Abstract
The stability of cropping systems in a changing climate context depends on numerous factors, including row orientation, to optimize the use of environmental resources. This study aimed to evaluate the performance of agricultural systems based on the row orientation of cowpea (Vigna unguiculata) and maize (Zea mays) in three different ecological conditions in Côte d’Ivoire, in 2020 and 2021. An experimental design with plots divided into three complete randomized blocks with six subplots per block and three repetitions was set up, varying the row orientation in intercropping and monoculture. Row orientation, cropping system used, and ecological zone significantly influenced crop yield and its components. In intercropping, cowpea (tiligré) showed the best yields when oriented North–South (NS) in the tropical rainforest zone and the sub-Sudanian savannah, while East–West (EW) orientation was optimal in the forest-savannah mosaic zone. In contrast, maize (GMRP18) yielded the best in intercropping when rows were oriented East–West, regardless of the season. In monoculture, cowpea yielded best when oriented EW in the tropical rainforest zone (488.17 kg ha−1) and in the forest-savannah mosaic zone (1262.12 kg ha−1), while NS orientations were preferable (228.6 and 259.54 kg ha−1) in the sub-Sudanian savannah zone. For maize monoculture, EW orientation was also preferable. Throughout the study, for both seasons, the highest yield advantages in maize-cowpea intercropping were obtained in EW orientation, regardless of the ecological zone. Overall, the forest-savannah mosaic zone yielded the best cowpea yields (1262.12 and 663.9 kg ha−1).
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1 Introduction
Agricultural production in Sub-Saharan Africa (SSA) is facing increasing pressure to meet the food and nutritional needs of a growing population, while grappling with the challenges posed by climate change and variability, as well as degraded and infertile soils [1, 2]. The phenomenon of climate change is having an increasingly significant impact on water resources and agricultural production [3]. One of the main reasons for this decline is the high pressure of climatic variations due to greenhouse gases, which lead to significant irregularities in rainfall. Smallholder farmers in Sub-Saharan Africa often resort to continuous monoculture of cereals such as maize (Zea mays L.) to ensure food security, even when profitability is limited [4]. These monocultural systems have increased crop yields with the discovery of pesticides. Today, the failure of this monocultural system is recognized by several researchers and policymakers. Indeed, this farming system requires a large amount of fertilizer and pesticides [5, 6]. This system promotes the genetic erosion of plants and land pollution. Thus, the interest in high-yielding agriculture that is mindful of biodiversity and the environment is constantly highlighted [7]. In particular, the development of cropping systems adapted to climate change and the nutritional and economic requirements of local populations has become a major challenge for scientists. In this context, local cropping systems consisting of crop associations that allow for the management of biodiversity and arable land should be promoted [8]. Crop association is a promising approach to achieve these goals in agriculture. Indeed, several researchers argue that polyculture systems enable efficient use of arable land and the preservation of agricultural biodiversity [9, 10].
These systems involve the cultivation of two or more plant species or genotypes together, coexisting for a certain period [11]. In practice, most intercropping systems involve only two crops, as including more crops incurs costs [12]. Intercropping with legumes and cereals is a popular option. Although both crops compete for soil nitrogen, as they both need it for growth, competition prompts legumes to fix atmospheric nitrogen in symbiosis with Rhizobium bacteria [13]. This results in complementary nitrogen use by crops, particularly crucial in soils where inorganic nitrogen is limited [14]. Many studies have shown that intercropping can increase crop yields [15] through efficient use of light [16] and enhanced positive interactions between crops [17]. However, most of these studies have focused on the effects of different intercropping species [18]. Nevertheless, despite the importance of intercropping, there are very few reports in the literature regarding the effect of planting row orientation on the productivity parameters of component species.
Commonly studied intercrops usually involve two species: the main cereal and the associated legume. In maize-legume intercropping systems, the extent of the intercropping advantage generally depends on the level of contribution of the legume component. However, legume components usually perform poorly due to the dominant nature of maize cultivation [19]. For example, in maize-soybean intercropping, the amount of available radiation for soybean has dropped by up to 90%, and consequently, grain yield has decreased accordingly [20]. Similarly, in a maize-cowpea intercropping, light interception by cowpea is reduced by up to 63%, leading to an associated productivity loss of 62% [21]. Effective use of solar radiation is one of the key criteria for achieving a yield advantage through intercropping and is more reliable compared to the large variability possible in water and nutrient supply [22]. The abundant radiation available in the tropics presents an excellent opportunity to increase its utilization for improved agricultural production [22]. It is therefore imperative to devise strategies to capture and use radiation as fully and efficiently as possible during a given growing season [19].
One possible way to increase light interception by the canopy is to manipulate the spacing and orientation of crop rows [23]. The response to row orientation can vary depending on location [24], season [25], and the availability of other growth resources [26]. The effect of row orientation varies depending on latitude and the seasonal tilt of the earth relative to the sun. Near the equator, a north–south orientation (as opposed to east–west) offers crops higher levels of light absorption for most of the year. In a maize-common bean intercropping system, row orientation showed negligible effects on fractional interception, radiation use efficiency, and intercropping advantage [20]. On the other hand, the fraction of intercepted light in a North–South (NS) orientation differed from the EW orientation by 10 to 23% depending on the seasons [25]. Additionally, there is a trend for better light infiltration in the morning and afternoon in EW orientation compared to NS in maize-common bean intercropping [27]. At higher latitudes (up to 55°), absorption is maximal for north–south crops in summer and for east–west crops the rest of the year. From 65° and beyond, an east–west orientation offers the greatest light absorption throughout the year (though the difference between orientations is minor) [28]. The latitude of the Western Australian grain belt (a region of large-scale cereal cultivation) ranges from 28° to 33°S. The cropping season occurs during winter and spring, indicating that east–west crops should receive the greatest light absorption [28].
These studies highlight the importance of orientation in light absorption in plant production. However, few farmers and researchers consider this factor in the stability and improvement of yields in cropping systems. Available data on intercropping systems mainly focus on the availability of plant resources, sowing date [29], plant density [30], competitiveness indices [31], and crop arrangement [14]. However, few studies have been conducted to investigate the effects of intercropping orientation, its interaction with agroecological zones, and cropping systems Existing data concern the effect of crop orientation on weed control [24, 32], a monoculture study on sugar beet in Hungary [26], and associations regarding orientation [19, 20, 33].
In mixed crop associations, several factors, including the orientation of rows of companion plants, must be considered to ensure better use of photosynthetic radiation and increased yields. Adequate knowledge of row orientation and its dependence on the cropping system and agroecological zone could be a promising approach to minimize inevitable interspecific competition when two species are grown together, leading to better resource utilization (active photosynthetic radiation) and, consequently, higher yields.
The study aimed to analyze the impact of row orientation on the yield of maize and cowpea, considering the agroecological zone and cropping system. These data are crucial for a more informed management of the agricultural system.
The research hypotheses have been formulated as follows: 1) The row orientation that would enhance productivity of the test crops would depend on the ecological zone. 2) The row orientation that would enhance productivity of the test crops would depend on the cropping system. 3) The row orientation that would enhance productivity of the test crops would depend on both the ecological zone and the cropping system.
2 Materials and methods
2.1 Growing conditions
The experiments were conducted in three distinct agro-ecological zones of Côte d'Ivoire: the zone of tropical rainforest zone, the forest-savannah mosaic zone, and the sub-Sudanian savannah zone, specifically in Adzopé, Bédiala, and Dikodougou respectively. Meteorological data related to precipitation, temperature, and humidity were provided by the Société d'Exploitation et de Développement Aéroportuaire, Aéronautique et Météorologique (SODEXAM) of Côte d'Ivoire over a period of two consecutive years (2020–2021) of experimentation in these three zones with different climatic and vegetal characteristics.
Maize is a staple crop grown in all three ecological zones. However, it is mainly grown in monoculture, except for a few farmers who grow it in association with rice or peanuts, especially in the sub-Sudanian savannah zone. This association is practiced without any particular orientation. In contrast, cowpea was more cultivated by allochthonous populations originating from Mali and Burkina Faso, settled in these zones. Today, cowpea has been adopted into the dietary habits and local agricultural practices by indigenous populations, especially those from the sub-Sudanian savannah and some from the forest-savannah mosaic zone.
However, the maize-cowpea association is mainly practiced for self-consumption, as farmers believe that its maintenance requires significant resources and that there is no suitable selective herbicide for both crops, as is the case for the maize-rice association, where a selective herbicide for grasses is usable. As for the tropical rainforest zone, the objective is to introduce agricultural practices that limit the harmful effects of climate variability.
The information on the three zones has been summarized in Tables 1 and 2.
The tropical rainforest zone, located in southern Côte d'Ivoire, benefits from a particularly rainy climate. Its geographical coordinates are as follows: latitude 6.10° North, longitude 3.87° West, with an average altitude of 119 m above sea level. The climate is of the Attéen type and characterized by its humidity. The vegetation is mainly composed of lush tropical rainforest, although it is heavily exploited [34]. Cash crops include cocoa, coffee, rubber, and plantain. In contrast, yam, rice, cassava, plantain, and maize are the main food crops (Table 1). The moderately leached ferralitic soils of the region are suitable for subsistence crops [35]. The area has a bimodal rainfall regime, with average annual precipitation totals of 1399.27 mm and 1518.68 mm in 2020 and 2021 respectively, average temperatures of 28.58 °C and 28.49 °C, and relative humidities of 71.24% and 72.46% (SODEXAM) (Fig. 1). The soils in the region are mainly sandy loams (Table 1), with high proportions of sand ranging from 44.59% to 58.39% and silt ranging from 36.39% to 48.70%. The study site had a labile carbon content of 371.01 mg kg−1 soil, a pH of 5.96, a coarse fraction content of 2.61%, and organic matter content of 3.57% (Table 2).
Diagram representing monthly precipitation (bars) and air temperature (line) in the tropical rainforest zone, for the years 2020 and 2021 (SODEXAM)
The forest-savannah mosaic zone, located in the central-west of Côte d'Ivoire, is characterized by a subequatorial Attéen climate. It serves as a transition zone between the forest and the savannah, with geographical coordinates of latitude 7° 10′ 00ʺ North and longitude 6° 18′ 00ʺ West, and an average altitude of 244 m above sea level [36]. Its vegetation is mainly composed of forests and wooded savannas. Cash crops include cocoa, coffee, rubber, cashew nuts, and food crops notably include rice, yam, cassava, plantain, peanuts, and maize (Table 1). The soil is mostly composed of remodeled soils [37], with low erosion due to the presence of vegetation cover [38]. The rainfall regime in this zone is bimodal, with average annual precipitation totals of 1328 mm and 1213.1 mm, average temperatures of 27.15 °C and 27.18 °C, and relative humidities of 72.74% and 72.93% in 2020 and 2021, respectively (SODEXAM) (Fig. 2). The soils in the forest-savannah mosaic zone have generally thick surface horizons, brown, humiferous, porous, sandy-clayey texture (Table 1) and gravelly structures, containing a sufficient number of millimetric to decimetric roots with sub-horizontal orientation. These characteristics diminish in depth in the soil [39]. The study site had a labile carbon content of 522.55 mg kg−1 soil, a pH of 6.62, a coarse fraction content of 11.35%, and organic matter content of 5.33% (Table 2).
Diagram representing monthly precipitation (bars) and air temperature (line) in the forest-savannah mosaic zone, for the years 2020 and 2021 (SODEXAM)
The sub-Sudanian savannah zone, located in the north of Côte d'Ivoire, is characterized by wooded savannah vegetation and shallow, often gravelly soils. The geographical coordinates of the site are as follows: latitude 6.10° North, longitude 3.87° West. Cash crops grown there include cotton, yam, cashew nuts, and shea butter, while yam, upland rice, sweet potato, maize, and cowpea are the main food crops (Table 1). The soils in this region are primarily ferralsols and ferruginous (lateritic) leached or impoverished soils [40]. Precipitation in this zone is monomodal, with average annual totals of 815.06 mm and 1311.5 mm in 2020 and 2021 respectively, average temperatures of 26.51 °C and 27.23 °C, and relative humidities of 65.12% and 64.80% (SODEXAM) (Fig. 3). This region is at an average altitude of 401 m above sea level. The soils in the study area are primarily clay-loamy in texture (Table 1), with a sand content of about 46.05%. The study site had a labile carbon content of 339.88 mg kg−1 soil, a pH of 6.4, a coarse fraction content of 39.26%, and organic matter content of 3.39% (Table 2).
Diagram representing monthly precipitation (bars) and air temperature (line) in the sub-Sudanian savannah zone, for the years 2020 and 2021 (SODEXAM)
2.2 Plant material
The plant material used in this study consisted of seeds of two varieties: a cowpea variety provided by the Institute for Environmental and Agricultural Research (INERA) of Burkina Faso and a maize variety from the National Center for Agricultural Research (CNRA) of Côte d'Ivoire. The maize variety known as GMRP-18, has corny, toothed, yellow, sweet-tasting grains. It is easy to grind and rich in amino acids. This variety has a short growth cycle (about 100 days) and shows resistance to lodging, rust, and streak diseases. Its potential yield is 5 tonnes per hectare. Information about the maize variety was provided by the National Agency for Rural Development Support (ANADER) of Côte d'Ivoire. The cowpea variety, known as KVX771-33-2G or “tiligré”, is characterized by large wrinkled white seeds. This variety is resistant to Striga, has a maturity cycle ranging from 65 to 70 days, and is well adapted to various ecological conditions [41].
2.3 Experimental design and cultivation practices
The experiments were conducted during the major rainy seasons (from July to September) in the forest-savannah mosaic zone and the sub-Sudanian savannah zone. However, in the humid tropical rainforest zone, the trials took place from June to August during the 2020 and 2021 seasons. For this study, an experimental design using plots divided into complete randomized blocks (CRB) comprising six (6) elementary plots per block and three replications was set up on a plot of 1006.2 square meters. Each treatment consisted of a sub-plot measuring 9.25 m by 4.2 m (38.85 square meters). The spacing between the sub-plots was one meter wide, and the plots were regularly weeded as needed. The intercropping method of the two crops was a simple approach between the rows. The treatments included East–West (EW) oriented intercrops, North–South (NS) oriented intercrops, East–West oriented pure crops, and North–South oriented pure crops. Each sub-plot received 96 planting points distributed in 12 rows of 8 holes, with a spacing of 0.4 m between holes in the rows and 0.75 m between rows, both in intercropping plots and in pure crop plots. The plots consisted of 50% maize and 50% cowpea in intercropped plots, and 100% maize or cowpea in pure crop plots. The association was additive, and cowpea and maize were introduced simultaneously. No fertilizer was applied to improve plant production. No plowing was done before sowing in the tropical rainforest zone and in the forest-savannah mosaic zone, but in the sub-Sudanian savannah zone, it was done after plowing. Planting was done with two seeds per hole. All plantings were done on the same day, after complete manual weeding of the entire plot. Staking was first done to delimit the sub-plots and spacings before planting. Fifteen days after planting, thinning was done to retain only the most vigorous seedling at each planting point. No irrigation was done. Harvesting was done when the pods and cobs had dried on the plant. Yield was evaluated by measuring the weight of the harvested seeds in the sub-plot, based on the species.
2.4 Data collection
In this study, six agronomic traits, three for cowpea (Vigna unguiculata) and three for maize (Zea mays), were examined. These characteristics included grain yield and 100-seed weight, which were measured for both cowpea and maize. Additionally, pod length was measured for cowpea and cob length for maize. The measurements (average pod and cob length, 100-seed weight of cowpea and maize) were taken on 20 randomly selected plants, whether grown in association or in pure culture, regardless of the species. After harvest, measurements were taken on pods and cobs, and then on seeds for grain yield.
2.5 Evaluation of the benefits of maize-cowpea intercropping system
The benefits of the cowpea-maize intercropping system were evaluated using the Land Equivalent Ratio (LER) [42]. The LER serves as an indicator of the efficiency of intercropping in utilizing environmental resources compared to monoculture [43]. When the LER is greater than 1, it indicates that intercropping promotes the growth and yield of the respective species. Conversely, when the LER is less than 1, intercropping offers no advantage, and interspecific competition outweighs interspecific interaction within the intercropping system [44]. The LER was calculated as follows:
Total LER: total land equivalent ratio, LERcowpea: land equivalent ratio cowpea, LERmaize: land equivalent ratio maize.
where Yc represents the yield of cowpea in monoculture, Ym represents the yield of maize in monoculture, Yci represents the yield of cowpea in the intercropping system, and Ymi represents the yield of maize in the intercropping system.
2.6 Statistical analysis of data
The analysis of variance (ANOVA) was conducted using three factors: the first factor represented the agroecological zone, the second factor represented the cropping system, and the third factor represented the orientation of planting rows. The statistical analysis was performed using RStudio [45] and interpreted at a significance level of P = 0.05. The objective of the ANOVA was to study the effect of the study zones, cropping systems, orientation of planting rows, and the cropping season of cowpea and maize, as well as their interactions, on the yield parameters of the two crops. When the ANOVA revealed a significant influence (P < 0.05) of one or more factors or their interactions on the yield parameters, the main effect and types of interaction were subjected to further analysis of variance to study the individual or combined effect of the factors on the studied agronomic parameters. Mean values and standard deviations were calculated for each trait, taking into account the agricultural season, agroecological zone, cropping system, orientation of planting rows, and season. The Tukey's honestly significant difference (HSD) multiple range test was used to identify differences between the mean values.
3 Results
3.1 Effect of tested factors and their combinations
The results of the ANOVA used to study the overall effect of the ecological zone, cropping system, orientation of planting rows, and cropping season on the yield parameters of maize and cowpea are presented in Table 3. This test showed that the different factors and their interactions had a significant effect on the evaluated yield parameters (P < 0.05). Given the significance of the interactions, interpretations focused on the interaction tables (ANOVA IV for cowpea), (ANOVA III for maize). And (ANOVA III and II for the Land Equivalent Ratio (LER)) (Table 4). For cowpea, the quadruple interactions were taken into account, and a table for yield, 100-seed weight (P100), and average pod length (Table 5) is presented. For maize, a triple interaction was considered, with Table 6 for yield and Table 7 for P100 and average cob length. There was also a triple interaction for cowpea LER and the total LER (Table 8), as well as a double interaction for maize LER (Table 9).
For cowpea, the interaction between the study zone, cropping system, orientation of planting rows, and cropping season was considered for the ANOVA test and Tukey comparison. However, for maize, it was the interaction between cropping season, cropping system, and orientation of planting rows that was considered for the ANOVA test and Tukey comparison, regarding maize grain yield. Nonetheless, the interaction between the zone, cropping system, orientation of planting rows, and season was used to perform an ANOVA and Tukey comparison for the other maize variables (Table 3).
3.2 Cowpea seed yield
The results indicate a highly significant difference (P < 0.001) in cowpea seed yield during the two cropping seasons (Table 5).
In 2020, the yield was higher in the forest-savannah mosaic zone, regardless of the cropping system and orientation of planting rows. The highest yield was observed in pure crops East–West (EW) orientation (1262.12 kg ha−1). East–West (EW) oriented crops yielded the best regardless of the system in the forest-savannah mosaic zone. In the Tropical rainforest zone, North–South (NS) oriented crops recorded the best yields, regardless of the cropping system. Statistically, there was no difference between cropping systems in this zone. However, in the sub-Sudanian savannah zone, cowpea monocultures had higher yields than intercropped crops, regardless of the orientation. While algebraically, North–South (NS) oriented crops expressed the highest yield in monoculture, compared to East–West (EW) in intercropping in this sub-Sudanian savannah zone. Overall, pure crops outperformed intercropped crops, regardless of the cropping zone.
In 2021, in the tropical rainforest zone, pure EW oriented crops expressed the highest yield (488.17 kg ha−1). The EW orientation yielded the best in pure culture (488.17 kg ha−1), and NS yielded the best in intercropped culture (340.57 kg ha−1), in this tropical rainforest zone. No significant difference was observed in yields for the other zones, regardless of the system and row orientation. However, in the sub-Sudanian savannah zone, cowpea monocultures had higher yields than intercropped crops, and the NS orientation was better than the EW orientation, regardless of the cropping system. Furthermore, in the forest-savannah mosaic zone, NS oriented crops expressed higher algebraic values (336.4 kg ha−1) than those oriented EW (179.73 kg ha−1) in cowpea monoculture, and those oriented EW yielded (324.61 kg ha−1) higher compared to NS in intercropping, in this zone (Table 5).
3.3 Cowpea 100-seed weight (P100)
The weight of one hundred cowpea seeds showed a highly significant difference (P < 0.001) based on the interaction between the zone, cropping system, and orientation of planting rows during the two cropping years (Table 3).
In 2020, during the first cropping season, the 100-seed weight was better (20.28 g) when the planting rows were oriented east–west (EW) in monoculture, in the sub-Sudanian savannah. In this same zone, regardless of the cropping system, EW-oriented plantings produced the highest 100-seed weights, while cowpea monocultures yielded higher 100-seed weights than intercropped crops. Furthermore, in the humid tropical forest, crops with north–south (NS) oriented plantings yielded the best 100-seed weights compared to those oriented EW, regardless of the cropping system. Additionally, pure crops achieved better values than intercropped crops in this tropical rainforest. In the forest-savannah mosaic zone, in pure culture, NS-oriented plantings yielded the highest 100-seed weights compared to EW-oriented ones, while the opposite was observed in intercropped culture, where EW-oriented plantings were superior to NS-oriented ones. Moreover, pure crops achieved higher 100-seed weights than intercropped crops in this mosaic zone (Table 5).
In 2021, the 100-seed weight was better (19.99 g) when the plantings were oriented NS, in cowpea monoculture, in the sub-Sudanian savannah. NS-oriented plantings produced the highest 100-seed weights, regardless of the cropping system, in this sub-Sudanian savannah zone, and pure crops yielded higher 100-seed weights than intercropped crops. Furthermore, in the forest-savannah mosaic zone, the 100-seed weight did not vary statistically based on the cropping system and orientation of planting rows. However, crops with NS-oriented plantings yielded higher 100-seed weight values than those with EW-oriented plantings. Additionally, intercropped crops expressed higher values than pure crops. Finally, in the tropical rainforest, NS-oriented cowpea yielded the best results in intercropped culture and EW in pure culture in this zone (Table 5).
3.4 Average length of a cowpea pod
During the 2020 and 2021 cropping seasons, the average length of pods showed a significant difference (P < 0.001) based on the zone, cropping system, and orientation of planting rows (Table 5).
In 2020, pods were longer in the forest-savannah mosaic zone, in EW oriented cowpea monoculture (18.09 cm). Examining the study zones individually, in the forest-savannah mosaic zone, pods were longer with an EW orientation (17.37 cm), and pods were longer in cowpea monoculture compared to intercropped crops. In the tropical rainforest, pods were longer in crops with EW orientation, regardless of the system, and intercropped systems recorded the longest pods. Conversely, in the sub-Sudanian savannah, pods were longer in crops with NS orientation, regardless of the system, and pure cropping systems yielded the longest pods compared to intercropped crops (Table 5).
In 2021, pods were longer in the tropical rainforest and sub-Sudanian zones, regardless of the cropping system and orientation of planting rows. In the tropical rainforest zone, pods were longer with an EW orientation (17.04 cm), in cowpea monoculture. However, it was the NS orientation that yielded the highest values for average pod length in pure cultures, respectively in the forest-savannah mosaic zone and the sub-Sudanian savannah (Table 5).
3.5 Maize seed yield
The interaction between cropping season, cropping system, and orientation of planting rows had a significant positive impact (P < 0.001) on maize seed yield in 2021 (Table 6). Maize seed yield was lowest in NS-oriented intercropping (964.62 kg ha−1). Pure crops, regardless of their orientation, as well as EW oriented intercropping, showed no statistically significant differences. In contrast, pure NS and EW oriented crops, as well as EW oriented intercropping, achieved the highest yields, in decreasing order of 1411.77 kg ha−1, 1356.59 kg ha−1, and 1225.98 kg ha−1 (Table 6).
Furthermore, in 2020, maize seed yield did not vary significantly, regardless of the cropping system or orientation of planting rows. However, pure crops showed higher yields compared to intercropped crops, and EW oriented crops yielded higher than NS oriented ones (Table 6).
3.6 Maize 100-Seed Weight (P100)
The 100-seed weight varied depending on the zone, cropping system, and orientation of planting rows (P < 0.001). The best 100-seed weights were recorded in the forest-savannah mosaic zone, regardless of the cropping system or orientation of planting rows (Table 7).
When examining the individual ecological zones: in the forest-savannah mosaic zone, the 100-seed weight was higher in intercropped crops compared to pure crops. Orientations differed depending on the systems. Thus, in intercropping, the NS orientation (26.06 g) yielded the highest 100-seed weight, while in pure cropping in this zone, the EW orientation (25.15 g) was preferred (Table 7).
In the tropical rainforest, regardless of the cropping system, the EW orientation produced the best 100-seed weights compared to the NS orientation. However, the intercropping system recorded the highest 100-seed weight (23.67 g).
In the sub-Sudanian savannah, the pure cropping system yielded the highest 100-seed weight, in the NS direction (20.64 g) (Table 7).
3.7 Average length of maize cob
The average maize cob length was influenced by the ecological zone, cropping system, and orientation of planting rows (P < 0.001). Thus, the forest-savannah mosaic zone recorded the longest cobs, with no statistical difference between cropping systems and orientation of planting rows. However, in this same mosaic zone, the highest value was observed in EW oriented intercropping (14.97 cm). The sub-Sudanian savannah zone followed the mosaic zone in the ranking, with no difference between cropping systems and orientation of planting rows. However, the highest value in this savannah zone was observed in EW oriented crops (13.54 cm). This same orientation recorded high average values of cob length compared to the NS direction, regardless of the cropping system (Table 7). In the tropical rainforest zone, the EW orientation in intercropping expressed the greatest value of average cob length (14.08 cm) (Table 7).
3.8 Maize land equivalent ratio (LERmaize)
The results reveal a highly significant difference (P < 0.001) in maize Land Equivalent Ratio (LERmaize) based on the ecological zone and row orientation (Table 8). LERmaize values were below 1 (LERmaize<1), Except for those obtained in the tropical rainforest zone and in the sub-Sudanian savannah zone when the planting rows were oriented EW (1.28 and 1.03, respectively). The same EW orientation expressed the highest LERmaize value (0.82) in the forest-savannah mosaic zone (Table 8).
3.9 Cowpea land equivalent ratio (LERcowpea)
The results reveal a significant difference (P<0.05) in LERcowpea based on the agroecological zone and row orientation, for both cropping seasons (Table 9).
In 2020, all LERcowpea values were below 1, regardless of the zone and row orientation. However, the highest values were obtained in the EW orientation in tropical rainforest zone (0.94) and sub-Sudanian savannah zone (0.94). For the forest-savannah mosaic zone, the highest value was observed in the NS orientation (0.62) (Table 9).
In 2021, LERcowpea values were above 1 when planting rows were oriented NS in the tropical rainforest zone and East-West in the forest-savannah mosaic zone (1.08 and 1.41 respectively), with no statistical difference between these two values (Table 9). LERcowpea values are statistically identical in the sub-Sudanian savannah zone; however, the NS orientation expressed the highest value (0.88) (Table 9).
3.10 Total land equivalent ratio (LERtotal)
The result shows a significant difference (P<0.01) only for the year 2020. All LERtotal values indicate an advantage of intercropping over monoculture systems (LERtotal >1), regardless of the ecological zone, row orientation, and cropping season. The advantage increases with the rise in the value of LERtotal (Table 9).
In 2020, the rows-oriented EW in tropical rainforest and sub-Sudanian savannah zones exhibited the best LERtotal values (1.98 and 1.78 respectively), with no statistical difference between these two values. However, in the forest-savannah mosaic zone, LERtotal values did not differ statistically based on the orientation of the rows, however the NS orientation had the highest value (1.42) (Table 9).
In 2021, no significant difference was found between LERtotal values, regardless of the zone and orientation of the planting rows. However, the forest-savannah mosaic zone recorded the highest value in the EW orientation (2.02). The sub-Sudanian savannah zone also obtained the highest LERtotal value in the EW direction (1.9), compared to NS (1.57) in the tropical rainforest zone (Table 9).
4 Discussion
Intercropping is a widely adopted practice in sustainable agricultural systems, playing a crucial role in enhancing productivity and yield stability while promoting better resource utilization and environmental preservation [46]. In this study, we evaluated the yield and its components in the maize-cowpea system, considering row orientation and agroecological zone.
4.1 Effect of growing seasons on the parameters of cowpea and maize studied in the three agroecological zones
Our results showed that, under our experimental conditions, cowpea yield and its components, as well as maize yield components, assessed during the 2021 growing season, were higher than those in 2020 in the sub-Sudanian savannah zone. However, these yields were higher in 2020 than in 2021 in the forest-savannah mosaic and tropical rainforest zones. These variations could be attributed to differences in precipitation between the two years, both in terms of quantity and distribution (Figs. 1, 2, 3). Changes in precipitation may have led to variations in cowpea populations, while temperature fluctuations between the two years could have influenced crop yields. The differences observed in 100-seed weight, average pod length, and cob length between the two growing seasons could also be attributed to the influence of the environment during seed development [47].
4.2 Effect of agroecological zones on the parameters of cowpea and maize studied over the two growing seasons
The forest-savannah mosaic zone recorded the highest yields for cowpea compared to the other two zones, while the lowest yields were observed in the sub-Sudanian savannah. This increase in yields in the forest-savannah mosaic zone could be explained by the fact that the soils in this zone have more developed physicochemical characteristics than the other two zones (Table 1). Indeed, the pH in this zone is 6.62, which is favorable for cowpea cultivation. Previous studies [48] have shown that pH levels between 6.6 and 7.6 promote the maximum growth of cowpea in terms of root length, plant height, number of nodules, and pods per plant. Furthermore, previous research [49] has revealed that soils rich in organic matter favor the cultivation of cowpea and maize. The low erosion resulting from vegetation cover in the forest-savannah mosaic zone promotes the accumulation of organic matter beneficial to crops [50]. Ref. [51] highlighted the importance of organic matter in maize and wheat cultivation. Additionally, [52] emphasized the significant role of organic matter in improving pH. They also stated that as pH increases in the soil solution, functional groups dissociate, thereby releasing cations. In contrast, the low yields observed in the other two zones are attributable to soil poverty (Table 1), with pH values of 5.96 and 6.4, and organic matter contents of 3.57% and 3.39%, respectively, in the tropical rainforest zone and sub-Sudanian savannah zone. These pH values are acidic [53], which hinders the availability of cations necessary for plant growth. A previous study [48] showed that cowpea plants grown under acidic pH conditions, between 4.2 and 5.8, had up to 83% fewer nodules than those grown in a pH range of 6.6 to 7.6, with a reduction in the number of pods. Additionally, the low yields observed in the two zones could be partly explained by the effects of local climate and environmental changes, which can lead to the emergence of new pests and alter the flowering and fruiting periods of crops. A survey conducted by [54] among farmers in Korhogo and Toumodi showed that they perceived the effect of climate change through the emergence of new pests, weeds, and changes in flowering and fruiting periods. These results confirm those of [55], who emphasized that cowpea plants face many ecological constraints such as diseases, climate, and pests. Furthermore, [56], in their studies on cowpea varietal diversity in Côte d'Ivoire, showed that the reduction in agromorphological characteristics of cowpea is due to low rainfall, a short rainy season, high temperatures, and the intensity of the long dry season characterizing the northern region of Côte d'Ivoire. These results seem to confirm the hypothesis of differences in assimilate mobilization efficiency and, consequently, in plants' ability to fill seeds. The ability to fill seeds, pods, and ears would therefore be greater in plants from the forest-savannah mosaic zone, which had the highest 100-seed weights and cob lengths for maize.
4.3 Effect of cropping systems on the parameters of cowpea and maize studied over the two seasons, in the three agroecological zones
In general, monocultures of cowpea have yielded better compared to cowpea intercropped with other crops. The cowpea yield appears to be clearly compromised when intercropped with maize, regardless of the ecological zone. Studies conducted in Sub-Saharan Africa (SSA) on maize-cowpea intercropping have reported low cowpea yields, mainly attributed to shading from maize, especially when cowpea was planted simultaneously with maize [57]. Furthermore, the efficient use of light and space by maize plants can explain the reduction in cowpea yield in favor of maize in intercropping. These two crops have different growth heights, which allows for a more efficient use of available sunlight. Maize, being the taller crop, can provide shade and reduce competition for light, compromising cowpea yield and benefiting maize [58]. Similarly, [59], in a maize-cowpea intercropping system in Zimbabwe, reported similar results. These authors attribute the low cowpea yields to the lack of differentiation in underground niches regarding root distribution between maize and cowpea. Maize, with its high root density, explored a larger soil volume, thereby acquiring more resources necessary for crop growth, resulting in lower cowpea yields.
In 2021, maize crops recorded the highest yields in pure culture, confirming the observations of [60] in a maize-cowpea association. A study by [61] showed that associating cowpea with maize reduces maize grain yield, regardless of the type of association. This decrease in maize yield in intercropping compared to pure culture could be due to a higher level of interspecific competition in mixed stands and the absence of interspecific competition in pure cultures, as highlighted by [62, 63]. The delayed senescence observed in pure cultures prolonged the grain-filling period, leading to an increase in grain size and thus contributing to higher maize yields, as indicated by [64].
Furthermore, in 2020, for maize, no significant difference was observed between the cropping systems, which is consistent with the observations of [65] in a maize-cowpea association. The high yield of maize in association could be explained by the fact that maize plants benefited from the nitrogen fixed by the legumes in this association.
4.4 Effect of row orientations on the parameters of cowpea and maize studied during the two seasons, depending on the cropping system and agroecological zone
Monoculture systems yielded the best results in the NS orientation in the tropical rainforest zone in 2020, while the EW orientation yielded the best results in the forest-savannah mosaic zone in the same year. The results reversed in 2021 for the same cowpea monoculture system: the NS orientation showed the highest yield compared to the EW orientation, respectively, in the forest-savannah mosaic zone and in the tropical rainforest. However, in the sub-Sudanian savannah zone, the yield was higher in pure culture oriented NS compared to EW. For maize monoculture, the best yield was obtained in the EW orientation, with no difference for maize monoculture. Similar results in the EW orientation have been reported by other authors [20, 24, 66].
Borger et al. [24], through a multifocal study in two locations (Beverley and Merredin) in Western Australia, found that wheat yield from EW oriented crops was higher than that from NS oriented crops in Beverley and Merredin. A result that is different from ours was reported in a study on sugar beet in Hungary, where root and sugar yields were consistently higher in NS orientations compared to EW [26]. The experiments were conducted under hotter and drier conditions than usual, hence under water deficit conditions. These authors explained that the reduced yield in EW rows was caused by more severe water stress than in NS rows. Higher temperatures in EW rows may also have led to reduced assimilate distribution to the roots, resulting in yield loss and reduced harvestable material. Soil temperatures were also affected by row orientation, with soil in EW rows being warmer, especially in windy weather. Greater transpiration of beets in EW rows could have led to early development of plant water stress and thus reduced root yields compared to NS beet orientation. In contrast, beets grown in NS rows would have transpired less and thus had greater reserves [26]. These parameters (soil temperatures, under-canopy temperatures, and plant transpiration based on orientations) were not measured in this study.
The weight of 100 seeds (P100) and the average pod length varied depending on the zones, cropping systems, and orientations. Thus, generally, the P100 of cowpea was higher in the sub-Sudanian savannah zone (20.28 g), while the pods were longer in the tropical rainforest zone (18.09 cm). Values were greater in pure culture compared to intercropping, and crops oriented EW showed the highest values for cowpea. This result clearly indicates that monoculture oriented EW favors cowpea. Probably in this pure culture system oriented EW, cowpea receives and utilizes the photosynthetic radiation necessary for its good growth, proper pod filling, and increased pod length in these ecological zones, as cowpea is a species that requires good solar radiation to achieve the expected yield [67].
Furthermore, maize showed the highest P100 and cob lengths in the forest-savannah mosaic zone, with the highest values obtained in intercropped cultures compared to pure cultures, except in the sub-Sudanian savannah zone where pure cultures yielded the highest values. Additionally, maize exhibited the highest P100 and cob lengths in the two former zones, and the NS direction yielded the highest values in the sub-Sudanian savannah zone. In an intercropped system, different species can have complementary nutrient needs, allowing for better soil resource utilization and vigorous plant growth. This can result in longer ears and larger seeds. Moreover, direct and indirect transfers of nitrogen from legumes to grasses can complement niche effects, improving nitrogen nutrition and grass growth [68]. However, it should be noted that these rates of nitrogen fixation transfer from legumes to cereal crops are generally considered modest [69, 70].
4.5 Effect of row orientations on the land equivalent ratio (LER) of cowpea and maize during the two seasons, depending on the agroecological zone
All LERtotal values were above 1. In 2021, the EW orientation improved the LERtotal (2.02) in the mosaic zone. However, it was in 2020 that the EW orientation improved the LERtotal (1.98) in the tropical rainforest and (1.78) in the sub-Sudanian savannah zones. Furthermore, the partial LER of cowpea (LERcowpea) were high (0.94) in the NS direction in the tropical rainforest zone and EW in the Sudanian savannah zone in 2020. The trend was reversed in 2021, with a predominant LERcowpea (1.41) in the EW orientation in the mosaic zone and NS in the tropical rainforest zone (1.08). Regarding the partial LER of maize (LERmaize), the LERmaize values were also high when the association was oriented EW, regardless of the ecological zone. These results show that row orientation depends on the zone, season, and timing [27] showed better light input in the morning and afternoon in the EW orientation compared to the NS orientation in maize-common bean intercropping in Kenya. According to [25], the fractional interception differed by 10 to 23% in the EW orientation depending on the seasons. Thus, the effects of row orientation could be inconsistent and variable depending on the location [24], season [25], timing of measurement [27], and availability of other crops.
The intercropping advantage was first contributed by the spatial complementarity that allowed greater capture of infiltrated light through the sequential growth of the two pulses [19]. A study conducted in southern Ethiopia by [19] showed that the extended use of the season by common bean for 6–7 weeks allowed further interception and use of incoming radiation. Consequently, intercropped crops captured the greatest amount of seasonal incident radiation compared to all single stands, including double-crop stands. The abundant radiation available in the tropics also provides an excellent opportunity to increase its utilization to boost agricultural production [22], and such cropping systems can offer the advantage of fully exploiting resources.
On the other hand, a result contrary to ours (LERtotal<1) would indicate a disadvantage of intercropping compared to monocultures of cowpea and maize in terms of utilizing environmental resources for plant growth [43]. These low values (LERcowpea and LERmaize) below 1 suggest that competition outweighs interspecific interaction or complementarity, leading to less efficient use of space by the species. Thus, low LER performance is observed in plant communities with intense competition [71]. A similar result was reported by [72] for bottle gourd (Lagenaria siceraria) grown in association with cassava.
Furthermore, higher values of LERtotal > 1 in EW-oriented crops, depending on the zones, indicate better utilization of environmental resources (water, photosynthetic radiation, and soil resources). Moreover, results from maize-cowpea intercropping show greater vegetation cover compared to pure crops, reducing soil evaporation and improving water use efficiency [73]. When LERtotal > 1, water was likely used more efficiently, as crops used more water through transpiration than through evaporation loss or weed growth. Therefore, yield (kg of grains/biomass) is higher in intercropping systems per unit of rainfall than in pure crops [59]. Cases of LERtotal values exceeding 1.00 have been reported in the analysis of sorghum (Sorghum bicolor) and bottle gourd (Lagenaria siceraria) yields in intercropping [10], cassava and Lagenaria siceraria [31], and maize (Zea mays) intercropped with watermelon (Citrullus lanatus) and cassava [74].
The EW orientation likely enhances the efficient use of active photosynthetic radiation by both species in the forest-savannah mosaic zone, regardless of the cropping system. Similar results, with better yields in the EW orientation, were obtained by [66] in Kenya. These authors reported that by planting rows of peas intercropped with sorghum in the EW direction, pea grain yield increased substantially, by 18%, compared to plants oriented NS. Planting in the EW direction allows the trailing plant (cowpea), associated with maize, an epigeal plant, in these zones and cropping systems to receive more active photosynthetic radiation than in the NS direction. Consequently, this enhances the rate of photosynthesis, seed size, weight, and overall yield [20].
5 Conclusion
For farmers with limited resources, the stability of income and yields from agricultural systems is of paramount importance. Polyculture systems have lower failure rates than monoculture systems in the face of disasters or climatic variability. The results of this study have demonstrated that orientation plays an important role in the stability of cropping systems. Thus, the production of cowpea and maize can be optimized through their intercropping with the adoption of appropriate row orientation for both crops.
The forest-savannah mosaic zone showed the best cowpea seed yield overall, while the sub-Sudanian savannah zone showed the lowest. In intercropping, cowpea yielded the best when the rows were oriented NS in the tropical rainforest zone and the Sudanian savannah zone, while EW-oriented rows were optimal in the forest-savannah mosaic zone. On the other hand, maize yielded the best in intercropping when the rows were oriented EW, regardless of the season. In monoculture, cowpea showed its best yields when the rows were oriented EW in the tropical rainforest zone and the forest-savannah mosaic zone, while NS-oriented rows were preferable in the sub-Sudanian savannah zone. For maize monoculture, EW-oriented rows were also preferable. Throughout the study, the highest yield advantages in maize-cowpea intercropping were achieved with EW-oriented rows, regardless of the agroecological zone.
However, further research in other agroecological zones of Côte d'Ivoire is recommended for a more comprehensive understanding. Additionally, variables such as photosynthetic radiation captured by species, temperatures, photosynthetic activity in different orientations, should be introduced into these research programs to aid in understanding and decision-making in Côte d'Ivoire.
Data availability
The data used in this research is available from the corresponding author on reasonable request.
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Acknowledgements
We would like to express our gratitude to the European Union and the African Union for their funding support through project number: AURG II-2-130-2018. We also extend our thanks to the Institute of Environment and Agricultural Research in Burkina Faso and the National Center for Agronomic Research in Côte d'Ivoire for providing the cowpea and maize seed varieties used in our experiments.
Funding
This project received funding from the European Union and the African Union through project number: AURG II-2–130-2018.
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The contribution of the authors was at all levels, from the conduct and realization of the work to the writing of the manuscript. YNC: Conducted the trials, collected the data, analyzed the results, and was one of the main editors of the manuscript. BSB, BJN, KKK: Helped carry out the protocol and conduct the trials. EJT: Helped carry out the protocol, he supervised the writing of the manuscript. IAZB: Was the investigator of the research project and was the artisan of its implementation by creating the framework for its realization. He also supervised the writing of the manuscript.
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Coulibaly, Y.N., Tondoh, E.J., Bonny, B.S. et al. Assessment of maize-cowpea intercropping response to row orientation in diverse agro-ecologies of Côte d'Ivoire. Discov Agric 2, 23 (2024). https://doi.org/10.1007/s44279-024-00025-2
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DOI: https://doi.org/10.1007/s44279-024-00025-2