁻¹) than the intercropped maize (Table 1). The higher grain of the sole-cropped maize might be due to the fact that the wider available space in may have reduced the competition for light, nutrients, and soil moisture, which may have provided favourable growing conditions for maize plants to produce higher yields. In agreement with this result, Negasa et al. [37] reported significantly higher grain yield (7.43 t ha⁻¹) for sole-cropped maize than for intercropped maize with soybean (7.19 t ha⁻¹). Similarly, Rezaei-Chianeh et al. [38] reported that intercropping maize with faba bean resulted in significant reduction in maize yield.

3.1.6. Harvest index

Cropping year had significant (p < 0.001) difference on harvest index of maize intercropped with faba bean effects. However, the main effect of faba bean varieties, plant density, cropping systems and the interaction effects of these factors had no significant effects on maize harvest index. A significantly higher harvest index (33.94%) was obtained in 2018 growing season than in 2019 cropping year (29.05%) (Table 1). The lower amount of rainfall received in 2018 cropping year may have resulted in a lower vegetative growth and aboveground dry biomass yield of the maize that may have led to increased harvest indices of maize. A similar result was reported by Alemayehu et al. [39] that maize harvest index was significantly affected by the growing season. Generally, even though it was not statistically significant, maize harvest index showed a decreasing trend with the increase in the density of intercropped faba bean plants possibly due to the decreased grain yield harvested as a result of increased inter-species competition.

3.2. Faba bean yield and yield components

3.2.1. Number of pods per plant

The main effects of densities of faba bean as well as cropping year and cropping system were significant (p < 0.001) on the number of pods per plant (NPPP). The NPPP of faba bean (10.04) in 2019 cropping year was higher by 16.7% than number of pods per plant obtained 2018 cropping year (Table 2). The results also revealed a higher NPPP (10.23) for the variety Gachena which amounted to17.4% more than that of variety Yehabesha Baqela intercropped with maize. The higher NPPP for Gachena variety may be attributed to higher number of primary branches bearing more pods. In line with this result, Tekle et al. [40] reported a significantly different NPPP among improved varieties of faba bean ranging from 18.61 to 21.56 indicating much higher NPPP than the present study due to the varieties used were an improved one.

Table 2.

Main effects of faba bean year, variety and plant density on yield and yield components of faba bean planted in sole and intercropped with maize at Haramaya.

Factor	NPPP	NGPP	HGW (g)	AGDB (t ha−1)	GY (t ha−1)	HI (%)
Cropping year
2018	8.36b	2.63b	56.63b	2.75b	0.67b	24.15
2019	10.04a	3.11a	59.48a	3.46a	0.81a	23.98
F-test	***	***	***	***	***	Ns
LSD (5%)	0.35	0.11	1.64	0.27	0.05	Ns
Faba bean varieties
M + Yehabesha Baqela	8.45c	2.77b	53.39b	2.62c	0.59c	22.7b
M + Yeferenji Baqela	8.98b	2.88ab	53.08b	3.07b	0.72b	23.31b
M + Batte	9.14b	2.88ab	63.23a	2.99bc	0.69b	23.5b
M + Gachena	10.23a	2.94a	62.51a	3.73a	1.96a	26.75a
F-test	***	*	***	***	***	*
LSD (5%)	0.49	0.16	2.32	0.38	0.07	2.59
Faba bean population
M + Faba bean (25%)	10.33a	3.01a	58.36	2.26c	0.54c	24.07
M + Faba bean (50%)	9.3b	2.81b	57.30	3.10b	0.72b	23.47
M + Faba bean (75%)	7.97c	2.79b	58.51	3.95a	0.96a	24.65
F-test	***	**	Ns	***	***	Ns
LSD (5%)	0.42	0.14	Ns	0.33	0.06	Ns
CV (%)	8.36	7.8	6.47	15.43	13.93	16.24
Cropping systems
Intercropped faba bean	9.2b	2.87b	58.05b	3.10b	0.74b	24.07a
Sole faba bean	13.82a	3.22a	65.71a	11.59a	2.08a	18.08b
F-test	***	***	**	***	***	***
LSD (5%)	1.03	0.16	3.23	0.92	0.19	1.86
CV (%)	21.2	11.8	12.0	37.7	38.6	17.6

Means in the same column followed by the same letter(s) are not significantly different at 5% level of significance according to LSD test; M = maize; NPPP = Number of pods per plant; NSPP = Number of seeds per plant; HSW = Hundred grain weight; AGDB = Above ground dry biomass; GY = Grain yield; HI = Harvest index; CV = Coefficient of variation; Ns = Not significant at p < 0.05; *, ** and *** indicates significant difference at probability level of 0.05, 0.01 and 0.001, respectively.

Generally, the highest mean NPPP (10.33) was obtained when 25% of faba bean population was intercropped with maize as compared to 50% (9.3) and 75% (7.97) of faba bean densities (Table 2). The decrease in the NPPP with increased plant density may be attributed to increased competition among plants for growth factors such as light, moisture and nutrients [30]. In agreement with the result, Abd El Lateef et al. [41] also found the reduction in the NPPP from 12.8 to 7.3 as plant density increased from 19 plants per m² to 44 plants per m². The sole-cropped faba bean had significantly higher NPPP (13.82) than intercropped faba bean (9.2) which might be due to higher primary branches resulting in more pods. The availability of photo-assimilates is reduced by low light intensity and shading during flowering, which causes abortion of flowers, resulting in lower number of pods per plant in the intercropping system [42]. Klimek-Kopyra et al. [43] also reported the highest average NPPP (8.79) of faba bean from sole cropping.

3.2.2. Number of seeds per pod

Number of seeds per pod (NSPP) was significantly (p < 0.0001) affected by the cropping year and cropping system with no significant interaction effects of the factors on this variable (Table 2). Higher NSPP (3.11) was recorded in 2019 cropping year than in 2018 cropping year (2.63). NSPP was significantly (p < 0.05) affected by plant population and faba bean variety. Maximum NSPP (2.94) was obtained from variety Gachena, which was in statistical parity with NSPP obtained from Yeferenji Baqela and Batte varieties while the lowest NSPP (2.77) was recorded when variety Yehabesha Baqela was intercropped with maize (Table 2). The Yehabesha Baqela is affected by the shading and competition effects resulting in lower NSPP. Likewise, Klimek-Kopyra et al. [43] reported that variety that recorded a higher NSPP (2.94) was obtained from the determinate variety than from an indeterminate variety.

The highest NSPP (3.01) was recorded for 25% faba bean density intercropped with maize as compared to 50% (2.81) and 75% (2.79) of faba bean densities (Table 2). Generally, with increasing density of the intercropped faba bean, a decreasing trend of NSPP was observed. The higher NSPP with less plant density could be due to less competition for growth resources. This result agrees with that of Mekkei [44] who reported that the NSPP of faba bean decreased from 3.63 to 2.30 with decreasing intra-row spacing from 25 cm to 10 cm. Khalil et al. [45] also reported minimum NSPP of 3.12 at highest density of faba bean and maximum (3.50) NSPP at lowest density of faba bean. Sole-cropped faba bean resulted in significantly higher NSPP (3.22) than the intercropped faba bean (Table 2). The reduction of faba bean NSPP under intercropping might be due to competition for light. Abou-Keriasha et al. [46] reported significantly higher NSPP (3.55) in sole cropped faba bean than faba bean intercropped with wheat (NSPP = 3.03).

3.2.3. Hundreds grain weight

Hundreds grain weight (HGW) was significantly (p < 0.01) affected by faba bean variety, cropping year and cropping system (Table 2). Maximum HSW (63.23 g) was recorded for Batte variety which was statistically at par with that of Gachena variety (62.51 g) while varieties Yehabesha Baqela and Yeferenji Baqela produced lowest HSW. Such difference might be due to the grain size differences among the varieties. Similarly, Dereje et al. [47] reported significant differences among faba bean varieties in thousands grain weight. A significantly higher HSW (65.71 g) was recorded in sole-cropped than the intercropped faba bean (58.05 g). This sole-cropped faba bean might have had no interspecific competition and also had enough sunlight to photosynthesize and the food translocates sufficiently to the sink and the endosperms of the grain become well developed that may have contributed increments in grain mass. Mahmoud et al. [48] also indicated that HSW of faba bean was higher (58.52 g) in sole cropping and significantly decreased when intercropped with onion (57.44 g), garlic (57.18 g) and fennel (56.03 g).

3.2.4. Aboveground dry biomass

The main effects of plant population, variety, cropping year, and cropping system had significant (p < 0.001) effects on aboveground dry biomass (AGDB) of faba bean intercropped with maize, while the interaction effects were not significant (Table 2). A higher AGDB of faba bean (3.46 t ha⁻¹) was obtained in 2019 cropping year than in the 2018 cropping year for the main effect of cropping year. Variety Gachena had the highest AGDB (3.73 t ha⁻¹) while variety Yehabesha Baqela produced the lowest (2.62 t ha⁻¹). This can be attributed to the inherent varietal characteristics that come from the maximum growth and development related to plant height and number of primary branches of variety Gachena. Similarly, Yirga and Zinabu [49] reported significant variations among faba bean varieties for AGDB ranging from 3000 to 4739.6 kg ha⁻¹ at Dehana, Ethiopia.

The highest mean AGDB (3.95 t ha⁻¹) was obtained with the 75% density of faba bean intercropped with maize as compared to 50% (3.10 t ha⁻¹) and 25% (2.26 t ha⁻¹) of density of faba bean (Table 2). The highest AGDB in 75% faba bean density may be due to higher number of faba bean population per unit area in the intercropping system [50]. Consistent with this result, Almaz et al. [51] obtained a higher AGDB (6909 kg ha⁻¹) in the high density (416667 ha⁻¹) and the lowest AGDB (4505 kg ha⁻¹) in lowest density (166667 ha⁻¹) of faba bean. A significantly higher AGDB (11.59 t ha⁻¹) was recorded for sole-cropped faba bean than intercropped faba bean with maize (3.10 t ha⁻¹) possibly due to their higher planting density and absence of inter-species competition. A similar result was reported by Oskoii et al. [52] where the highest (451.9 g m⁻²) and the lowest (125 g m⁻²) AGDB were achieved in sole cropping and, maize and faba bean intercropping, respectively.

3.2.5. Grain yield

The grain yield of faba bean was significantly (p < 0.001) affected by plant population, faba bean variety, cropping year, and cropping system. However, this variable was not affected by the interaction effects of the factors (Table 2). A significantly (p < 0.001) higher grain yield (0.81 t ha⁻¹) was obtained in 2019 cropping year than in the 2018 cropping year (0.67 t ha⁻¹) (Table 2). Inadequate soil moisture as a result of low amounts of rainfall that fell during the 2018 cropping year might have resulted in stiff competition among the components crop plants for uptake of soil moisture, resulting in law hydraulic conductivity in the leaves, slowed cell growth, slower leaf expansion for enhanced photosynthesis. The 75% density of faba bean intercropped with maize produced the maximum grain yield (0.96 t ha⁻¹) as compared to 50% (0.72 t ha⁻¹) and 25% (0.54 t ha⁻¹) (Table 2). This could be due to higher number of plants in spite of lower number of pods per plant and grain per pod at the 75% plant density In line with this result, Derogar et al. [53] found the maximal and minimal faba bean grain yields of 487.49 and 369.69 g m⁻²for density of 12 plants m⁻² and 8 plants m⁻², respectively. Similarly, Wondimu et al. [54] reported that grain yield increased for all tested faba bean varieties with increasing plant density from 166666 up to 250000 plants ha⁻¹.

Intercropping different faba bean varieties with maize also resulted in significant differences in grain yield of faba bean (Table 2). The maximum grain yield (1.96 t ha⁻¹) was recorded for Gachena variety while the lowest grain yield (0.59 t ha⁻¹) was recorded for Yehabesha Baqela variety intercropped with maize. The significant differences observed among the varieties in grain yield might be related to genetic differences as reflected in the number ofbranches, grain size, and yielding capacity. In line with this result, Ashenafi and Mekuria [55] reported significant yield differences among faba bean varieties, which ranged from 4886.8 kgha⁻¹for variety Shallo to 878.6 kgha⁻¹for variety Tumsa. A higher mean faba bean grain yield (2.08 t ha⁻¹) was recorded for the sole-cropped than for the intercropped faba bean (0.74 t ha⁻¹) possibly due to higher population density of the plant and absence of inter-specific competition among the faba bean plants. Corroborating this result, Rezaei-Chianeh et al. [38] reported a decline in the grain yield of faba bean in an intercropping system (175 g m⁻²) as compared to a sole cropping system (280 g m⁻²).

3.2.6. Harvest index

The analysis of variance revealed no significant differences in harvest index of faba bean due to cropping year as well as faba bean density and their interactions. However, there was a significant difference in harvest index of faba bean due to variety and cropping systems (Table 2). Harvest index of faba bean variety Gachena intercropped with maize was the highest (26.75%) while intercropping with variety Yehabesha Baqela produced the lowest harvest index (22.7%). The highest harvest index for variety Gachena might have resulted due to high partitioning of assimilates to the grain as it is a high yielding improved variety. A similar result was reported by Abdalla et al. [56] that the harvest index varied for different faba bean varieties ranging from 12.2% to 32.7%. These results also revealed that intercropped faba bean had a significantly higher harvest index (24.07%) as compared to the sole-cropped faba bean (18.08%). This indicates the ability of intercropped faba bean to partition more of it's assimilates into grain than biological yield [49]. This was attributed to the efficient utilization of growth resources by the intercropped faba bean. The current result is in line with Negasa et al. [37] who reported higher soybean harvest index (28.12%) in intercropping with maize compared to sole cropping (25.7%).

3.3. System productivity and economic return

3.3.1. Land equivalent ratio (LER)

The productivity of intercropping was evaluated using land equivalent ratio (LER) as an index. The combined analysis of variance showed that the LER was not significantly affected by plant population, faba bean variety, and growing season (Table 3). However, LER showed an increasing trend with an increase in faba bean plant populations. Thus, minimum LER (1.168) and maximum LER (1.211) were recorded in response to intercropping maize with faba bean plant populations of 25% and 75%, respectively. LER was greater than 1.0 in all intercropping treatments (Table 3), indicating that intercropping of maize with faba bean was more productive than sole cropping of each crop. The higher LER in response to intercropping might be due to efficient resource utilization by the component crops. In agreement with this result, Rezaei-Chianeh et al. [38] reported total land equivalent ratios of greater than 1.22, showing 22% yield advantages due to faba bean intercropping with maize compared with sole cropping of each crop. Even though it was not significant, the highest LER (1.22) was recorded in response to intercropping Batte variety with maize, indicating that an additional 22%ha of land would have been needed to get an equal yield to planting maize and faba bean in pure stands. Similar results were reported by Solomon [57] that intercropped maize with soybean varieties yielded 14–32% and 6–28% of its sole-cropped yield in terms of soybean varieties and planting densities, respectively. On the other hand, intercropping had a LER of 1.184 which had an 18.4% more advantage in terms of saving land than sole cropping. Similarly, Matusso et al. [58] reported a LER of 1.11–1.55 in intercropping system of maize-soybean in Kamujine.

Table 3.

Main effects of faba bean year, varieties and their plant population on land equivalent ratio, maize equivalent yield, gross monetary value, and monetary advantage index of maize-faba bean intercropping system.

Factor	LER	MEY (kg ha−1)	GMV (ETB ha−1)	MAI
Cropping year
2018	1.180	5511.05b	82666b	+12512
2019	1.188	6672.03a	100081a	+15352
F-test	Ns	***	***	Ns
LSD (5%)	Ns	315.05	4725.7	Ns
Faba bean varieties
M + Yehabesha Baqela	1.186	5820.98b	87315b	+13913
M + Yeferenji Baqela	1.188	5905.63b	88584b	+13695
M + Batte	1.222	6018.35b	90276b	+16120
M + Gachena	1.139	6621.2a	99318a	+12001
F-test	Ns	**	**	Ns
LSD (5%)	Ns	445.55	6683.2	Ns
Plant population
M + 25% faba bean	1.168	6138.76	92081	+12998
M + 50% faba bean	1.173	6049.33	90740	+13158
M + 75% faba bean	1.211	6086.54	91298	+15641
F-test	Ns	Ns	Ns	Ns
LSD (5%)	Ns	Ns	Ns	Ns
CV (%)	11.37	10.89	10.89	75.72
Cropping systems
Intercropping	1.184	6092a	–	–
Sole maize	1.0	5544b	–	–
F-test		*
LSD (5%)		755.3
CV (%)		14.7

Means in a column for treatments with the same letter or no letter are not significantly different; *, **, *** Significant at p ≤ 0.05, p ≤ 0.01, p ≤ 0.001 probability levels; M = Maize; Ns = not significant; LSD = Least significant difference; ETB = Ethiopian Birr = 1 US dollar = 32.2 ETB; LER = land equivalent ratio; GMV = gross monetary value; MEY = maize equivalent yield; MAI = monetary advantage index.

3.3.2. Maize equivalent yield (MEY)

A significantly higher maize equivalent yield (6092 kg ha⁻¹) was obtained from intercropped maize than sole-cropped maize (Table 3). A yield advantage of 19.4% was obtained from maize intercropped with Gachena faba bean variety relative to sole-cropped maize. Similarly, there was a yield advantage of 10.7% from maize intercropped with 25% of the recommended faba bean compared to sole-cropped maize. The highest maize equivalent yield for the intercrop treatments compared with sole crop maize was possibly because additional faba bean yields were obtained from the intercrop treatments. The reduced intercropped maize yield compared with the sole-cropped maize was compensated for by the additional yield of faba bean in the intercropping system. Consistent with this result, Alemayehu et al. [33] found a lower intercrop maize yield (4.3–4.9 t ha⁻¹) but a higher maize equivalent yield (5.4–6.2 t ha⁻¹) relative of sole maize (5.4 t ha⁻¹) in a maize-common bean intercropping system. Similarly, Paudel et al. [59] also reported a higher maize equivalent yield in intercropping (8.19 t ha⁻¹) than sole-cropped maize (5.83 t ha⁻¹) in a maize-soybean intercropping system.

3.3.3. Gross monetary value (GMV)

Evaluating the economic benefit of intercropping system is helpful in identifying the most rewarding treatments. Results showed that intercropping of maize with faba bean variety Gachena produced a significantly higher gross monetary value (99318 ETB ha⁻¹) than the other varieties of faba bean intercropped with maize (Table 3). This may be due to high yielding capacity of Gachena variety. Consistent with the present result, Mitiku and Getachew [60] reported the highest GMV of 30,883 ETB ha⁻¹ was obtained from common bean variety Awash Melka while the lowest Goss Monetary Value of 17,356 ETB ha⁻¹ was obtained when bean variety Red wolaita was intercropped with rice. However, in this study, there was no significant difference in the gross monetary value in plant population of faba bean intercropped with maize. A significantly higher gross monetary value (GMV) (100081ETB ha⁻¹) was obtained in the 2019 cropping year than in the 2018 cropping year. The higher maize and faba bean grain yields obtained in the 2019 cropping year were a possible reason for the higher GMV recorded in 2019. In agreement with this result, Getachew et al. [17] reported a significant difference in GMV for cropping year in maize-common bean intercropping system.

3.3.4. Monetary advantage index (MAI)

Monetary advantage index (MAI), which is an indicator of the economic feasibility of an intercropping system, was positive for all intercropping systems including faba bean varieties, plant populations, and cropping years (Table 3). However, the two seasons combined analysis of variance showed no significant differences of MAI due to faba bean varieties, plant population and cropping year. The positive MAI values in all intercropping systems and growing years show a yield advantage compared with the respective sole cropping systems implying the suitability of intercropping maize with faba bean. Intercropping advantage may come from better resources (moisture, light and nutrient) utilization with low interspecific interaction and better complementary effect of the component crops. A similar result was reported by Ghosh [61] that when land equivalent ratio is higher there is also significant economic benefit expressed with higher effective monetary advantage index.

4. Conclusion

The results of this study demonstrated that maize intercropped with 25% of sole-cropped faba bean varieties resulted in the highest number of ears per plant, thousands kernels weight, aboveground dry biomass, and grain yield. Sole-cropped maize produced significantly higher grain yield than intercropped maize. For the faba beans, variety Gachena produced the highest number of pods per plant, aboveground dry biomass, grain yield, and harvest index. Maximum grain yield was obtained from the 75% faba bean plant population intercropped with maize. Yield and yield components were significantly higher for the sole-cropped than intercropped faba bean. However, a higher land equivalent ratio, and maize equivalent yield were obtained from the variety Gachena intercropped with maize. In conclusion, intercropping of Gachena faba bean variety with maize can be used by smallholder farmers to obtain the higher maize equivalent yield and gross monetary value of the system and enhance food and nutrition security in the study area. Furthermore, it is important to develop varieties of maize and faba beans that are successful and compatible in intercropping system through breeding program; determining the appropriate time (stage of growth) of introducing a component crop such as faba bean to the system for successful intercropping; and including the impact of maize-faba bean intercropping on major soil nutrient contents.

Author contribution statement

Negera Nurgi: Performed the experiments; Wrote the paper.

Tamado Tana: Conceived and designed the experiments.

Nigussie Dechassa: Conceived and designed the experiments.

Yibekal Alemayehu: Contributed reagents, materials, analysis tools or data.

Bulti Tesso: Analyzed and interpreted the data; Wrote the paper.

Funding statement

The study was supported by Ministry of Science and Higher Education of Ethiopia and Collaborative Crop Research Program of the McKnight Foundation (CCRP) under the Legume Diversity Project funded (2/2018).

Data availability statement

Data will be made available on request.

Declaration of interest’s statement

The authors declare no competing interests.