Week 6 (Unit 12)
Sorghum (Sorghum bicolor L. Moench) & Millets
Sorghum bicolor is divided into three subspecies:
There are four races of the subspecies arundinaceum, all of which can cross with cultivated sorghum (race aethiopicum, arundinaceum, verticilliflorum, and virgatum).
Domestication of sorghum
Sorghum was domesticated around 3,000 BC, in the savanna zone of Africa, south of the Sahara and north of the equator. From there it spread through Arabia around 1000 to 800 BC, through India (1st century AD) and China (3rd century, AD). It was introduced to the US with the slave trade.
Ethiopia can be considered to be one center of diversity for sorghum, but it is not the sole center of origin or diversity.
Several wild grasses belonging to the subspecies S. bicolor subsp. arundinaceum have been proposed by various research workers as possible progenitors of cultivated sorghum - these include the races verticilliflorum, aethiopicum, and arundinaceum.
Cultivation in the US became important as settlers moved to the western, drier states of Oklahoma and Texas. Acreage increased again during the 1950's with the introduction of commercial hybrids. Nearly all of the sorghum in the US is the dwarf type that can be readily harvested with a combine. It is used almost exclusively for animal feed, either as grain or as forage. In the developing world, nearly all of the crop is used for human food.
Sorghum is largely self-pollinating (~6% outcrossing). Some fodder sorghums and members of the guinea race may have up to 30% outcrossing. Hybrids are produced using a cytoplasmic male sterility system that prevents selfing.
The grain is partially covered by glumes, but the lemma and palea are removed during combining. The seeds are round, from 4 to 8 mm in diameter.
Sorghum is a C4 species with wide adaptation, from temperate to tropical climates, under rainfed and irrigated conditions. It has a dense, deep root system and can tolerate high temperatures, low rainfall, and low soil fertility. Sorghum is a short-day species, but many cultivars are daylength insensitive.
Sorghum has a competitive advantage over maize on marginal lands in dry, hot areas. However, it is best adapted to slightly cooler, dry climates. Optimal growth occurs at 30 °C. In very hot, dry areas, pearl millet is a better alternative. About 17 to 25 inches of rainfall per year (450 to 1000 mm) are required to grow a sorghum crop.
Lack of cold tolerance limits adaptation and production of sorghum. Soil temperatures of 60-65 °F are required for good emergence. It is adapted to a wide range of soil types, from waterlogged to sandy soils, and low to high fertility.
Yield potential is similar to maize and wheat under many conditions, but somewhat less than maize and wheat in better environments (3-4 t/ha). When moisture is limiting, yields of 0.3 to 1 t/ha are typical. Highest yields reported are about 11 t/ha (246 bu/a).
Sorghum is a fast-maturing crop with high photosynthetic efficiency and a high rate of dry matter accumulation. Most varieties will mature within 90 to 140 days.
Sorghum has good drought tolerance and high water use efficiency. Sorghum has several mechanisms for drought tolerance:
Sorghum varieties differ in their response to nitrogen. Some will respond to high fertility levels and produce higher yields and some will not.
Sorghum can be ‘ratooned’ like sugarcane. If the heads and stems are cut back, the plant can resprout from the roots. This is a useful characteristic both for subsistence farmers and for sorghum breeders.
Growth and development of sorghum is similar to maize. A plant will generally have a single stem, but may tiller profusely. Plant height may vary from 0.5 to 6 m, depending on the cultivar and growing conditions. Modern sorghums typically have 2-3 dwarfing genes, and are 2-4 ft in height.
Grain color of sorghum is variable and may be white, yellow, or brown. Brown-seeded types are generally high in tannins and lower in palatability.
Green plants contain a glucoside ‘dhurrin’, which converts to prussic acid (HCN), which is toxic to livestock. Grain sorghums are not suitable for pasturing. Forage types have been selected for lower HCN levels.
Traditional types have been selected for:
There is a need to develop locally adapted varieties with improved yield and yield stability, disease resistance, drought tolerance and quality.
Sorghum is the fifth most important cereal crop, and is grown on about
42 million hectares worldwide. Average annual production from 1997-2001
was 59 million metric tons. About 90% of the world crop is grown in developing
countries, where it is a dietary staple food for over 500 million people.
It is estimated that 80% of the crop is produced by subsistence farmers,
who often use local landraces that provide low, but stable yields under
marginal conditions. Sorghum is the 2nd most important cereal crop in
Africa, after maize. Thus, although the total production of sorghum and
millet is far less than for wheat, rice and maize, these crops play a
vital role for farmers in dry areas where little else can grow.
Mexico experienced a rapid increase in sorghum production from 1960 to 1980, and is currently the fourth leading country in terms of total production. Sorghum largely replaced maize because it required fewer irrigations than maize or wheat. A crop could be grown with two to four irrigations per year, whereas wheat required six to seven irrigations. Sorghum is also more heat tolerant than wheat.
In the USA, the area planted to sorghum depends a great deal on the corn/sorghum price differential and moisture conditions at planting time. Highest acreages are planted in the Central and Southern Plains States, with some irrigated areas in the Southwest. The trends in the US have been for decreased acreages of sorghum over the past several decades, while yields have increased due to adoption of hybrids.
International agencies involved in sorghum breeding
International Sorghum and Millet Collaborative Research Support Program
|Striga hermonthica on sorghum
Photo courtesy IITA
Shattercane (Sorghum bicolor ssp. drummondii) is a major weed problem in US maize and sorghum fields. It resembles forage types of maize and sorghum, and Johnsongrass. See the Extension Bulletin from West Virginia University for tips on how to distinguish these species. Shattercane is an annual plant that results from crosses between cultivated sorghum and johnsongrass or as a spontaneous appearance of "wild'' (ancestral) genes in cultivated sorghum through genetic recombination. It can cross readily with cultivated sorghum.
Johnsongrass (Sorghum halepense) is a tall, perennial weed of the Central and SE USA. It can be distinguished from cultivated sorghum by its large, loose panicle. It is a common weed in cultivated annual crop fields, along roadsides and in other disturbed sites. It spreads by thick rhizomes and can also reproduce by seed.
Greenbug (Schizaphis graminum) is a type of aphid that is particularly problematic in sorghum. The aphid injects a toxin into plants as it sucks the juices from them. Greenbugs also transmit maize dwarf mosaic virus.
Small grains, primarily wheat, are the winter host. Where the growing season of wheat does not overlap that of sorghum, grasses such as johnsongrass, are interim hosts.
Sorghum hybrids with genetic resistance to greenbug are available. However, greenbugs seem to be able to develop new biotypes that overcome the resistance.
The composition of sorghum is similar to maize or wheat. The grain has more protein and fat than maize, but lower vitamin A content (than yellow maize).
Earlier maturing types have superior mold resistance and are best suited for beer. Cultivars with higher tannin levels have superior malting ability and hydrolyzable starch. Like barley, sorghum has some diastatic activity (capacity for enzymatic conversion of starch into sugar).
Brewing raises nutritional value of sorghum:
Sorghum is uniquely suited to hot, dry conditions. It will remain a key food security crop in Africa. Investments in research and development contribute directly to alleviation of poverty.
There are limits for genetic improvement of drought tolerance in maize. Water constraints are likely to impact decisions regarding the relative acreage of sorghum and maize.
Sorghum is increasingly used as stockfeed.
Sorghum is not a ‘preferred target’ for biotech products.
"Millets" refer to the grain of a number of small-seeded grasses that are grown as cereal crops. The most important species is pearl millet.
Pearl Millet - Pennisetum glaucum L.
Pearl Millet is the 6th most important cereal crop in the world. The 28 million ha of production are split equally between Africa and the Indian subcontinent. It is a staple and primary foodstuff for 90 million people. It is also used as animal fodder, fuel, and building materials.
The common name for pearl millet in Hindi is 'Bajra'.
Photo courtesy ICRISAT
Pearl millet is an annual species, with 2n=14 chromosomes. Flowers are perfect, but are predominantly cross-pollinating (selfing is typically ~20%). Anthesis (pollen shed) occurs from 1-4 days after stigmas are exerted. This characteristic, called protogyny, helps to promote outcrossing. Plant height can vary from 50 cm to 4 m.
Pearl millet was domesticated in Africa 3000 to 5000 years ago, on the southern edge of the Sahara, west of the Nile.
The genus Pennisetum contains about 140 grassy tropical species. Wild relatives are problems for weed control and as crop contaminants.
Pearl millet is well adapted to areas of high temperatures, low and erratic rainfall, and poor soils. It is grown in areas with as little as 150 mm precipitation, but generally requires from 200 to 700 mm per year. It is the last crop for arable farming on the edge of the desert.
It is tolerant of acid, sandy soils with very low organic matter. Time to maturity varies from about 60 days to 180 days, depending on the variety and the environment. Pearl millet has a faster growth rate than any other cereal crop, enabling it to respond to very brief periods of favorable conditions.
Photo courtesy IITA
Pearl millet can germinate at high soil temperatures, and tillers profusely, enabling it to compensate for irregular plant stands. Warm temperatures are required for the grain to mature, typically greater than 30 °C.
With concerns about global warming, erratic weather patterns, and water scarcities, pearl millet may well be the crop of the future. It offers some promise for the drier areas of the central US and Australia. Yields are generally low (500 to 600 kg/ha on average), but it is more reliable than maize or sorghum in marginal areas.
Pearl millet acreage has declined in many areas, especially in southern Africa. This is largely due to a preference and trend for increased maize production.
Pearl millet possesses an enormous range of genetic variability, including genes for reduced height, early maturity, flowering and tillering synchrony, etc.
ICRISAT is the international center with a mandate for improvement of pearl millet. About 22,000 accessions of pearl millet are maintained in its genebank in Hyderabad, India.
Traditional cultivars are random-mating populations, which are highly variable. Modern varieties may be open-pollinated cultivars, synthetics, or hybrids.
Four cytoplasmic–genic systems for male sterility (CMS) are available. These male sterility systems are not stable in all environments.
Heterotic effects are large: good hybrids will yield 20 to 30% more than the best open-pollinated cultivar. Inbreeding depression is also large - there may be a 30% reduction in yield potential with one generation of selfing.
Photoperiod response is critical to adaptation and yield. Varieties may be
Harvest index is as high as 40% in modern cultivars, when grown under optimal management.
Standability is crucial; peduncle and stem lodging resistance are needed.
Pearl millet rust (Puccinia substraita)
Blast (Piricularia setariae) – dominant resistance genes
Downy mildew - major disease on Pearl Millet in Africa (not in US)
Longevity of hybrids in India is only 3-5 years due to instability of resistance to downy mildew
Ergot - related to protogyny mechanism
Striga – most landraces are tolerant to this root parasite, but production of tolerant varieties leads to increased Striga seed bank in the soil. Striga seeds can persist and remain viable for many years.
Chinch Bug and False Chinch Bug are important insect pests of pearl millet in the USA. Damage may occur any time from the seedling stage to the soft dough stage.
The term ‘Pearl’ is derived from the glistening appearance of the grain.
Photo courtesy ICRISAT
Flour deteriorates after a few days, so it must be ground frequently. This deterioration is associated with oxidation of lipids and ‘apigenin’, a flavanoid compound. The added need for food processing (frequent milling) is a disadvantage of millet as a crop.
ICRISAT’s pearl millet genotypes with yellow endosperm (right)
appear to have beta-carotene levels comparable to those of 'Golden
Rice'. Yellow endosperm is a naturally occurring trait and is not
genetically engineered like ‘golden
Photo courtesy ICRISAT
Finger millet (Eleusine coracania) - Major food crop in parts of Africa. It is adapted to more humid growing conditions. It stores well and accounts for 10% of global millet production
Foxtail millet (Setavia italica) - Originated in China
Proso millet (Panicum miliaceum) - Grown for Birdseed
Review at least two of your classmates' group projects on the discussion board, and send them specific comments and suggestions that you think will improve their project.
Take the quiz on this Unit on the Blackboard.
Andrews, D.J., J.F. Rajewski, and K.A. Kumar. 1993. Pearl millet:
New feed grain crop. p. 198-208. In: J. Janick and J.E. Simon (eds.),
New crops. Wiley, New York.
Center for New Crops & Plant Products, Purdue University. 2002.
Website on sorghum.
Dendy, DAV. 1995. Sorghum and millets: chemistry and technology. Amer. Assoc. Cereal Chemists, St. Paul, MN
ICRISAT. 2006. Sorghum.
ICRISAT. 2006. Pearl millet.
Lee, D., W. Hanna, G.D. Buntin, W. Dozier, P.
Timper and J.P. Wilson. 2004. Pearl Millet for Grain. Bulletin
1216. Cooperative Extension Service, University of Georgia and USDA-ARS.
National Research Council. 1996. Lost Crops of Africa: Volume I,
Grains. National Academy Press.