Trends in varietal diversity of main staple crops in Asia and Africa and implications for sustainable food systems

 Marcel Gatto, Stef de Haan, Alice Laborte, Merideth Bonierbale, Ricardo Labarta, and Guy Hareau   |  

Agricultural diversity can be beneficial for environmental health, resilience, and food production. Biodiverse cropping systems can suppress pests more effectively than single species or varietal stands because they harbor populations of natural enemies that protect. In addition, genetic diversity within crops can suppress diseases by blocking the ability of pathogens to freely transmit diseases and evolve new pathotypes.

Agricultural diversity can be beneficial for environmental health, resilience, and food production. In this context, crop species diversity and intraspecific diversity are often identified as resulting in both nutrition security and ecological resilience.

Biodiverse cropping systems can suppress pests more effectively than single species or varietal stands because they harbor populations of natural enemies that protect. In addition, genetic diversity within crops can suppress diseases by blocking the ability of pathogens to freely transmit diseases and evolve new pathotypes.

In this context, cropping systems with mixed varietal arrangements are found to be more resilient, particularly under biotic stresses, and have been shown under favorable conditions to produce 2.2% higher yields than monocultures. Positive effects of diversity have also been reported for coping with abiotic stressors.

Climate change will drive extreme weather events and the range expansion of infectious plant diseases and pests, which suggests that managing genetic diversity within crop species and keeping crop varietal diversity a part of the agricultural landscape can be an increasingly important focus to enhance food system resilience, i.e., the capacity to respond and recover from shocks.

Reduced levels of varietal diversity within and among fields result in increased vulnerability to biotic and abiotic stressors, posing a threat to food security. Examples of the devastating effects of low crop varietal diversity over the past century have been extensively reviewed.

Diseases such as a new race of stem rust of wheat are spreading rapidly on a global scale. In developing countries, crop losses to diseases may have devastating effects on an already alarming food security situation. For example, wheat blast reached Bangladesh for the first time in 2016, causing major production losses.

The fall armyworm has damaged maize crops especially in Africa and is now wreaking havoc in Asia. The developed world is not spared as seen from a recent outbreak of stem rust reported in Italy. On a global scale, approximately 10–16%of annual yields are lost to plant diseases, and current trends project diseases will continue to spread.

Meanwhile, higher levels of climate variability will increasingly impact crop performance and food security. A few so-called mega varieties of major crops have come to dominate agricultural landscapes globally. Mega varieties are generally widely adaptable, exhibit preferred quality characteristics, and are traded in established marketing channels. Structural evidence of the degree to which mega varieties of major food crops dominate national agricultural landscapes, especially in developing countries, is lacking.

A study reported that only four varieties are planted on 65% of the world’s total rice area and six varieties on 71% of maize area. Another strand of literature focuses on the interplay between the displacement of landraces and the resulting genetic erosion. Along with their implications for increased food security, factors such as seed delivery systems, crop improvement practices, industry preferences, consumer behavior, ecological diversity, and capacity of extension services may all influence varietal diversity.

The importance of varietal and seed turnover has been analyzed in-depth in the context of Africa, including policy options for smallholders. However, data are scarce as only a few analyses have been conducted in a handful of countries, and these at different aggregate levels. Major gaps in crop varietal release registries and unavailability of varietal adoption estimates have prevented a structural overview of the varietal diversity in current cropping systems.

In addition, the strong emphasis on landraces and their importance for resilience has not been met by parallel literature on how improved varieties are shaping diversity portfolios and resilience.

A lack of robust baseline data on spatial and temporal diversity is the key factor underlying this gap. The objective of this study is to explore and unravel the relationship between modern crop improvement and varietal diversity of major staple food crops: cereals, pulses, and root and tuber crops for Africa and Asia.

The results of this study show an increasing reduction of crop varietal diversity linked to the spatial displacement of traditional landraces. This trend occurs at a faster rate in Asia than in Africa. In addition, mega varieties tend to increasingly dominate agricultural landscapes, adding to spatial homogeneity. The results further revealed a negative association between varietal richness and its relative abundance, which challenges the relationship between crop improvement and varietal diversity.

In different words, a higher number of adopted varieties are positively associated with the incidence of mega varieties. Whereas crop improvement will remain of critical importance to equip smallholder farmers with new technologies to improve incomes and nutritional status, questions addressing how mega varieties emerge, remain “mega,” and can be replaced are important avenues of future research.

We acknowledge that there are several limitations. Despite the streamlined methodology of the databases used, three common biases exist. First, the importance of crops differs across countries resulting in a crop-importance bias. Experts’ knowledge of varietal release and adoption estimates is likely greater and more accurate for more dominant crops. This bias infrequently led to difficulties identifying experts and thus low expert workshop attendance.

Second, the number of dominating varieties also largely varies across our sampled countries resulting in a crop-dominance bias. Estimating varietal adoption of more evenly adopted varieties resulted in much less bias compared with estimations of unevenly (i.e., many varieties adopted on small areas) adopted varieties.

Third, country size and number of agro-ecologies differ largely across our sample, which makes it more difficult to estimate adoption rates. This country-size bias was generally minimized by inviting more experts and by organizing several expert elicitation workshops in very large countries, such as China or India.

In addition to challenges associated with data collection, our article is an application of the use of big data, which has its own challenges. To smoothen any future analyses of release and adoption data, the use of survey instruments that are standardized in terms of format, units, language, etc., is recommended.

We acknowledge that data constraints limit the analysis for comparisons of crop–country combinations and varietal diversity among fields and that the quadrants are based on the internal distribution of our data rather than external reference points.

Whereas solving the latter limitation will be challenging, in future applications of the crop diversity framework, the spatial diversity could be based on comparisons of intrafield genetic diversity and using lower aggregates. In addition to this avenue, future research may investigate total varietal diversity (and link to resilience) by including landraces in the analysis. Especially for Africa where more than 50% of the agricultural area is planted to landraces, this may result in interesting findings.

More research is needed to better understand threshold levels and dimensions of cropping systems that support resilience, and more insights are needed into the resilience of agricultural systems that are composed of various cropping systems that each has its own spatial and temporal diversity levels. Combined analysis of data on pest and disease levels and dynamics and our spatial diversity data would also be an interesting avenue for future research, enabling a test of the relationship between crop diversity and agricultural resilience that our study assumed.

In addition, future research is warranted to collect better data on the varietal category “other varieties.” The objective of the three datasets we accessed was to collect data for the most dominant varieties. Those varieties not considered and captured in the “other varieties” category may thus affect the diversity indices. To get a more accurate picture of varietal diversity, an exhaustive varietal list/database is required, which will remain a challenge for future research.

A further limitation of our study is that national-level analyses may not reflect varietal diversity at lower aggregate levels. However, as a tradeoff, the national-level lens is relevant because pests and diseases may spread effectively and at a high pace with increasing levels of seed system connectivity, better transportation infrastructure, and climate change.

An increasing body of literature has emerged using varietal adoption estimates based on expert opinions, which acknowledge the reliability of perceived adoption in relation to household survey estimates.

While expert survey provides only indirect estimates, it is important to note that household surveys may also overestimate or underestimate true adoption rates. Methods such as DNA fingerprinting will allow for high precision and accuracy in determining genetic identity and unique varieties but require significant additional resources.

Nevertheless, surveys based on expert opinion of the extent to which varieties are grown may fail to correlate with actual diversity because different varieties may be given the same name; the same variety may be given different names, or as mentioned previously, varieties may be very similar to each other. Additional efforts are needed to precisely measure genetic diversity within and among fields dedicated to crop production by, for instance, using DNA fingerprinting methods.

Finally, we acknowledge that providing a clear definition of a mega variety is challenging, in particular, because of its context specificity. Defining and quantifying mega varieties deserve more attention in future research. There is no single solution for enhancing crop diversity. We need to acknowledge that what works for one crop and context, or for the same crop in a different situation, may not work as well for another. The crops analyzed in this study are of different nature (i.e., grains, vegetatively propagated) and of different use (e.g., food crop, cash crop) and thrive in different agroecological conditions.

Rather than focusing on single solutions, crop diversity will likely only be successfully increased by identifying a range of innovations and incentives suitable for different sectors and crops, at different aggregation levels, and under varying agroecological conditions. In the end, food system sustainability depends on enhancing and maintaining species and varietal diversity.

Read the full study:
Gatto M, de Haan S, Laborte A, Bonierbale M, Labarta R, Hareau G (2021) Trends in varietal diversity of main staple crops in Asia and Africa and implications for sustainable food systems. Front. Sustain. Food Syst. 5:626714.

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