Maize (Zea mays L.) and common bean (Phaseolus vulgaris L.) are important food and cash crops globally. Maize is the main staple crop in eastern and southern Africa (ESA) and accounts for 19% of the average calorie intake per person per day (Santpoort, 2020). Maize is also grown for livestock feed and as industrial raw material (Badu-Apraku and Fakorede, 2017). Common bean is a source of dietary calories and protein for more than 200 million people in ESA (Nkhata et al., 2021a). It is a vital commercial crop and a source of income in ESA, where 40% of the total annual production has been traded in the region. There is a burgeoning demand for common bean at local, regional, and international markets (Sichilima et al., 2016; Nkhata et al., 2021b). Common bean contributes to biological nitrogen fixation through a symbiotic association with Rhizobium leguminosarum biovar phaseoli. Hence, it is a valuable component in cereal-legume-based cropping systems to improve soil fertility and enhance yield gains of companion crops (Latati et al., 2016).

Maize and common bean are predominantly cultivated under low input and small-scale farming systems in ESA which are characterized by low yield gains (Nassary et al., 2020b). Annual total production of the two crops falls short of demand in ESA partly due to a shortage of agricultural lands, high human population pressure, urbanization, and an array of production constraints, including low soil fertility, drought, weeds, insect pests, and diseases (Sileshi et al., 2010; Singh, 2013; Jayne et al., 2014; Conceição et al., 2016; Badu-Apraku and Fakorede, 2017). For effective resource utilization and enhancing soil fertility and yield gains, intercropping of maize and common bean has long been recognized and practiced in ESA (Alemayehu et al., 2017; Nassary et al., 2020b).

Intercropping is the practice of growing two or more crops on the same land simultaneously (Machado, 2009). There are four main intercropping types: inter- or intra-row, strip, mixed, and relay intercropping. Intra-row intercropping used in the present study is the practice of growing two or more crops simultaneously on the same field within a row. The various advantages of the cropping system include suppressing weeds, insect pests, and disease (Fininsa, 2003); enhancing diversity and ecosystem functions; altering soil biota; and protecting and improving soil structure (Finn et al., 2013; Duchene et al., 2017).

In crop production systems, intercropping has been commonly practiced to maximize available resources and labor (Coll et al., 2012). It allows efficient use of resources such as soil moisture, solar radiation, and soil nutrients (Alemayehu et al., 2017; Nassary et al., 2020b). It also reduces insect pests and disease problems significantly when the intercropped species are affected by different causative agents (Theunissen and Schelling, 1996; Lopes et al., 2016). Intercropping permits higher and stable yields in many crop combinations with minimal production inputs such as fertilizers and crop protection chemicals. Intercropping has been an economical option of crop production systems in low input agricultural systems prevalent in ESA (Dyer et al., 2012; Abera et al., 2017). It is a viable strategy to minimize the risk of crop failure in unfavorable environmental conditions such as drought and heat stress.

Studies on bean intercropping with maize pinpointed the high grain yield (GYD) response and land-use efficiency compared with the corresponding monocultures (Worku, 2008; Nassary et al., 2020a,b). Nassary et al. (2020b) reported a two-fold GYD increase in the bean-maize intercropping system. The GYD increase in the reported studies could be attributable to the high crop growth rate; low incidences of insect pests, disease, and weeds; and practical resource use due to the difference in resource utilization and conversion among the companion crops (Alemayehu et al., 2017; Nassary et al., 2020b).

The productivity of monocropping or intercropping systems is dependent on crop variety, location, year, and their interaction. The benefits accrued from intercropping depend on the companion cultivars' compatibility in resource utilization such as nutrients, water, and light; tolerance to insect pests, diseases, and weed infestation; and various planting times, planting densities, and maturity periods. High yield advantage occurs when component crops have different growing periods, making their major demand of growth resources at different times (Seran and Brintha, 2010). Legume crops such as cowpea (Vigna unguiculata L.), groundnut (Arachis hypogaea L.), and common bean have been widely reported to be compatible with maize intercropping (Ghosh, 2004; Latati et al., 2014; Nassary et al., 2020b). However, compatibility varies among crop species and cultivars, necessitating detailed evaluation on agronomic performance and yield advantages before large-scale production is recommended (Worku, 2008; Abera et al., 2017).

The International Center for Tropical Agriculture (CIAT)/Malawi recently developed new common bean varieties denominated as “NUA45” and “SER83.” The two bean varieties are widely cultivated in southern African countries. NUA45 has medium maturity with mean days to maturity of 90 days, a large red mottled seed with bush growth habit, while SER83 is an early maturing (70 days) with a small red seed and bush growth form. NUA45 was released in Malawi (in 2009), the Democratic Republic of Congo (2012), Eswatini (2013), Lesotho (2011), Mauritius, Mozambique (2011), Zambia (2012), and Zimbabwe (2010); SER83 in 2018 in Malawi. NUA45 occupies 20% of the area allocated to bean production in Malawi. The total share of this variety compared to other bean varieties is 12 and 8% in Zimbabwe and Mozambique, respectively (Chirwa, personal communication). The average yield for NUA45 is 2,000 kg/ha in pure stand, experimental, or commercial production conditions, while SER83 recorded a yield of 2,500 kg/ha under the same conditions (CIAT, unpublished data). Most farmers in the region widely intercrop these varieties with maize (MOAFS, 2015; Silberg et al., 2017). However, their performance under intercropping systems has not been well-documented for large-scale recommendation and production. Therefore, the objective of this study was to determine the productivity of the newly released common bean varieties NUA45 and SER83 under sole cropping and intercropping with a maize hybrid variety SC672 to guide large-scale production.

The significant interaction effects of location, year, genotype, and cropping system for the evaluated common bean agronomic traits (Table 4) suggest that environment, genotype, and cropping system effects determined the performance of the tested common bean varieties. Common bean-maize intercropping studies (Santalla et al., 2001; Worku, 2008; Nassary et al., 2020b) showed interactive effects among the component crops. The significant location × year × cropping interaction effects on NPP indicate that the test location and year (environment) influence GYD production in a bean–maize intercrop. This interaction would complicate selection due to crossover ranking in different environments. This finding corroborates earlier reports that suggested that cropping season, location, and cropping system greatly influence the overall performance of the common bean (Santalla et al., 1999; Worku, 2008; Nassary et al., 2020b). Multiple seasons and locations testing trials are necessary before selecting the variety and cropping systems for recommendation to farmers. Grain yield and other agronomic traits varied significantly between the test genotypes. The evaluated bean genotypes are divergent and belong to distinct gene pools; SER83 is a Mesoamerican bean genotype, while NUA45 is an Andean genotype (Nkhata et al., 2020). Mesoamericans have a significantly small seed size when compared with Andean (Santalla et al., 2005). Mesoamerican has a higher yield potential than the Andean beans (Polania et al., 2016).

The significant impact of location by cropping system for GYD of maize may be attributable to different soil properties (Table 4). Linthipe has loamy clay soils, possibly with good water retention. In addition, Linthipe soils were rich in organic matter, organic carbon, and nitrogen compared to Chitedze. These mineral elements are essential for maize growth and productivity. The significant location × cropping interaction on maize yield implies that a particular cropping system could be site-specific. Atuahene-Amankwa et al. (2004) reported a significant location × cropping system interaction in a similar study. Significant effects exerted by the cropping system on maize yield performance suggest that maize performed differently between the two cropping systems. The attributing factor could be interspecific competition for growth resources of maize grown in association with common bean, which was absent in maize grown in the pure stand (Cardoso et al., 2007; Nassary et al., 2020a).

Grain yield and other agronomic traits varied between the cropping systems (Table 5; Figures 2, 3). Common bean intercropped with maize recorded a lower yield than the sole bean. The results corroborate previous maize–bean intercrop studies (Worku, 2008; Yilmaz et al., 2008). The factors contributing to lower bean GYD could be stiff interspecific competition belowground and aboveground for growth resources between the two species in the intercrop (Nassary et al., 2020a). Common bean has a short root system compared to maize. This likely reduced the competitive advantage for essential growth factors such as water, nutrients, and mineral salts.

Similarly, above-the-ground competition for air, water, and light favored maize. Additionally, maize has a height advantage over common bean, reducing adequate light for the bean plant to optimal photosynthesis. The arguments are similar to the findings in previous studies (Brooker et al., 2015; Nassary et al., 2020b). Maingi et al. (2001) reported that C4 species outcompete C3 plants, and the yield of C3 is suppressed in intercropping systems. Both varieties suffered approximately the same yield loss in the intercrop system compared to yield in pure stand. The yield response suggests that the genotypes have the same competitive ability when planted in association with maize. On the contrary, common bean genotypes suffered a further yield loss in a similar study (Atuahene-Amankwa et al., 2004).

Grain yield of maize, when cultivated in association with SER83, was higher than that of maize in a pure stand (Table 8; Figure 4). The results are in line with previous findings in maize–bean intercrop (Alemayehu et al., 2017) and maize–soybean intercrop (Amini et al., 2013). Nitrogen from inorganic fertilizers plus nitrogen fixed by common bean increase total GYD of maize in the intercrop compared with sole crop due to the efficient utilization of growth resources. Nitrogen fixed by common bean supplemented the inorganic nitrogen-added enhanced maize production (Brooker et al., 2015; Kermah et al., 2017). SER83 is drought tolerant with high water use efficiency, which has likely reduced competition for water between the maize and SER83 bean variety (Nassary et al., 2020a). Conversely, GYD of maize is low under maize–NUA45 intercrop. NUA45 is a late-maturing variety compared to SER83 which probably formed nodules during vegetative growth used by the maize crop.

The total LERs of 1.59 and 1.77 for NUA45 and SER83 (Table 6) indicate that intercropping enhanced the productivity of the common bean varieties NUA45 and SER83 compared to sole cropping. In an intercrop system, if LER is >1, the intercropping has advantages over sole cropping. Similarly, higher LER-values of 1.60, 1.85, and 2.47 were reported by Santalla et al. (2001), Latati et al. (2016), and Nassary et al. (2020b), respectively, in maize–bean intercrop studies. Total LERs greater than the unit value in maize–bean intercrop have been reported widely in similar studies (Worku, 2008; Abera et al., 2017; Nassary et al., 2020a). Overall, in the study, intercropping maize with common bean rendered higher yield outputs in the SER83/SC672 intercropping system than the sole crop in the study areas. Therefore, intra-row intercropping of the newly released common bean variety SER83 with a maize hybrid variety SC672 is recommended in the study area and other similar agro-ecologies for stable and sustainable production of both crops.

The maize yield did not vary between sole cropped and intercropped systems. The total LERs for NUA45 and SER83 were 1.59 and 1.77, respectively, suggesting considerably higher benefits from maize and common bean intercropping. Overall, intercropping maize with common bean showed a considerable yield advantage in the SER83/SC672 intercropping system than the sole crop in the study areas. Therefore, intra-row intercropping of the newly released common bean variety SER83 with a maize hybrid variety SC672 is recommended in the study area and other similar agro-ecologies for sustainable production of both crops. Further research on the two cropping systems' social and economic benefit analysis would be of paramount importance.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

WN, HS, and RC contributed to the writing of this article. HS and RC contributed to the editing and reviewing of the article. WN contributed to the original draft and data analysis. All authors contributed to the article and approved the submitted version.

This research publication was made possible with support from the United States Agency for International Development (USAID) through the Africa Research in Sustainable Intensification for the Next Generation (Africa RISING) program as part of the US Government's Feed the Future Initiative. The authors also acknowledge various contributions by the Alliance of Bioversity and CIAT.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.