Diversification of Rice-Based Cropping Systems with Vegetables and Legumes in Asia and Africa

Abstract

Rice is one of the most important staple foods worldwide. Asia, particularly South and Southeast Asia, is a major rice producer, and rice production is also gradually increasing in Africa. However, rice cultivation poses economic and environmental challenges, which are exacerbated by climate change. Hence, diversification of rice-based production systems is highly imperative to improve soil health and thus sustain productivity while also enhancing income opportunities. Vegetables and pulses are crucial components for diversifying rice-based production systems as they have the potential to increase income and improve soil health. The World Vegetable Center has introduced mungbeans and vegetable soybeans to diversify the cereal-based production systems in Asia. About 27–93% of the mungbean area in India, Pakistan, Bangladesh, and Myanmar is planted with varieties containing improved germplasm developed by WorldVeg in collaboration with national agricultural research systems. Additionally, the introduction of vegetables and legumes is highly remunerative and improves dietary diversity, leading to better nutrition. For instance, the productivity of vegetable crops increased by 200–350% when they were combined with improved production practices. Such diversification also holds great promise for improving income and nutrition in Africa. It also enhances the resilience of farming systems, particularly in a changing climate. Hence, governments should prioritize system diversification to enhance the income and livelihood opportunities for smallholders in Asia and Africa.

1. Introduction

Rice is one of the most important staple crops in the world, with a cultivation area of approximately 168.36 million hectares (Mha) and an annual production of almost 800 million tons [1]. Asia is the world’s largest rice producer, accounting for over 85% of the global rice cultivation area. Within Asia, South Asia is the leading rice producer, accounting for 39.6% of the worldwide rice acreage, followed by Southeast Asia, which holds 26.7% of the production area. Despite an increase in total rice production and productivity, intensive rice-producing countries in Asia have experienced a significant decline in the growth of their rice production areas over the last decade [1]. For instance, the rice production area in Asia increased by 9.72 Mha between 2004 and 2013, whereas it increased by only 1.50 Mha between 2014 and 2023. This has become particularly evident in Southeast Asia, where 1.26 Mha of land shifted away from rice cultivation between 2014 and 2023. However, South Asia increased its rice acreage by over 1 Mha during the same period. China, one of the world’s major rice producers, with 17% of the global rice area, has also shifted more than 1.86 Mha from rice cultivation in the last decade. On the other hand, Africa has significantly expanded its rice cultivation area by over 5 Mha, although productivity has remained relatively stable in recent decades [1].

Like rice, Asia is also the top producer of vegetables, accounting for approximately 71% of the global vegetable cultivation area. Interestingly, South Asia increased its vegetable production area by 815,000 hectares between 2014 and 2023, whereas it was approximately 290,000 hectares in Southeast Asia and 2.38 Mha in Africa. Similarly, Asia accounts for nearly half of the global pulse production area [1], followed by Africa, which has about 32% of the area. In the last decade, Africa has significantly expanded its pulse production area by about 5.35 Mha, and South Asia by over 6.39 Mha, whereas Southeast Asia has decreased its pulse cultivation by 778,000 ha. Thus, Africa and South Asia have expanded their cultivation areas for rice, vegetables, and pulses. In contrast, Southeast Asia has increased its vegetable cultivation area but drastically reduced its rice and pulse production areas.

Despite shifts in rice, vegetable, and pulse cultivation among regions in Asia and Africa, global rice acreage has increased by 5.6 Mha in the last decade. Rice remains the primary staple food crop for more than half of the world’s population. However, the main challenges for rice cultivation include reduced rice yields in intensive rice production systems, rising production costs and low economic returns, biotic and abiotic stresses, and increasing public concern about the environmental impacts of rice farming due to the release of greenhouse gases into the atmosphere. The average rice yield grew at a rate of 3.68% per year in the early 1980s but significantly decreased to 0.74% per year in the late 1990s [2]. In addition, warming climates also reduce rice productivity. For instance, rice yield declines by 10% for every 1 °C increase in night temperature [3]. A recent study has shown a strong correlation between climate variability and declining rice yields, especially in rainfed regions. This decline is attributed to factors such as water scarcity, soil degradation, and increased pest infestations [4]. Each year, a total of 22 Mha of rice fields are negatively affected by flooding, impacting the livelihoods of over 100 million people worldwide [5]. Climate change and other abiotic stresses will also reduce the available land for rice cultivation. The rainfed rice cultivation area in Cambodia is predicted to decrease by 20% by 2050 [6]. In contrast, approximately 100,000 hectares of rice in the coastal regions of the Mekong Delta in Vietnam are affected by increased water salinity due to sea-level rise [7]. Among the biotic constraints, plant diseases seem to have a much greater impact on rice production than insect pests currently [8]. In addition, the rice–wheat cropping sequence, the world’s largest agricultural production system, has also been linked to soil degradation [9]. Hence, it has become highly imperative to diversify the rice-based cropping systems in tropical Asia and sub-Saharan Africa.

Crop diversification is an approach that involves cultivating multiple crops on a farm, rather than relying solely on a single crop. In addition to increasing income opportunities, crop diversification enhances soil health, reduces the risk of crop failures due to biotic and abiotic stresses, promotes biodiversity, and reduces reliance on chemical inputs, thereby strengthening both sustainability and resilience. For instance, diversifying the rice–wheat cropping systems in the Indian subcontinent to include legumes and vegetables has increased dietary diversity and improved soil health, leading to greater productivity and sustainability within the agroecosystem [10]. In response to recurring seasonal droughts in a warming climate, Chinese rice farmers have gradually moved away from double-cropping rice production. Instead, they have diversified their crops to include rice, vegetables, cotton, and coarse cereals, aiming to minimize drought risks and maximize economic returns [11]. Agroforestry systems that integrate trees with maize, beans, and other staples have significantly improved soil fertility, moisture retention, and crop yields and generated additional income in sub-Saharan Africa [12].

Hence, this article underscores the importance of diversifying rice-based cropping systems by integrating vegetables and legumes to enhance resilience, food security, and sustainability, drawing experiences and evidence primarily from the World Vegetable Center (WorldVeg) in tropical Asia and sub-Saharan Africa.

2. Mungbean Transformation in Cereal Fallows in South and Southeast Asia

Mungbean (Vigna radiata (L.) Wilczek) is one of the mandate crops of WorldVeg, formerly known as the Asian Vegetable Research and Development Center (AVRDC), headquartered in Taiwan. WorldVeg’s gene bank maintains the world’s largest public collection of vegetable germplasm, comprising over 70,000 accessions covering 330 species from 155 countries. Of these, 10,542 accessions belong to the mungbean, which is the second-largest collection in legumes, second only to the soybean (Glycine spp.; 13,712 accessions) [13]. WorldVeg’s mungbean breeding program focused on the most important traits, including high yield, bold seeds, wide adaptation to agro-climatic conditions, early and synchronous maturity, and resistance to the most important diseases in East and Southeast Asia (Cercospora leaf spot and powdery mildew) and South Asia (Mungbean Yellow Mosaic Virus—MYMV) as well as insect pests such as bruchids (Callosobruchus spp.) [14]. WorldVeg introduced these improved mungbean lines to countries in South and Southeast Asia. International mungbean nurseries served as the first conduits for distributing the improved lines to national partners, who then tested them for local adaptation. By the end of 2023, 35 countries had released about 150 improved mungbean varieties to their farmers. The improved mungbean varieties were well-adapted to local conditions and offered significant benefits to farmers, who readily adopted them. For rapid adoption across a larger area, seed multiplication of the released mungbean lines was carried out through the Seed Village Program, in which farmers produced their own seed and shared it with others [14].

The rice–wheat cropping system is practiced on millions of hectares in Asia. For instance, the rice–wheat cropping system occupies approximately 16 Mha in Bangladesh, India, Nepal, and Pakistan, with about 85% of this area located within the Indo-Gangetic Plains [15]. Hence, WorldVeg introduced the improved mungbean varieties in the Indo-Gangetic Plains of South Asia. The inclusion of mungbeans in the rice–wheat cropping system has not only diversified and strengthened the cropping system but also improved soil health. Mungbean enriches the soil and breaks the soil fatigue caused by cereal–cereal rotations [14]. For example, the introduction of mungbean in a rice rotation increased the yield of paddy and the income of farmers in Punjab [16]. The improved mungbean lines were also introduced into China, Myanmar, and Thailand. A recent study found that 93% of the mungbean area in Pakistan, 77% in Myanmar, 67% in Bangladesh, and 27% in India is planted with varieties containing improved germplasm developed by WorldVeg in collaboration with national agricultural research systems [17]. Besides its contribution to soil health in cereal-based cropping systems, mungbean is also an important source of protein and iron, which is a significant addition to the cereal-based diets of low-income people in Asia [18]. The introduction of mungbean in rice–wheat systems also increased the system’s productivity. A study in the eastern Gangetic Plains of South Asia found that the rice–wheat–mungbean system, combined with conservation agriculture, yielded the highest rice equivalent yield (REY) and the second-highest gross margin [19]. Thus, mungbeans fit well into Asia’s rice-based farming systems due to their short duration, low fertilizer requirements, and good performance under abiotic stresses [20,21], thereby contributing to the improvement in dietary quality.

3. Diversification in Vegetable and Legume Production in South and Southeast Asia

The continuous cultivation of rice has led to a decline in soil quality, posing serious threats to agricultural sustainability in the intensive rice production areas of tropical Asia. Hence, the diversification of rice-based farming systems has become highly imperative. The inclusion of vegetables and legumes can contribute to poverty alleviation while also enhancing the resilience of cereal-based production systems. Vegetables are high-value crops and a crucial source of essential micronutrients, including vitamins, minerals, and dietary fiber. Vegetables and fruit are also rich sources of antioxidants. Hence, vegetables have the potential to alleviate both poverty and malnutrition. It has been reported that the returns from small-scale rice farming are generally low, ranging from USD 200 to USD 600 per hectare per season [22]. However, vegetable production has led to 3–14-times-higher profits per hectare than rice farming in Cambodia and Vietnam, while earnings per labor day are double those of rice farming [23]. Additionally, vegetables offer more employment opportunities per hectare than cereals, as fruit and vegetable production require two to four times more labor than cereal production [24]. For instance, vegetable production in Asian countries such as Bangladesh, Cambodia, India, Laos, the Philippines, and Vietnam requires 124–468 labor days per hectare per season, compared to 80–216 labor days for cereal production. Another study from the Red River Delta in Vietnam found that permanent vegetable cultivation required more labor than traditional cropping systems, which involved rice followed by seasonal vegetables [25]. However, it should also be noted that vegetables have a lower comparative advantage when labor and access to inputs become the major limiting factors [23]. Hence, vegetable production can be considered as a means to diversify rice-based production systems, provided sufficient inputs are available.

WorldVeg introduced cropping system diversification approaches in the rice production systems of Eastern India. For instance, the state of Jharkhand in India has approximately 1.8 Mha of cultivated area, of which 90% is under rainfed conditions, and the majority of this area is used for monocropped rice cultivation. Efforts were made to diversify the cropping system with a suitable crop that requires less water, particularly during unexpected dry periods under rainfed conditions, to provide both economic and nutritional benefits [26]. Over the five years (2008–2012), the vegetable soybean was successfully introduced into rice-based production systems and as an alternative protein source in the diets of target communities in Jharkhand. It is quite common to see legumes in cereal-based cropping systems in South and Southeast Asia. For example, soybeans are cultivated in rice-based cropping systems or on bunds and dikes in lowland rice fields [27]. Besides their nutritional value, vegetable soybeans also improve soil fertility by fixing atmospheric nitrogen, boost the yield of subsequent cereal crops, and provide up to 30 t/ha of highly nutritious stover or green manure [28]. Hence, vegetable soybeans can easily fit into the rice-based production systems of Asia, where they are currently uncommon.

In addition to vegetable soybeans, several vegetable crops, including tomatoes, eggplants, okra, garden peas, cowpeas, and bottle gourds, were also introduced in the lowlands and mid-altitudes of Jharkhand, which are usually dominated by cereals, especially rice. When improved varieties of these crops were combined with good agronomic practices, productivity was increased by 200–350% [29]. Thus, the farmers in Jharkhand were able to significantly increase their income while also improving the quality of their diets. Similar diversification efforts were piloted in Assam, which has approximately 0.54 Mha of rice fallow land, a considerable portion of which can be utilized for cultivating vegetables and legumes. New climate-resilient technologies, such as tomato production under minimum tillage conditions to utilize rice fallows in Assam, were piloted by WorldVeg. As a result, farmers in various districts of Assam generated an additional income of INR 7000 to INR 24,000 from tomato cultivation following the adoption of minimum tillage practices in rice fallows. New garden pea varieties, such as Kashi Sambridhi, Kashi Udai, and Kashi Ageti, were also introduced to Assam, yielding a maximum of approximately 1.5 tons per hectare. Consequently, these varieties are being considered for introduction to farmers for rice fallow cultivation in Assam.

Other institutions in South and Southeast Asia have also undertaken similar efforts. A study in Gujarat found that the rice–fenugreek–okra system and the rice–onion–cowpea system were the most productive, sustainable, resource-use efficient, and remunerative cropping systems [30]. The introduction of green manure crops (Sesbania rostrata) or legumes (mungbean and cowpea) into the existing rice–wheat system not only increased grain yields but also improved the physicochemical properties, organic matter content, and nutrient availability in the soil in Pakistan [31]. Another study in Eastern India demonstrated the potential of growing a minimum of two crops in rainfed rice production systems, utilizing residual soil moisture to achieve higher productivity and profitability in the cropping systems [32]. It has been demonstrated that system productivity can be enhanced by up to 6–6.5 t/ha after incorporating a single dry-season crop, which typically yields 3.5–5.5 t/ha when a single crop of rice is grown. A recent study investigated the effects of ten different cropping systems on productivity, soil quality, family nutrition, livestock nutrition, and income improvement in western India. The findings revealed that the rice–fenugreek–cluster bean system significantly outperformed the traditional rice–mungbean–fallow system, showing increases of about 261% in REY, 182% in nutrient use efficiency, and 160% in field water use efficiency. Additionally, this innovative system yielded a net profit of USD 4742 compared to the traditional approach [33]. Thus, the introduction of vegetables and legumes into rice-based cropping systems can enhance the resilience of production systems. These crops provide smallholder farmers with additional income, contribute to dietary diversity for vulnerable communities, and improve soil health.

4. Diversification of Rice-Based Farming Systems with Vegetables in Africa

Rice has become an increasingly important staple food in the African diet, particularly in West Africa, accounting for 37% of cereal food consumption [34]. Some of the major rice-producing countries in Africa include Nigeria (4.51 Mha), Guinea (2.35 Mha), Madagascar (2.07 Mha), the Democratic Republic of the Congo (1.91 Mha), Tanzania (1.20 Mha), Sierra Leone (0.92 Mha), and Mali (0.91 Mha) [1]. However, Africa still heavily relies on rice imports, as domestic production is unable to meet demand. The productivity of rice in Africa (2.31 t/ha) is less than half that of Asia (4.99 t/ha). For example, the average yield in the Democratic Republic of the Congo (DRC), one of Africa’s major rice-producing countries, is only about 951 kg/ha. This yield is significantly lower than the average yield of rainfed lowland or upland rice on the continent [35]. In Africa, rice is predominantly produced by smallholder farmers. Although technological innovations and agricultural initiatives have been promoted to intensify rice production, a substantial gap exists between potential and actual yields in irrigated lowland rice systems [36,37]. Hence, it is important to bridge the yield gaps in rice by addressing the critical constraints across Africa. However, it is equally important to diversify rice-based production systems with high-value commodities such as vegetables, since rice as a sole crop is not profitable, for instance, in Benin and Mali [38].

The diversification of rice-based production systems with vegetable crops contributes to food and nutritional security while also substantially increasing farm income in Africa. It also helps to conserve natural resources and ensure the sustainability of farming systems, particularly in reducing soil and water resource degradation and the biotic stresses associated with monocropping rice. Hence, rice and vegetable cultivation are attractive alternative options for smallholder producers in Africa [38]. According to a recent study, Senegalese farmers reported transitioning towards less rice production during the wet season, using short-duration rice varieties, and increased vegetable production, although rice remained essential to their livelihoods [39]. An earlier study also found that more diverse farming systems can contribute to household food security [40]. In addition, diversification is also imperative in ensuring nutritional security and addressing the problems of ‘hidden hunger’ (malnutrition) [41], which continues to affect an increasing human population in Africa. Hence, significant policy and research funding adjustments are needed to accommodate explicit investments in diversification, as well as varietal improvements in major staples and nutrient-dense vegetable crops that are more climate-resilient [40].

5. Conclusions

In conclusion, the diversification of cereal-based cropping systems, particularly through the inclusion of vegetables and legumes such as mungbeans, presents a viable climate-smart strategy to enhance agricultural sustainability in tropical Asia and Africa. The integration of mungbean not only addresses critical challenges such as soil degradation and nutritional deficiencies but also improves resilience to climate-related stresses. Our findings from research in Benin, where intercropping maize with chili and mungbean led to significantly increased yields and gross returns, underscore the economic potential of such diversification. Specifically, gross returns were USD 7,797 per hectare for maize–chili and USD 1,301 per hectare for maize–mungbean [42]. Furthermore, mungbean intercropping positively impacts soil health by boosting levels of mineralizable carbon and enhancing total nitrogen and potassium concentrations. To effectively promote these agroecological approaches and fulfill nutritional goals in cereal-based agri-food systems, systematic integration of mungbean within sustainable intensification frameworks is essential. This is particularly paramount in Africa, where the adoption of improved mungbean varieties remains low. As only 2% of the mungbean area is planted with improved varieties developed by WorldVeg, compared to 61% in Southeast Asia, there is a pressing need for initiatives to bridge this gap [43]. For instance, initiatives aimed at integrating vegetables and legumes into cereal-based production systems are currently being implemented in countries such as Benin, Tanzania, the Philippines, and Sri Lanka. These efforts involve collaboration between WorldVeg, CGIAR institutes like CIMMYT, IWMI, and IFPRI, and national agricultural research and development institutes, NGOs, and the private sector in each respective country. However, to fully realize the potential benefits of system diversification, it is crucial to adjust policy frameworks and increase investments in agricultural research and development. By doing so, we can enhance food security, create economic opportunities for smallholder farmers, and support the sustainable future of agriculture in these regions.