Millets: Types, Benefits, and History of Domestication

Millets: Types, different species, their history and domestication

In recent years, millets have emerged as a beacon of sustainable agriculture, offering a plethora of benefits to farmers, the planet, and consumers (human health). These ancient grains, once sidelined (forgotten) by modern agriculture, are now being recognized globally for their nutritional richness, adaptability to diverse climates, and minimal environmental impact.

The main types of millets include:

• Pearl millet (Cenchrus americanus, or Pennisetum glaucum)
• Sorghum (Sorghum bicolor)
• Finger millet (Eleusine coracana)

Additionally, there are several lesser-known varieties known as minor millets:

• kodo millet (Paspalum scorbiculatum)
• arnyard millet (Echinochloa crusgalli)
• proso millet (Panicum miliaceum)
• little millet (Panicum sumatrense)
• browntop millet (Brachiaria ramosa)

These small millets are often referred to as "wonder cereals" and are also known as "Nutri-Cereals" (or Shree anna in India) due to their exceptional nutritional and health benefits (Barretto et al., 2021).

Types of Millets

Millets encompass a diverse group of small-seeded grasses cultivated worldwide, especially in semi-arid regions (with rainfall of 200-700 mm), primarily for human consumption and animal fodder. The most common types include:

• Pearl Millet (Pennisetum glaucum (L.) R. Br., [syn. Cenchrus americanus (L.) Morrone]):

Pearl millet stands as a vital staple crop in both sub-Saharan Africa and tropical India, playing a crucial role in food security (Stevens and Fuller 2018). The wild precursor of cultivated pearl millet is identified as Pennisetum violaceum (Lam.) Rich., also known as P. americanum subsp. monodii (Maire) Brunken (D'Andrea and Casey 2002). This species naturally occurs within the Sahelian zone, spanning from Senegal to northern Sudan, with indications suggesting its domestication likely took place in the western part of this range, specifically between Niger and Mauritania (Upadyaya et al., 2017; Dupuy, 2014; Fuller & Hildebrand, 2013).

Pearl millet thrives in semi-arid regions and is renowned for its drought resistance. It is valued not only for its ability to grow in challenging environments but also for its rich nutritional composition. This crop is notably high in protein, fiber, and essential minerals such as iron and magnesium, making it a significant contributor to balanced diets in regions where it is cultivated.

• Sorghum (Sorghum bicolor):

The genus Sorghum, part of the Poaceae (Gramineae) family, falls under the subfamily Panicoideae, tribe Andropogoneae, and subtribe Sorghinae (Clayton & Renvoize, 1986). This tribe includes significant crops like sugarcane (Saccharum spp.) and maize (Zea mays). Sorghum's diversity complicates the classification of its domesticated and wild varieties (Wiersema & Dahlberg, 2007). It consists of 25 species grouped into 5 subgenera based on morphology: Chaetosorghum, Heterosorghum, Parasorghum, Stiposorghum, and Eusorghum, with cultivated sorghum belonging to Eusorghum (Celarier, 1958; Price et al., 2005a; USDA ARS, 2015).

Sorghum's complexity is evident in its chromosome numbers. Parasorghum and Stiposorghum have the lowest haploid chromosome number of five, with most polyploid species being autopolyploids (2n = 10, 20, 30, 40). Eusorghum species have a minimum haploid number of ten and are allopolyploids (2n = 20, 40). Chaetosorghumand Heterosorghum are 2n = 40 allopolyploids (Celarier, 1958). The taxonomy of Sorghum remains debated. Morphological classifications of the five subgenera do not entirely align with molecular phylogenetic analyses. One study suggested dividing Sorghum into three genera (Spangler, 2003), while another supported maintaining it as a single genus, proposing two main lineages within the 25 Sorghum species: one containing Eusorghum, Heterosorghum, and Chaetosorghum, and another comprising Parasorghum and Stiposorghum (Dillon et al., 2007a).

Subgenus Eusorghum

Eusorghum (or Sorghum/Eu-sorghum) includes all cultivated sorghum races and their close wild relatives. Species in this subgenus are inter-fertile, allowing gene flow between cultivated sorghum and wild relatives (de Wet, 1978). Eusorghum contains three species: S. halepense (Johnson grass, a significant weed), S. propinquum, and S. bicolor (Ejeta & Grenier, 2005). S. halepense and S. propinquum are rhizomatous perennials, while S. bicolor, usually cultivated as an annual plant, is a short-lived perennial without rhizomes.

Sorghum bicolor is divided into 3 subspecies (Wiersema & Dahlberg, 2007):

1. S. bicolor subsp. bicolor - includes all cultivated sorghum.
2. S. bicolor subsp. arundinaceum - consists of wild and weedy annuals or weak biennials, found mainly in Africa but also in tropical Australia, India, and the Americas.
3. S. bicolor subsp. drummondii - includes annual weedy derivatives from hybridization between domesticated sorghum and subspecies arundinaceum, such as forage sudangrass and weedy shattercanes (de Wet, 1978; Dahlberg, 2000).

S. bicolorsubsp. bicolor (referred to as cultivated sorghum) consists of five basic races and ten intermediate races, identifiable by spikelet/panicle morphology, reflecting their original environments and the nomadic peoples who first cultivated them (Harlan & de Wet, 1972; Kimber, 2000).

Other Sorghum Subgenera in Australia: Australian Sorghum species are primarily found in the monsoonal region of the Northern Territory. They are key components of the understory in grassland, woodland, and forest communities. This region is a diversity center for Australian Sorghum species across four subgenera: Chaetosorghum, Heterosorghum, Parasorghum, and Stiposorghum (Lazarides et al., 1991). Of the 25 Sorghum species, 17 are native to Australia and Southeast Asia, with 14 endemic to Australia.

• Finger Millet (Eleusine coracana):

Finger millet was domesticated in western Uganda and the Ethiopian highlands at least 5000 years ago before being introduced to India about 3000 years ago (Dida et al., 2008). Its name comes from the inflorescence, which resembles human fingers. The morphology of the inflorescence helps differentiate between its two subspecies, africana and coracana, each of which can be further divided into several races (Dida & Devos, 2006). Finger millet is an allotetraploid, with Eleusine indica and Eleusine tristachya likely being the genomic donors of the "A" genome. However, the "B" genome remains unidentified and may have come from an extinct ancestor (Liu et al., 2014). Finger millet offers notable health benefits, including anti-cancer and anti-diabetic properties, due to its polyphenol content and high fiber, respectively (Chandrasekara & Shahidi, 2011; Devi et al., 2014). Under optimal conditions, it can produce up to 5 tons/ha and requires minimal nitrogen fertilization, with the most economical application rate being 20 to 60 kg/ha (Dida & Devos, 2006; Hegde and Gowda, 1986; Pradhan et al., 2011). The plant is highly tolerant to drought and salt stress, though resistance varies across genotypes (Bhatt et al., 2011). Unlike many crops consumed by subsistence farmers, finger millet remains socio-economically important in the semi-arid tropics of India and Africa and has been well-studied compared to its relatives (Gull et al., 2014).

• Foxtail Millet (Setaria italica):

Foxtail millet, named for the bushy, tail-like appearance of its immature panicles, has garnered significant research interest. Domesticated in China around 8700 years ago, it is one of the world's oldest crops and ranks second in global millet production, providing six million tons of grain, particularly in southern Europe and Asia (Li & Wu, 1996; Yang et al., 2012). It is a staple in northern China’s dry regions (Wang et al., 2012). It is also cultivated in North America for silage, bird seed, and as a cover crop, maturing in 75-90 days (Baltensperger, 2002).

Unlike finger millet, which had a single domestication event (Dida et al., 2008), foxtail millet's domestication history is complex. A study of 250 Chinese genotypes found high sequence diversity, suggesting two possible domestication events in China (Wang et al., 2012). There is also evidence suggesting independent domestication in Europe (Hirano et al., 2011). Foxtail millet is closely related to the weed Setaria viridis, or green foxtail, which often grows near cultivated millet and exhibits herbicide resistance (Heap, 1997). Genetic studies imply frequent hybridization between S. viridis and modern S. italica (Jusuf & Pernes, 1985), though hybrids often retain undesirable weedy traits and fertility losses (Wang & Darmency, 1997).

Modern foxtail millet diversity includes two phenotypically different varieties: waxy and non-waxy grain types. Waxy grains have lower amylose levels, resulting in a sticky texture when cooked, preferred in East and Southeast Asia, where sticky cereals are favored due to chopstick use (Van et al., 2008). The non-waxy phenotype is more widespread across Eurasia and parts of Africa (Kawase et al., 2005).

• Kodo Millet (Paspalum scrobiculatum):

Kodo millet, domesticated approximately 3000 years ago in India, remains primarily cultivated as a grain in the Deccan plateau region, making India the only country where it is harvested in significant quantities today (de Wet et al., 1983b). This grain boasts a diverse range of high-quality proteins (Kulkarni & Naik, 2000) and exhibits notable antioxidant activity, with potential anti-cancer benefits, even when compared to other millets (Chandrasekara & Shahidi, 2011b). Like finger millet, kodo millet is rich in fiber, making it beneficial for diabetics (Geervani & Eggum, 1989). Its drought tolerance allows it to thrive in various poor soil types, from gravel to clay.

Based on panicle morphology, the Kodo millet is classified into three races—regularis, irregularis, and variabilis (de Wet et al., 1983b). In southern India, there are recognized small-seeded (karu varagu) and large-seeded (peru varagu) varieties, often cultivated together in the same fields (de Wet et al., 1983b). Despite its ancient domestication, kodo millet is sometimes referred to as "incompletely domesticated," with some experts describing it as "pseudo-cultivated" (de Wet, 1992; Blench, 1997). Consequently, systematic breeding of kodo millet has been largely neglected, although limited efforts have shown promise.

• Proso Millet (Panicum miliaceum):

Proso millet, also known as broomcorn and common millet, was domesticated in Neolithic China around 10,000 years ago  (Lu et al., 2009). Adapted to dry, sandy soils, it may be the earliest dryland-farming crop in East Asia (Baltensperger, 2002; Lu et al., 2009). Proso millet is notable for its low water requirement, needing only 330–350 mm of annual rainfall to produce harvestable grain, which might be the lowest among cereals (Hunt et al., 2011). It matures quickly within 60–90 days, a trait that enhances its drought resistance and makes it an excellent catch crop (Hunt et al., 2014).

Cultivated proso millet is classified into 5 races (Reddy et al., 2007). The race miliaceum resembles wild proso with large, open inflorescences and sub-erect branches with few subdivisions. Patentissimum is very similar to miliaceum, with narrow, diffuse panicle branches. These two races are widespread across the Eurasian range of proso millet and are considered primitive. The other three races—contractum, compactum, and ovatum—have more compact inflorescences that are drooped, cylindrical, and curved, respectively (Reddy et al., 2007).

• Barnyard Millet (Echinochloa spp.):

Barnyard millet comprises two species within the genus Echinochloa:

1. Echinochloa esculenta (syn. Echinochloa utilis, Echinochloa crusgalli)
2. Echinochloa frumentacea (syn. Echinochloa colona)

E. esculentais cultivated in Japan, Korea, and northeastern China, while E. frumentaceais found in Pakistan, India, Nepal, and central Africa (Yabuno, 1987; Wanous, 1990). These species have overlapping morphological traits, making differentiation challenging. They can be visually identified by the presence or absence of an awn and subtle differences in spikelet and glume morphology (de Wet et al., 1983c). To simplify research, the names Japanese and Indian barnyard millet have been proposed (Yabuno, 1987).

Japanese Barnyard Millet (Echinochloa esculenta):

Originating in eastern Asia from its wild relative E. crusgalli (Yabuno, 1987; Hilu, 1994), it is distinguished by larger, awned spikelets with papery glumes (de Wet et al., 1983c). Tolerant to cold, it was traditionally grown in northern Japan, where rice cultivation was not viable (Yabuno, 1987). This species shows high morphological and physiological diversity, with variations in flowering time, inflorescence shape, and spikelet pigmentation (Nozawa et al., 2006). It is classified into the races utilis and intermedia (Upadhyaya et al., 2014).

Indian Barnyard Millet (Echinochloa frumentacea):

Domesticated in India from its wild relative E. colona(Yabuno, 1987; Hilu, 1994), it is either harvested as a weed or grown with finger millet and foxtail millet (Gupta et al., 2009b). It is primarily cultivated on hilly slopes in tribal regions and is crucial in the northwest Himalayan area (Gupta et al., 2009b). Quick maturity makes it well-suited to regions with low rainfall (Channappagoudar et al., 2008). Indian barnyard millet shows significant phenotypic variation, classified into four races (laxa, robusta, intermedia, and stolonifera) based on characteristics such as flag leaf length, peduncle length, inflorescence, raceme, plant height, and basal tiller number (de Wet et al., 1983).

• Little Millet (Panicum sumatrense):

Found across India, little millet is a highly nutritious grain, rich in B vitamins, essential minerals, and antioxidants. Its ease of digestion and low glycemic index make it an excellent dietary choice for diabetic individuals.

Commonly known as Sama, little millet is cultivated in various regions, including India, Sri Lanka, Pakistan, Myanmar, and other Southeast Asian countries (Hiremath et al., 1990). In India, it holds significant importance for the tribes of the Eastern Ghats and is typically grown alongside other millets (Hiremath et al., 1990). This domesticated form of the wild species Panicum psilopodium is distinguished by its adaptability and nutritional value (de Wet et al., 1983a).

Little millet is classified into two races based on the morphology of its panicles:

1. nana and
2. robusta

Race nana is characterized by its faster maturation and lower biomass production compared to race robusta(de Wet et al., 1983a).

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