The concept of marginal environments is subjective and therefore defined from diverse perspectives, including societal, infrastructure, health, education, political, economic, environmental, and development index (Gurung and Kollmair, 2005). In the context of agriculture, we consider marginal environments as lands that are limiting in the provision of optimal conditions required for crop growth and productivity as well as those that result in poor economic returns when used for crop cultivation. A comprehensive definition of marginal environments was provided by Pender and Hazell (2000), who referred to these environments as less-favorable agricultural areas (LFAAs) characterized by constrained agricultural potential and resource degradation attributable to biophysical and politico-socioeconomic factors. The authors indicated that the low production potential of these LFAAs is driven by rugged terrains, extreme weather conditions, poor soil and water quality, lack of socioeconomic connectivity, and limited exposure to agricultural intensification opportunities. They also added that drought and erratic rainfall, salinization, and other factors present significant constraints for intensive agriculture in LFAAs.

According to Dawson et al. (2007), orphan crops are those that have either originated from a geographical location or become “naturalized” due to many decades (usually greater than 10 decades) of cultivation alongside natural selection and/or artificial selection by farmers. The term “orphan” is derived from the state of neglect and abandonment of the crops by the scientific community despite their grossly underexploited food and nutritional potential that can contribute to food and nutrition security, healthy living, improved livelihood of farmers, and improvement of the environment (Tadele and Assefa, 2012; Tadele and Bartels, 2019). Orphan crops are also referred to as underused, lost, indigenous, minor, promising, or future crops (Tadele, 2019). Orphan crops have a history with indigenous people and are generally accepted among the rural populace for their nutritional and health values as well as adaptation to prevailing local stresses and growing conditions (Kamei et al., 2016). With renewed awareness of the potential of orphan crops in terms of being nutrient-dense, amenable to diverse food systems, and tolerant of suboptimal growing conditions, research attention is beginning to shift in the direction of these crops, for which the knowledge of indigenous people would be invaluable.

Although number of orphan crops are native to different continents or regions like Africa, Asia, South America, North America, South Pacific, and Australia, among others (Davies et al., 2005; Foyer et al., 2016; Mueller et al., 2017; Kagawa-Viviani et al., 2018; Dawson et al., 2019; Kehinde et al., 2022), but a plant species may be called orphan for one particular region may not be necessarily orphan crop for others. Several orphan crops listed for Africa include crops that are commonly grown in South Asia and other parts of the world. For example, the African Orphan Crops Consortium (AOCC) listed okra, onion, cashew, custard apple, jack fruit, papaya, watermelon, coconut, pumpkin, finger millet, sweet potato, lentil, mango, bitter gourd, drumstick, mulberry, banana, guava, etc., as part of the 101 most important orphan crops in Africa¹, whereas these crops are widely cultivated in other areas of the world and are not considered as orphan crops in those regions. In the past couple of decades, crop species from different parts of the world have been gaining attention in other regions, that is, outside of their niche. Quinoa (Chenopodium quinoa) is one of the most captivating cases, moving out of South America (Bolivia, Peru, and Chile) and expanding globally. Quinoa varieties have been bred specifically for the European and North American environments (Alandia et al., 2020). Similarly, amaranth, buckwheat, millets, and other crops have gained attention outside of their centers of origin and domestication (Rodríguez et al., 2020).

It is noteworthy that the AOCC selected the 101 priority orphan crops based on three primary criteria: (1) being rich in micro- and macronutrient contents, (2) being relevant to Africa, and (3) having a need for developing breeding resources (Hendre et al., 2019). In a similar manner, the Food and Agriculture Organization of the United Nations Regional Office for Asia and the Pacific (FAO/RAP) identified and designated traditional underused crops as Future Smart Food (FSF) based on four criteria: high nutritional profile, climate resilience, local availability, and economic viability. This was in line with the FAO/RAP Initiative for Zero Hunger (Li and Siddique, 2018). Thus, in this review, we identified 13 orphan crops based on four criteria: (1) resilience to salinity, drought, and/or heat stress, which are prevalent in marginal environments; (2) high nutritional profile; (3) amenability to diverse cropping systems; and (4) local availability for economic growth and social development. These crops were assigned to three categories: cereals and pseudo-cereals, pulses, and oil crops (Table 1).

It suffices to indicate that the intention is not for orphan crops to dominate the diet or compete with the major food crops but to complement production to meet the food requirements of the fast-increasing population as soon as possible. Orphan crops are also outstanding in performance and could thrive better than major food crops in environments where they are indigenous and widely cultivated. Promotion and adoption of orphan crops are expected to achieve direct and indirect impacts on many global challenges.

While maize, rice, and wheat dominate cropping systems; these crops do not perform well in marginal areas. Orphan crops can be grown successfully as sustainable alternatives to satisfy the calorie requirements of communities. The use of orphan crops to occupy niches in production in multiple cropping systems has several advantages. For instance, niche management using alternative orphan crops addresses the food production challenges in locations where climate and other environmental factors can no longer support traditional cereals. Second, selected orphan crops can be used to explore sustainable crop diversification opportunities for multiple cropping, thereby supporting yield stability, building up of disease and pest resistance, higher resource use efficiency, and intensification where conditions allow for this. Local crop intensification and the use of diverse crop cycles have been put forward as a solution to improve food security without increasing the area under cultivation subject to site-specific productivity and actual environmental costs (Waha et al., 2020). As the frequency of climate extremes increases, mixed crop production practices that include wide adaptation traits are important for maintaining food security (Iijima et al., 2016). Therefore, the inclusion of orphan crops can increase both food security and opportunities for designing sustainable crop intensification strategies. Alternative crops in the form of orphan crops score highly in all areas of the four pillars of food security: access, availability, use, and stability. In particular, the majority fit in sustainable production practices and support dietary diversity. Orphan crops could have a different supplemental requirement (fertilizers/minerals) for their growth and development as compared with the staple cereals that largely dominate food systems, but definitely not exhaustive requirements. Soil nutritional status affects the nutritional profile of crop produce (Rekik, 2020). Therefore, it is imperative to work out soil health needs and define specific fertilizer requirements prior to including orphan crops in food systems.

Nutritional requirements significantly affect food systems. Small millets, which are mostly orphan crops, play significant roles in the diets and nutrition of local indigenous people in Africa, Asia, and South America. Compared with traditional cereals, orphan crops such as finger, foxtail, and proso millet, among others, have high potential and value as food sources as they provide gluten-free flour, are higher in dietary fiber and essential amino acids (leucine, isoleucine, and lysine), and have good lipid content (Dias-Martins et al., 2018). For example, finger millet has comparable carbohydrate, protein, and fat content as wheat and rice, but the crop is by far the richest source of calcium (300–350 mg/100 g), as detailed in Table 2A. Finger millet also has comparable and, in some cases, higher essential amino acids, vitamins, and micronutrients than wheat and rice (Table 2B). Other small millets also serve as abundant sources of phosphorus and iron. In a study conducted by Devisetti et al. (2014), the protein content of foxtail and proso millets was found to be high (e.g., 14.8 g/100 g), and measured total and soluble dietary fiber was significantly high. In the brown and whole grains, both small grains had significant phenolic compounds and phytic acid, which can be overcome by processing, while the nitrogen solubility of their flours was good (e.g., up to 16.4 mg/g) in water (Devisetti et al., 2014). Work of Das et al. (2019) has shown that proso millet, for example, provides vitamins (niacin, B-complex vitamins, and folic acid), minerals (P, Ca, Zn, and Fe), and essential amino acids (methionine and cysteine). Proso millet also has comparable contributions to protein, energy, and carbohydrates.

Orphan crops support sustainable production practices of diverse cropping systems, have high feed value in livestock and fisheries, and contribute to soil health. They also contribute to net-carbon sequestration (Toensmeier et al., 2020), a goal important for climate adaptation and mitigation. These system attributes are fundamental in pursuit of good human health as supported by agriculture and related ecosystem services. The nutritional status of the crops can be considered as an overlap between agriculture and health. In this process, system performance and the ability to provide nutritious food become a “central dogma.” It is also evident that evolution of a few dominant crops largely contributes toward a risk of malnutrition and dietary diseases. Here, we give some examples of direct health benefits from agricultural products beyond nutrition. For instance, the low glycemic index of their flour products has been found to decrease the risk of type-2 diabetes mellitus and cardiovascular disease in human adults (Das et al., 2019). Foxtail millet products have been found to contribute to human health through low glycemic index and hypolipidemic and antioxidant attributes (Sharma and Niranjan, 2018). Several diseases in humans (i.e., cancer, cardiovascular disease, dental disease, diabetes, obesity, and osteoporosis) are on the increase in developed and developing countries and diets supported by orphan crops are an extremely important consideration for diminishing these diseases. The prominence of diverse foods that include orphan crops has potential to decrease the impact of epidemics from diet-driven non-communicable diseases (Watkins and Daignault, 2020; Åhlberg, 2021; Carazo et al., 2021).

Indigenous communities living in remote areas often grow orphan crops for their food and livelihood development (Kuhnlein et al., 2013). The proliferation of orphan crops in remote areas is driven by low adoption of modern farming methods because of limited access to agricultural services and a disconnect from input and output markets. For these indigenous communities, orphan crops are central to religious beliefs, rituals, and customary practices. These crops have also been used to develop recipes for dishes served on special occasions, in the cure of disease bouts, and as currency for the barter system of exchange for other commodities. Together with wild fruits and vegetables collected from forests, orphan crops constitute a source of the entire livelihood system for poor rural people living in remote areas.

With higher incomes, a spin-off from improved literacy and rural-urban migration, most of the youth do not consume traditional dishes (Baada et al., 2019; Marchetti et al., 2020). In urban centers, consumer tastes are shaped by increased access to meat products and other dairy products at the expense of products from orphan crops (Akinola et al., 2020; Lillford and Hermansson, 2021). Large well-developed commodity value chains dictate consumption of rice, maize, and wheat products at the expense of orphan crops in urban centers (Reardon, 2015). Generational differences have not supported the use of orphan crops on a large scale. Orphan crops have maintained the diversity of food chains although they have limited taste preferences.

Some of the challenges could be as follows:

Global agricultural systems are at a critical juncture. Agricultural policies, market incentives, trade forces, agricultural subsidies, and years of intensive research targeting agricultural productivity growth have skewed crop production portfolios toward a handful of high-yielding crops that can be grown to scale on the world’s mainstream agricultural lands (Tadele, 2019; Borelli et al., 2020). Policy support has been mainly directed at promoting the production and use of conventional crop species (e.g., wheat, rice, and maize) that dominate global food markets and underpin global diets that are becoming increasingly homogeneous (Borelli et al., 2020). Meanwhile, increasing evidence has come to light suggesting that the narrowing of diversity in crop species poses a significant threat to global food security and environmental sustainability (Khoury et al., 2014). From an environmental standpoint, these specialized modern production systems dominated by conventional crops require high-tech and high-input production systems, leading to the overexploitation of the natural resource base and ecosystems (Willett et al., 2019; Borelli et al., 2020). Given the high-level adaptation of orphan crop species to heterogeneous and harsh agro-ecological conditions, promoting orphan crops on a large scale could encourage the use of regenerative and ecological processes instead of relying entirely on intensification of conventional crops requiring a significant number of external inputs (i.e., chemicals and fertilizers) that are harmful to the environment. From the poverty elimination perspective, orphan crops are seen as “poor peoples’ crops,” which have the potential to not only enhance the food and nutrition security of the rural poor but also offer income-generating opportunities (Naluwairo, 2011). Investment in orphan crops could improve the livelihoods of socially marginal and vulnerable groups, including indigenous people and women (Egeland and Harrison, 2013; Azam-Ali et al., 2021).

In spite of the increased political will throughout recent years to promote investment in orphan crops, numerous studies generate evidence pointing to the lack of support for them, all of which recognize the need for change in the nature and scope of the policy environment (Mabhaudhi et al., 2017; Tadele, 2019; Tadele and Bartels, 2019; Kamenya et al., 2021). Previous literature on neglected crop species points to several impediments and challenges that limit the integration of orphan crops into commercial food systems (Fanzo et al., 2013). Inappropriate agriculture and food security policies that favor large cereal crops often diminish the dietary importance of orphan crops. There is still a lack of policies to support and recognize the nutritional value of neglected crops, which makes it difficult for locally produced traditional products to enter international marketplaces. There is very little research tying orphan crops to various diets and enhanced nutrition or comparing cost-effective strategies of using species for nutritional outcomes. Most of the essential data are dispersed and inaccessible through a single platform, which frequently goes unnoticed by policymakers. More crucial is the lack of infrastructure and markets nationally and regionally to channel neglected crop products to major food markets (Hunter et al., 2019; Kumar and Bhalothia, 2020).

The State of the World’s Biodiversity for Food and Agriculture highlighted a rapid decline and global disappearance of many local varieties of domesticated plants, many of which are neglected in local production systems and are mainly maintained by custodian farmers exclusively for subsistence and informal trade (FAO, 2019; Borelli et al., 2020). These plants nevertheless have a tremendous potential to redress key challenges in sustainable development, such as the vulnerability of production systems to climate change, disempowerment of vulnerable groups (women and indigenous peoples), widespread poverty, shrinking food biodiversity, and pervasive malnutrition (Egeland and Harrison, 2013; Azam-Ali et al., 2021). With the effects of climate change on the horizon posing critical challenges to agricultural systems, orphan crops have recently attracted increasing attention at the policy level because of their remarkable resilience to biotic and abiotic stresses and importance for rural food security, especially in marginal areas where production is constrained by biotic and socioeconomic constraints (Mabhaudhi et al., 2017).

In pursuit of achieving the Sustainable Development Goals (SDGs) related to the food security-poverty-environment nexus, the United Nations General Assembly (UNGA) has adopted several conventions to address the problems faced by global food production systems, including the lack of policy support for orphan and underused crops. The UN community has widely advocated investing in conserving these crop species and making greater use of the abundance and diversity of orphan crops in promoting sustainable diets (United Nations, 2019a,b). Subsequently, the UNGA conventions recognized several priority actions to transform production systems, especially to make underused and neglected crop species available to producers and consumers. These actions include directing increased research investments to explore their climate resilience and nutritional characteristics; providing greater support for their mainstreaming in food security policies and programs (e.g., public procurement); encouraging their use to diversify farming systems and create more biodiverse landscapes and healthier ecosystems; and upgrading their value chains and markets to ensure their sustainable use (FAO, 2019; United Nations, 2019a,b).

Similarly, the Second Global Plan of Action for Plant Genetic Resources for Food and Agriculture and the International Treaty on Plant Genetic Resources for Food and Agriculture lay out a series of agreed priority plans and actions that can protect the rich portfolio of diverse genetic resources, especially promoting the conservation and sustainable use of neglected and underused species (FAO, 2012). The Second Global Plan of Action aims to promote cost-efficient and effective global efforts to conserve and sustainably use orphan and neglected crops, link conservation with use for a greater use of plant germplasm, reinforce crop improvement and seed systems to foster economic development, create capacity, and strengthen national programs and widen partnerships for orphan crop management. The Second Global Plan of Action is a strategic framework that comprises 18 priority activities organized into four areas: (1) in situ conservation and management, (2) ex situ conservation, (3) sustainable use, and (4) sustainable institutional and human capacity. The agreed priority actions are instrumental in reorienting and prioritizing research and development (R&D) agendas.

Several national and international research and policy institutions have dedicated efforts to investigate and identify best management practices to improve orphan crops. Among others, these include CGIAR, the African Orphan Crops Consortium (AOCC), Crop Diversity Trust, Svalbard Global Seed Vault, Crops for the Future (CFF), and Tef Improvement Project. In addition, several funding agencies provide financial grants and aid for R&D of orphan crops, including the Bill & Melinda Gates Foundation (BMGF), McKnight Foundation, and Biotechnology and Biological Sciences Research Council (BBSRC) (Tadele, 2019). CGIAR plays a vital role among the international institutions involved in germplasm collection and conservation, but it focuses more on the major staple crops. Only a few of the 15 CGIAR centers are mandated to do R&D for orphan crops. The AOCC is an international effort to improve the nutrition, productivity, and climatic adaptability of some of Africa’s most important food crops; its work involves sequencing, assembling, and annotating the genomes of 100 traditional and underused African food crops, which will enable higher nutritional content for society over the decades to come (Jamnadass et al., 2020).

To put neglected crops back on the national agenda and for orphan crops to see a resurgence in local production systems and global food demand, strategic actions are required at different levels to mainstream orphan crops. These actions should target combining intensification with sustainable solutions (Tadele, 2017). A major part of the recent literature exploring producers’ and consumers’ behavior and preferences advocates for a three-pronged approach to establish enabling environments for mainstreaming neglected crops for food and nutrition in local food systems (Hunter et al., 2019; Borelli et al., 2020; McMullin et al., 2021). This three-pronged approach (Figure 1) revolves around three avenues of R&D activities: (1) generating increased evidence about the nutritional value and biocultural importance of neglected crops, (2) linking research to policy and strategic plans to ensure that underused crop species are considered in national food and nutrition security strategies and actions, and (3) improving the knowledge base to increase consumer awareness about the nutritional desirability of underused crops to incorporate these crops into their food systems, markets, and diets.

The three-pronged strategy to investigate orphan crop mainstreaming solutions reveals innovative research pathways for examining producer and consumer behavior and preferences. Further research is essential to broaden our understanding of the incentives and factors that influence diverse groups of farmers’ productive activities (Gassner et al., 2019; Borelli et al., 2020; McMullin et al., 2021). It is necessary to take into account a considerably broader range of additional aspects, including long-term aspirations of the farming communities (Mausch et al., 2018). In order for orphan crop foods to compete more effectively with other food crops in more complex market-based food systems, a deeper knowledge of consumers’ preferences and behavior in relation to orphan food crops is required (Revoredo-Giha et al., 2018). These must be considered in relation to the consumers’ age, education, income, location, and media access. Because of the importance of orphan crops in traditional food systems, it is also necessary to analyze and understand consumers’ cultural backgrounds (McMullin et al., 2021).

Future strategies should highlight and promote the design and execution of a variety of interventions aimed at enhancing and improving value chains for neglected crops. The development of quality assurance measures to indicate enhanced nutritional value, demand creation through marketing campaigns and food product development, supply chain development to provide planting materials to farmers, and stimulating private sector involvement to create “shared values” are some examples (Waized et al., 2015). Orphan or indigenous crops are associated with some desirable properties that can be useful in the elevation of the crops to a higher rate of production (Tadele, 2019). However, low yield remains a challenge for producers and policymakers to take orphan crops to the next level. The application of modern genetic and genomic tools to the breeding of these crops can provide enormous opportunities for ensuring world food security (Tadele, 2018). Technological developments in terms of breeding and varietal development should be tailored to the context of harsh and fragile environments where orphan crops can strive. It is also vital to strengthen the seed system for orphan crops to maintain local production systems. Public-private partnerships in the multiplication and distribution of better seed to marginal and isolated agricultural production locations should be encouraged through breeding programs.

With the impending effects of climate change, abiotic stresses such as drought, salinity, and heat, as well as climate change, have a significant impact on crop yield and food security. Future research should focus on creating resistance to or tolerance of these environmental stresses. Several orphan crops are nutrient-dense and their ability to adapt to harsh conditions suggests that they can be deployed as part of efforts to champion climate-change adaptation, improved agriculture, and economic advancement for smallholder farmers residing in low-production environments. However, some of the crops’ negative characteristics (for example, low production) must be improved (Tadele, 2019). Future policies must integrate intensification with sustainable solutions (Tadele, 2017). Future agricultural policies targeting orphan crops should also encourage agro-ecological practices by encouraging the use of orphan crops in operations with the goal of decreasing external inputs and maximizing plant hardiness in vulnerable, drought-prone areas where underused crops can exploit residual soil moisture and scarce rainfall.

Building social capital can help farming communities and grower associations strengthen their governance and technical capacity, particularly for indigenous farmers with limited access to information. Strengthening the capacity of farming communities including women, youth, and indigenous people is fundamental for boosting production efficiency, along with improved post-production, technologies, business and entrepreneurship skills, markets and market information, and sustainable investments in physical infrastructure. This could be pursued as part of a larger public-private partnership initiative and investment goal in agricultural marketing of orphan crops.

Local and indigenous knowledge should be valued as a vital instrument in future policies for agriculture and food security and should be carefully considered and documented, when possible, with the cooperation of indigenous and local populations. Indigenous farmers who typically grow traditional and neglected crops are losing valuable expertise about these crops; thus, traditional knowledge linked with orphan crops is in grave danger of eroding and disappearing. Traditional knowledge on orphan crops is critical not only for the rural farmers who produce them but also for scientific study on crop improvement in general. Future policies should encourage investing in the preservation and protection of traditional knowledge.