1 Introduction

In sub-Saharan African (SSA) countries, agricultural sector constitutes the major primary source of employment and income [1]. However, a significant part of the population in SSA faces food insecurity. In Côte d'Ivoire, for instance, the count of peoples experiencing food insecurity continues to rise [2]. The adverse effects of climate change are leading to a decline in agricultural yields [3], thus exacerbating food insecurity. In such contexts, the agricultural system must be adapted to climate change to address major challenges, such as the need to increase production to feed a rapidly growing population [4, 5].

Cowpea is grown in the tropical regions for its seeds, which represent a valuable source of proteins in the diets of millions of people [6]. The seeds of this leguminous plant are consumed as a source of high-quality vegetable proteins [7, 8]. Cowpea plays a crucial role in nutritional balance and the economy of rural populations. Moreover, its integration into the agricultural system significantly improves soil nutrients especially nitrogen [9]. This leguminous is a resilient crop and an interesting nutritional supplement accessible to the poor people [10]. West Africa remains the main cowpea production area, accounting for over 80% of the African production [9]. However, in Côte d'Ivoire, cowpea production remains low and is nearly absent in agricultural statistics.

Maize is one of the most cultivated cereals in the world. It holds high value and is of significant economic importance [11]. It is the most energy-dense cereal and is rich in starch, proteins, and minerals [12]. In most SSA countries, maize constitutes the staple food for populations [13]. Maize is used for human and animal consumption and serves as a raw material in some industries (brewing, soap-making, and oil extraction) [14]. Despite its numerous advantages, corn production remains low in Côte d'Ivoire, amounting to 1.1 million tonnes out of a global production of 1.2 billion tonnes [15].

Improving cowpea and maize yields in Côte d’Ivoire, is therefore necessary to achieve sustainable food and nutritional security. The utilization of high-yield varieties of cowpea and maize in agricultural practices has the potential to enhance productivity, income, and food security in vulnerable production systems [16]. Consequently, cowpea and maize are among the key crops to be promoted due to their nutritional significance. Cowpea and maize are extensively cultivated in rural areas. However, local farmers cultivate cowpea and maize without considering the varieties that better adapt to the specific agroclimatic conditions of their regions, often resulting in yield reductions. The selection of suitable varieties within specific zones holds paramount importance [17], as it allows for the improvement of agricultural yields.

Several studies have been conducted on cowpea and maize, focusing mainly on the effect of cowpea and maize mixed cropping on light intensity, the benefits of maize-cowpea intercropping, the quantification of nutrient loss through soil erosion, as well as on productivity and biological nitrogen fixation [18,19,20,21].

To the best of our knowledge, no study has been conducted to identify the suitable cowpea and maize varieties according to the climatic and vegetation zones in Côte d’Ivoire. This study aimed, in general, to improve the yield of cowpea and maize in Côte d'Ivoire. More specifically, it sought to assess cowpea and maize varieties in different ecological zones in order to identify the resilient varieties with respect to zone.

2 Materials and methods

2.1 Description of the study areas

Field experiments were conducted at Adzopé (6°06′24ʺ N, 3°51′42ʺ W) in the tropical rainforest zone, at Bédiala (7°10′0ʺ N, 6°18′0ʺ W) in the forest-savanna mosaic zone and at Dikodougou (8°30′30ʺ N, 5°52′0ʺ W) in the sub-Sudanian savannah zone. The experiment was conducted over two consecutive years of cultivation (2020–2021) in these three different climatic and vegetation zones. The meteorological data (precipitation, temperature, and humidity) for the three zones were provided by the meteorological station of the Cotton and Cashew Council (CCA) of Côte d'Ivoire. The tropical rain forest zone is located in the Southern part of Côte d'Ivoire. It is characterized by a bimodal rainfall pattern, with an average annual precipitation reaching up to 1800 mm (Fig. 1). The average temperature was 27 °C with a relative humidity of 85%. The zone is characterized by moderately leached and acidic ferralitic soils, which are favorable for subsistence crops [22]. The soils in this zone have low nitrogen (N% = 0.17) and organic matter (OM = 2.0) contents, with a cation exchange capacity (CEC) value of 10.77 cmol kg−1. They exhibit high proportions of sand (44.59–58.39%) and silt (36.39–48.70%) [23]. The forest-savannah mosaic zone is located in the Centre-West of Côte d'Ivoire. This zone is characterized by very deep ferritic soils, rich in organic matter, suitable for all types of crops. Erosion is low due to the presence of vegetation cover. The soils are generally thicker, humus-bearing, porous, with a sandy-clay texture and gravelly structures, and exhibit intense biological activity in the soil. Their cation exchange capacity (CEC) ranges from 7.3 to 8.8 cmol kg−1, with a nitrogen (N) content of around 0.06% [24]. The forest-savannah mosaic zone is influenced by a bimodal rainfall pattern [25]. Annual precipitation varies between 1200 and 1300 mm, with an average temperature of 27 °C and a relative humidity of 76% (Fig. 2). The sub-Sudanian savannah zone is located in the Northern part of Côte d'Ivoire. The entire area is under the influence of unimodal rainfall pattern and receives an average annual rainfall of 900 mm (Fig. 3). The average temperature is about 29 °C, with a relative humidity of 59%. The soils in this zone are of the ferralitic and ferruginous depleted types. These soils exhibit 11.87 to 14.77% silt, 7.67 to 31.91% clay, and 56.42 to 77.54% sand. Their cation exchange capacity (CEC) ranges from 5.72 to 7.28 cmol kg−1, with a nitrogen (N) content varying between 0.06 and 0.054%. They are characterized by a very low organic matter content [26].

Fig. 1
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Graph representing monthly precipitation (bars) and air temperature (line) in the tropical rain forest zone, for the years 2020 and 2021 (CCA)

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Fig. 2
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Graph representing monthly precipitation (bars) and air temperature (line) in the forest-savannah mosaic zone, for the years 2020 and 2021 (CCA)

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Fig. 3
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Graph representing monthly precipitation (bars) and air temperature (line) in the sub-Sudanian savannah zone, for the years 2020 and 2021 (CCA)

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2.2 Planting materials

At total of eight varieties, including four varieties of cowpea and four varieties of maize, constituted the plant material for this study (Fig. 4). The origins and characteristics of the studied cowpea and maize varieties are recorded in Table 1.

Fig. 4
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Seeds of cowpea varieties and maize varieties

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Table 1 Origins and characteristics of cowpea and maize varieties studied
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2.3 Experimental design and agronomic practices

In the three zones where the experiments were conducted, cowpea and maize varieties were grown in an intercropping system with alternating rows. Each cowpea variety was combined with each maize variety. This study does not evaluate cropping systems, but rather the different cultivated varieties. The experimental design was a completely randomized block plan conducted on a plot size of 50 m × 40 m, equivalent to 2000 m2 in each zone. This plot received 16 intercropped subplots (cowpea and maize), each replicated three times. Each subplot had dimensions of 4 m × 3.6 m, resulting in an area of 14.4 m2. The subplots were arranged randomly, and each received six rows. The rows were spaced 60 cm apart, and within each row, there were ten planting points, spaced 40 cm apart from each other. Three seeds were sown per planting point. After germination, one seedling was removed to maintain two seedlings per planting point. Weeds were regularly removed to prevent competition with crops. The experiment was repeated twice in each of the three different zones and was conducted under natural conditions without the addition of fertilizers.

2.4 Data collection

The collected data primarily focused on the grain yields of cowpea and maize varieties in the three study areas. Grain yields were determined based on the gram masses of the dry seeds produced by the plants. The gram masses of the seeds were measured using the SARTORIUS ENTRIS201 digital balance with a precision of 0.01 g and then expressed in kilograms. The grain yields of cowpea and maize in each subplot were converted to kilograms per hectare using the following formula:

$$\text{Grain yield }\left(\text{kg }{\text{ha}}^{-1}\right) = \left(\frac{\text{Grain yield }(\text{kg})}{\text{subplot area }(3.6\text{ m }\times 4\text{ m})}\right)\times \text{ 10,000}$$

2.5 Statistical analysis

The obtained data was subjected to statistical analysis of variance (ANOVA) using the R software (R version 4.1.3). After verifying the homogeneity of variance across years and study areas, a combined analysis for both years and areas was performed using the analysis of variance procedure. The means yields were compared using the Duncan’s multiple range test (DMRT) and least significant difference (LSD) at p ≤ 0.05. Graphs were constructed using Microsoft Office Excel 2016 software, utilizing the average grain yields of all cowpea or maize varieties. These graphs were utilized for comparing the study zones.

3 Results

3.1 Comparison of grain yield of cowpea and maize varieties in the tropical rainforest zone

The results show a significant difference (p < 0.05) among cowpea varieties (Table 2). Indeed, the highest grain yields of 324.36 kg ha−1 in 2020 and 593.30 kg ha−1 in 2021 were recorded for variety KVX780-6. On the other hand, variety KVX745-11P had the lowest yields with 163.73 kg ha−1 in 2020 and 496.84 kg ha−1 in 2021. As for the maize varieties, the results are documented in Table 3. These results demonstrate a significant difference (p < 0.05) among the varieties. Variety Violet de Katiola exhibited the highest grain yields with 1199.02 kg ha−1 in 2020 and 1270.54 kg ha−1 in 2021. Meanwhile, the lowest grain yields were observed for variety Barka with 847.36 kg ha−1 in 2020 and 892.73 kg ha−1 in 2021.

Table 2 Mean values (± standard deviations) of grain yield of cowpea varieties in 2020 and 2021 in tropical rainforest, forest-savannah mosaic and sub-Sudanian savannah zones
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Table 3 Mean values (± standard deviations) of grain yield of maize varieties in 2020 and 2021 in tropical rainforest, forest-savannah mosaic and sub-Sudanian savannah zones
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3.2 Comparison of grain yield of cowpea and maize varieties in the forest-savannah mosaic zone

The results comparing the grain yields of cowpea varieties are presented in Table 2. These results demonstrate that the grain yields of cowpea varieties differ significantly (p < 0.05) from each other. Indeed, the highest grain yields (761.82 kg ha−1 in 2020 and 486.03 kg ha−1 in 2021) were observed for variety KVX745-11P. However, variety Tiligre exhibited the lowest yields during both years of cultivation with respective values of 393.18 kg ha−1 and 383.76 kg ha−1. As for the maize varieties, the results show a significant difference (p < 0.05) in grain yields among the varieties (Table 3). Indeed, variety SR21 had the highest yields in 2020 and 2021 with 1633.05 kg ha−1 and 2212.99 kg ha−1, respectively. On the other hand, variety Violet de Katiola had the lowest yield in 2021 with a value of 1590.61 kg ha−1. In 2020, variety Barka also achieved a low grain yield of 1457.82 kg ha−1.

3.3 Comparison of grain yield of cowpea and maize varieties in sub-Sudanian savannah zone

The results comparing the grain yields of cowpea varieties are presented in Table 2. The results show that the grain yields of the varieties differ significantly (p < 0.05. Indeed, the highest grain yields of 161.08 kg ha−1 in 2020 and 327.31 kg ha−1 in 2021 were observed for variety Tiligre. However, variety KVX745-11P recorded the lowest grain yields during both years of cultivation, with 131.50 kg ha−1 in 2020 and 253.52 kg ha−1 in 2021. For maize, the results comparing the grain yields of the varieties are presented in Table 3. The results reveal a significant difference (p < 0.05) among the varieties. Indeed, variety Barka had the highest yield in 2020 with 950.72 kg ha−1, and variety SR21 also had the highest yield in 2021 with 1797.31 kg ha−1. The lowest yields were observed for variety Violet de Katiola during both years of cultivation with values of 800.56 kg ha−1 in 2020 and 1514.10 kg ha−1 in 2021.

3.4 Comparison of climatic and vegetation zones based on grain yields of cowpea varieties and maize varieties

The results of comparing the three ecological zones (tropical rainforest, forest-savannah mosaic, and sub-Sudanian savannah) based on the grain yield of cowpea and maize varieties during the two years of cultivation are presented in Figs. 5 and 6. The results indicate a significant difference (p < 0.05) among the three ecological zones. Forest-savannah mosaic zone recorded the highest cowpea grain yields in 2020 and 2021, with respective values of 513.20 kg ha−1 and 454.99 kg ha−1. The same zone recorded the highest maize grain yields during the two years of cultivation, with values of 1530.54 kg ha−1 in 2020 and 1757.13 kg ha−1 in 2021. However, sub-Sudanian savannah zone exhibited the lowest cowpea grain yields, with values of 156.49 kg ha−1 in 2020 and 301.64 kg ha−1 in 2021. This same zone recorded the lowest maize grain yield at 870.87 kg ha−1 in 2020. Tropical rainforest zone also experienced a low maize grain yield at 1110.33 kg ha−1 in 2021.

Fig. 5
figure 5

Grain yield for cowpea varieties in tropical rainforest, forest-savannah mosaic and sub-Sudanian savannah zones. Vertical bars show standard error of mean and same letter above the bar show no significant differences

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Fig. 6
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Grain yield for maize varieties in tropical rainforest, forest-savannah mosaic and sub-Sudanian savannah zones. Vertical bars show standard error of mean and same letter above the bar show no significant differences

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4 Discussion

4.1 Tropical rainforest zone

The productivity of cowpea and maize is strongly influenced by the types of cultivated varieties and the growing zone. In tropical rainforest zone, the highest grain yields were achieved with the cowpea variety KVX780-6.

Tropical rainforests are characterized by a warm and humid climate, along with heavy rainfall, which could impact crop growth. The success of the KVX780-6 variety suggests that it would be well-suited to the specific climatic conditions of this area and would efficiently utilize the available resources. However, the work of Olorunwa et al. [27] shows that excessive humidity negatively impacts cowpea yield. Mulungu et al. [28] demonstrated that intense rainfall decreases bean yield. In the same area, the maize variety Violet de Katiola resulted in the highest yield. Maize plants of the Violet de Katiola variety efficiently utilize environmental resources, including water, mineral elements, and light. This corn variety would be genetically adapted to withstand stress related to excessive humidity. It would have a capacity for resistance to common diseases favored by moisture and would be more apt to thrive in humid conditions. The study conducted by Ojara et al. [29] demonstrated that extreme precipitation decreases maize yields. According to Li et al. [30] excessive rainfall can reduce maize grain yield by up to − 34%.

4.2 Forest-savannah mosaic zone

Among the four varieties of cowpea cultivated in the forest-savannah mosaic zone, the cowpea variety KVX745-11P demonstrated the highest yield. The plants of this cowpea variety are upright, allowing them to benefit from increased solar radiation. Similar observations were made by Zhang et al. [31]. Additionally, the physicochemical characteristics of this area would favor the growth of the KVX745-11P variety. The favorable production of cowpea variety KVX745-11P could also be attributed to its adaptation to the agroclimatic conditions of the region, where the annual rainfall reaches 1300 mm of water. Our results differ from those of Johnson et al. [32], who demonstrated that cowpea yields are higher in regions with low rainfall (less than 600 mm per year).

Among all maize varieties cultivate, the SR21 variety showed the best grain yield in the forest-savannah mosaic zone. These results could be attributed to the strong adaptability of SR21 to the specific climatic conditions of the forest-savannah mosaic, including precipitation and temperature. This variety of maize demonstrates increased resistance to common diseases in this region, promoting plant health and, consequently, yield. Additionally, the SR21 variety is well-suited to the physicochemical characteristics of the soil in this area, fostering healthy plant growth. The high productivity of the SR21 variety in this zone could be linked to its enhanced genetic traits. Swarup et al. [33] reported that the improved genetic characteristics of cultivars enhance agronomic performance, particularly by increasing the yield.

4.3 Sub-Sudanian savannah zone

The cowpea variety Tiligre, exhibited higher grain yield performance in Sub-Sudanian savannah zone. Dominated by relatively low annual precipitation, high temperatures, moderate humidity, and soils with very low organic matter content, the Sub-Sudanian savannah zone is favorable for cultivating this cowpea variety. The adaptation of this variety in this zone could be attributed to its ability to leverage available nutrients and maximize the absorption of elements present in impoverished soils. Murdock et al. [34] state that cowpea adapts to semi-arid tropics and thrives even in soils that are relatively poor and experience low precipitation. The maize varieties Barka and SR21, showed the best yields in Sub-Sudanian savannah zone. This area is characterized by low rainfall and nutrient-poor soil. However, the maize varieties Barka and SR21 have shown the best yields. This result can be explained by the capabilities of these varieties to overcome abiotic constraints. Our findings differ from those of refs. [35, 36]. These authors demonstrated that successful maize production on nutrient-poor soils necessarily involves amending these soils.

4.4 Three ecological zones

The choice of an appropriate écologisa zone for a species is essential for optimal production. Our findings have demonstrated that forest-savannah mosaic is the best zone cowpea and maize. The high yields of cowpea and maize in this area may be attributed to the presence of organic-rich soils. Previous research conducted by Adediran et al. [37] revealed that soils rich in organic matter exhibit excellent agricultural suitability and promote the cultivation of maize and cowpea. Moreover, the low erosion resulting from vegetative cover in the forest-savannah mosaic zone fosters the accumulation of organic materials beneficial to crops, as reported by Bayer et al. [38]. Kaur et al. [39] emphasized the importance of organic matter in maize and wheat cultivation. Additionally, the average annual rainfall in forest-savanna mosaic zone, ranging between 1200 and 1300 mm, provides favorable conditions for maize productivity. Prior studies have indicated that maize cultivation is highly sensitive to water deficits [40]. Intercropping was employed in the field experiment not for comparison purposes, but because intercropping cereals with legumes is a common agricultural practice adopted by local farmers in various climatic zones. This practice reflects the real-world agricultural strategies used in these regions, making our results more applicable and relevant to local farming conditions. The intercropping system can influence crop performance by improving resource use efficiency, enhancing soil fertility [41], and reducing pest and disease incidence [42], which aligns with the practices and experiences of local farmers [43].

5 Conclusion

The most productive varieties of cowpea and maize have been identified for three ecological zones. In the tropical forest zone, the cowpea KVX780-6 and the maize Violet de Katiola achieved the highest yields. In the forest-savanna mosaic zone, the cowpea KVX745-11P and the maize SR21 demonstrated the best performance. In the sub-Sudanian savanna zone, the cowpea Tiligre and the maize SR21 reached the highest yields. The forest-savanna mosaic zone recorded the highest overall yields for both cowpea and maize. Future research should focus on the continuous optimization of cultivation practices for these varieties, and it would also be advantageous to investigate the economic impact of adopting these high-yield varieties on local agricultural communities.