Sorghum: one of the world's great cereals
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Sorghum (Sorghum bicolor (L.) Moench) is a crop of world-wide importance. It is unique in its ability to produce under a wide array of harsh environmental conditions. The crop has great genetic diversity, nmking selection for most traits of economic importance possible. However, in spite of the significant improvements in yield, isease and pest resistance and utilization/quality traits, there is need to transfer these technologies into farming and user communities.
Key Words: Crop improvement, genetic diversity, pest resistance, Sorghum b:color, utilization
Le sorgho (Sorghum bicolor L.) est une culture mondialement importante. Elle est unique quant a la capaci te de prod uire sur une grande gamme de conditions drastiques de l'environnement. La culture posse de une grande diversite genetique, ce qui permet de rendre possible la selection des traits d'importance economiques. Cependant, en depit d'ameliorations significatiques en rendement, en resistance aux maladies et insectes, et au point de vue de l'utilisation et de la qualite des prodnits, ii s'avere necessaire de transferer ces technologies aux communautes d'agriculteurs et de consommateurs.
Mots Cids: Amelioration des plantes, diversite genctique, resistance aux pestes, Sorghum bicolor, utilisation
Sorghum (Sorghum bicolor (L.) Moench) belongs to the grass family and was domesticated in different areas of Africa. Recently, seeds of sorghum, several millets, legumes and other edible plant remains were found at an ancient site, Nabta Playa, in southern Egypt close to the border with the Sudan. These remains were carbon dated to be approximately 8,000 years old. Sorghum seeds were similar in size and structure to the modern wild types, but infrared spectrographic results suggestthat they arecloserto present day cultivated grains, particularly the hexane extracts. This suggests the possibility of some cultivation for food and selection that may have contributed to domestication (Wendorf et al., 1992). This is the earliest known archaeological record.
There are five basic races of sorghum: bicolor, guinea, caudatum, durra, and kafir (Harlan and de Wet. 1972). A major step in the process of domestication is the loss of the seed shattering characteristic (Mann etal., 1983). Harlan (1975) asserted that domestication of sorghum occurred 0 ver time and in several areas where it was probably ennobled many times over several years. He envisions that early domestication occurred in an area extending from near the Ethiopian border, west through Sudan and up to Lake Chad. There is great diversity in this area as well as the presence of the primitive race bicolor (Harlan and de Wet, 1972). It is likely that this race arose from the
domestication of the aethiopicum verticilliflorum complex some 3,000 to 5,000 years ago. The finding at Nabta Playa may cause some rethinking of dates; however, it is not clear if these 8,000 year old seeds were from plants that did not shatter grains, although altered chemical composition would indicate some selection.
Bicolor sorghum has spread over much of the old sorghum growing world, being found in India. and is the likely progenitor of the kaoliongs of China (Mann et al., 1983L The race guinea arose from bicolor with the possibility of interaction with the wild race arundinaceum in the higher rainfall areas of West Africa. The guinea are now the dominant sorghum of West Africa but have spread and are found in Tanzania and Malawi. The guinea race arose more than 2,000 years ago. The race caudatum also likely arose from bicolor: it is found in the same area. Today, the caudatums are most abundant from east Nigeria to eastern Sudan and southward into Uganda. The race durra was selected from early bicolor that had moved into India some 3,000 years ago. With Arab migration, the durras moved into Ethiopia around 615 A.D.; and is today the dominant race in India, Ethiopia, the Nile Valley of Sudan and Egypt. Race kafir was probably derived from bicolor but there is also evidence of association with the wild race verticilliflorum. The kafirs are found primarily in eastern and southern Africa (Mann etal., 1983). Sorghum found its way into the Americas after 1850.
Much of the agricultural history of sorghum has been for food, beverage, feed and bui Iding material. It has been used as an industrial crop during the last 100 years. Mechanization of its cultivation and harvesting has occurred primarily in the last 60-70 years. Industrial uses of the crop have been for feed, some for food, starch, the chemical industry. and for fuel alcohol.
Maize, sorghum and pearl millet are the most important dry land summer cereals. Maize is generally cultivated where moisture is adequate, without water-logging, and under moderate temperatures. Sorghum is generally cultivated in dry, hot areas (38-40 C) with an averge annual rainfall of 400-750 mm, though it can be grown where rainfall is much higher. Sorghum also withstands water logging. Pearl millet is frequently cultivated on lightly textured, sandy soils with annual rainfall in the 300-500 mm range. Under comparable moisture conditions, sorghum will do better under cooler temperatures, while pearl millet does better under warmer conditions.
For centuries, farmers selected seed from the best plants for sowing the next seasoffs crop. They selected for traits ensuring some production; i.e., avoidance of insect pests. grain moulding from late rains, drought tolerance, etc. They also natrowed the genetic base. thus restricting their opportunities for selection. The consequence was varieties with a fairly reliable but low production of grain. While there have been significant changes from this base resulting in higher yields. there are still many places where traditional cultivars are used in much the same way as in the past. The advent of hybrids in the mid-l950's has been an important factor contributing to the expanded use and higher yields of the crop.
In terms of acreage. sorghum ranks fifth among the world's cereals following wheat, maize, rice and barley- World production at the beginning of the 1960's was about 40 million metric tonnes. Production increased and, by the mid-1970's, was some 65 million metric tomes. It reached peak production of around 80 million metric tomes in the mid-1980's and then declined to 66 million metric tomes in 1992 (Dendy, 1995). The rate of increase in production of sorghum has been relatively low (2.4% yr^-1) while that of other important cereals has been better; i.e., 3.4 yr^-1 for wheat (Dendy, 1995).
On a world basis, sorghum represents 3.5% of total cereal production. While this figure is small, there are countries where it is of great importance: Burkina Faso (52.8%). Sudan (71.6%), Chad 1.0%), Cameroon (39.9%), Botswana (84.4%), and Rwanda (51.5%)- In Africa as a whole, the proportion is 17.6% (Dendy, 1995).
Where traditional agriculture predominates, average yields have ranged between 200 to 1500 kg ha^-1 depending largely on moisture availability. This average has remained relatively flat over long periods of time. By contrast, in the USA. yields ranged from 630-1260 kg ha^-1, prior to hybridization (belore 1960), to 3775-4400 kg by the 1980's (Maunder, 1990). However, in the USA, there has been a decline in production in the 1992-94 period; from 22.5 to 16.5 million metric tonnes. This is expected to change in 1995 because export demand is up by 40% over last year and, on average, production is projected to increase by 25 % over the next four years (Maunder, personal communication). Prior to the availability of hybrids in India Imid-l96ffs) average rainy season yield was in the neighbourhood of 515 kg ha^-1, but this has increased during the 1986-90 period to 878 kg ha^-1 (Murty, 1992). There is considerable variation in yield, with some districts in India averaging over 2500kg ha^-1. Of concern in much of the sorghum growing world has been the drop in increased sorghum production below population growth. During the period 19721992, the average annual per capita growth rate for sorghum production for sub-Saharan Africa was 1.04%, south Asia -2.07%, and for Latin America -2.03%.
The use of grain as an animal feed has been an important stimulus lot the global use of sorghum (Dendy, 1995). Feed use was relatively minor until the mid-1960's when there was a rapid expansion in this use. particularly in North America. Feed utilization overtook food use for the first time in 1966. Over the past 25 years. Feed use has risen from 15 to 40 million tomes. Up to 97% of this use has been in developed countries, but it has also been used in some higher income developing countries, particularly in Latin America where it constitutes about 80% of sorghum utilization. In several countries. there has been interest in producing sorghum for feed and maize for food.
A number of factors have contributed to the recent decline in sorghum production. Particularly in a number of developing countries, programmes to develop and spread new sorghum cultivars have been lacking, only recently initiated, and/or lower priority than for other crops. There has been lack of high yielding cultivars and appropriate management practices.
In countries with poorly developed infrastructure for the dissemination of research accomplishments, scientists need to participate in the production and marketing of seed, as well as with the extension agency, to get new developments to farmers. This is essential in generating interest among farmers and others concerned with implementing the new technology. The approach is becoming increasingly important as donors want clear indication of accomplishment. Care must, however. be taken to ensure that accomplishments do not get stalled at the research level.
Sorghum farmers are frequently distant from markets which complicates the dispersal of development to them and their produce to markets. Policies are needed which provide price incentives as weII as stable prices to farmers.
Uncertainties in the supply of good quality grain have contributed to a slow down in commercial use. For a number of places, improvement in marketing grades and standards would be helpful.
Urbanization and the availability of inexpensive wheat and rice has contributed to discouraging production and marketing of sorghum. As an indication. when Nigeria placed a ban on imports of wheat and rice in 1985, production increased from 7.9 million metric tomes on 7 million hectares in 1979-1981 to 11 million metric tomes on 10 million hectares in 1989 (Dendy, 1995L Urban people prefer wheat and rice partly because less energy is required to prepare it for fbod compared to sorghum. There is also lack 0f processed foods, such as flour, in markets compared to othercereals. In southern Africa this is particularly true where millimeal from maize is widespread.
In the USA, lack of flexibility in the farm programme where a farmer's cotton acreage base is jeopardized if sorghum is includ~d in the rotation the Conservation Reserve programme providing a guaranteed price for hectares taken out of production, and also advancesin d~;veloping maize for drier areas. have contributed to reduced sorghum production (Maunder, personal communication).
Sorghum will continue to be an important food crop in areas where it is better adapted than other cereals, particularly in drought prone areas and under high temperatures, and water logging. Availability of varieties andhybrids with resistance to yield limiting factors and good feed quality traits have contributed to its production.
Important steps in the improvement of sorghum have been the diversification of the genetic base, improved hybridization, modification in plants so as to shorten their time to maturity, shorter stature than locals (better grai n/straw ratio). control of insects, diseases and weeds, better response to drought, temperature and acidic soils, and improved grain and forage qualities. We might give consideration to some aspects important to improvement.
The genetic resource. One recognized contribution of sorghum improvement has been the establishment of a world sorghum collection. This collection is an outgrowth of an effort which began in 1959. to collect the sorghums and several of the millets in India, funded by the Rockefeller Foundation. The collection was obtained by request from diflerent sorghum growing countries. The International Crops Research Institute for the Semi-Arid Tropical (ICRISAT) was responsible for this collection. and the collecting programme continues to this day. The collection has grown from about 10,000 accessions in 1972 to 33,766 in 1994 (Mengesha and Appa Rao. 1994). These are maintained at ICRISAT Centre. at the long term storage facility of the USDA at Fort Collins, Colorado, USA, and a set of every country collection remains with the country.
These collections have been systematically evaluated at ICRISAT and in as much as possible in the area of collection using a set of descriptors that was recently revised (IBPGR and ICRISAT, 1993). New accessions are catalogued using these descriptors and are available from ICRISAT.
From the collection, ICRISAT has distributed 237,265 samples of sorghum (Mengesha and Appa Rao, 1994). Besides the extensive use of these accessions in breeding programmes to broaden the genetic base for yield, resistance and quality traits, 21 accessions or selections from them have been released in nine countries. ICRISAT is actively involved in germplasm enhancement so that valuable accessions for yield, resistance and quality traits are more usable to breeders around the world. The conversion programme of Texas A&M University and USDA, initiated in 1962, has been another valuable source of improved accessions (this programme converts selected tropical lines to temperate adaptation).
Several categories are maintained in the world collection: accessions from farmer's landraces, wild types, sources of useful traits, and released or elite varieties and parents of hybrids. Concerned scientists in many countries have contributed their collections to the world collection and have facilitated collection in their country. Almost all sorghum breeding programmes have benefited from introductions from the collection. and from other breeders. in diversifying their germplasm base. Hageen Durra-l. the first hybrid released in the Sudan, is from introduced parents. Kuyuma and SIMA released in Zambia are from introductions. as was SV-2 in Zimbabwe. The major proportion of varieties and hybrids released in India since 1960 involve introductions directly or from crosses with local varieties. The tremendous source of variability available in the world collection has made a significant contribution to the more than three fold increase in yield during the past 45 years. and has become one of the most environmentally friendly crops because of its in-built resistance traits (Maunder. personal communication).
Crop improvement. As mentioned earlier, traditional sorghum cultivars have been farmerselected over long periods of time, greatly narrowing the genetic base and leading to cultivars that yield reliably. but at a low level. Traditional varieties are usually tall, 2.5 to 4 m, and frequently long season, requiring 120 to 180 days to mature. A common breeding objective is to reduce plant height and select plants that mature 2 to 5 weeks earlier than locals. A general observation is that farmers in dry conditions select for mid-season maturity (100-115 days) and not for very shortseason types. Going to earlier types brings problems, including changes in susceptibility to insect, disease. and Striga pests, as well as grain quality for the production of food and beverage.
Improvement by selection within traditional cultivars or by selecting progeny from crosses between similar traditional cultivars has generally not been promising in enhancing yield. It is necessary to diversify the genetic base by introducing exotic cultivars. Generally, these introductions include breeding stocks, accessions from the world collection, and converted lines from the Texas A&M - USDA conversion programme. Selection in progeny from crosses of traditional x exotic or exotic x exotic cultivars has been rewarding.
The opportunity to improve sorghum has not been the same in all situations. For example, the All India Sorghum Improvement Programme ratherquickly developed lines and hybrids superior to locals for the rainy season; but have not had nearly the same degree of success in the post rainy season. Direct use of introductions was not helpful and crosses between locally adapted and introduced cultivars frequently did not produce progeny from which useful selections were forthcoming. It has been difficult to make beneficial changes in the race guinea common in West Africa. There has been little improvement of the tall late sorghums found along the Ivory Coast across Africa into the high rainfall zones of southern Sudan. Maize has replaced sorghum in some of these areas. These are challenging situations in which more imaginative, novel approaches may be required. For example, (Cox et al., 1984) introduced a few genes from wild types into cultivated sorghum. The phenotype of the cultivated type was not greatly changed but the expression of some traits of interest (yield) was enhanced.
On the other hand, substantial gains have been made in many areas. Orrin Webster. years ago, introduced yellow endosperm sorghums (Korgi and Kaura) into the USA from Nigeria. These were developed into breeding stock from which came the pollinator parent for CSH-I; the first hybrid released in India. From a large nursery of yellow endosperm types came Karper 1597, the pollinator parent of the first hybrid released in Sudan. These yellow endosperm types contributed to drought resistance and grain quality and have been extensively used in crosses in many places in the world, to select new varieties and parents of hybrids.
A landrace type, zera zera, found in the low lands (Gambella area) of Ethiopia extending into a portion of central Sudan, has been of great importance in providing a pearly white grain of good food and feed quality with reasonable resistance to grain weathering. Theavailability of this grain type has virtually spread over the world and has, for example, contributed much to the spread of white seeded hybrids in the USA.
The advent of hybrids from the pioneering work of Stevens and Quinby at the Chillicothe Station in Texas, has provided breeders with one of the most powerful tools for sorghum improvement. Hybrid sorghum almost universally out-yields non-hybrid varieties; and, as growing conditions become stressed, hybrids out-yield varieties by an even greater margin. Most people believe that hybrids require optimal growing conditions but, in my experience, they are beuer than other varieties as growing conditions become stressed. A concern with hybrids is the need to provide farmers with hybrid seed each year - they should not save seed. While this is a major undertaking, it will result in amuch better situation to provide adequate quantities of good quality seed on a timely basis. This can be a major contribution in having a positive impact on the farming community.
I had the opportunity to initiate a sorghum improvement programme for the Southern African Development Co-operation (SADC) region under the auspices of ICRISAT. Approximately 5,500 breeding lines and accessions from the world collection were introduced including 35 lines that had been introduced into India some 25 years earlier. The difference in the performance of these lines compared to introductions of current breeding stock from 25 stations, was obvious in demonstrating the significant gain in the general performance of the crop.
There is a range of problems related to resistance and quality considerations. too lengthy to go into here, and some will be discussed in other papers at this meeting. I would like to make several comments relevant to working with these traits.
Crop improvement: Some considerations. Looking to the future, it is going to be important to increase yield to keep sorghum competitive w!th othercrops. However, the crop improvement process also involves resistance to yield limiting factors, end use and quality traits. A team approach is recommended.
When dealing with resistance traits. it is necessary to know what problems exist in a region and to prioritize research on them. This sounds easy but in southern Africa. it took several years including the evaluation of uniform nurseries grown in select locations in the region. Today. many of these problems as well as a reasonable idea of their importance are known. The identification of sources of reststance generally begins with the more elite breeding stock. then to germplasm collections to broaden the genetic base of the trait. Procedures to evaluate the trait need to be available. They should be simple. reliable. affordable, repeatable and useful to evaluate a large number of entries. Screemng nurseries are generally conducted in areas where the trait is well expressed. and usually more than one location is involved to be sure that satisfactory results can be obtained. since there is risk of poor expression or over expression of the problem of interest so that differences are not obviously apparent.
Generally, techniques such as spreader rows. use of irrigation or water with-holding, and artificial inoculauon are used to ensure a uniform spread of infection over the nursery. A standard, usually susceptible, cultivar is sown periodically to help measure the degree of uniformity. Generally, it is more difficult to realize unilormity in a nursery evaluating soil borne prc, blcms. The nearest neighbout technique was used at ICRISAT Centre to evaluate resistance to Striga asiatica. Laboratory and greenhouse testing can be of help, and at times the evaluation of more easily measured highly correlated traits is helpful. This, generally, requires a good resource base.
Collaboration with regional stations, international centres, and universities can be helpful.
It is important for the research team to be vigilant after a new cultivar reaches the fanner's fields. In spite of years of testing, the first pearl millet hybrid released in India, while high yielding, was extremely susceptible to downy mildew and failed in some of the traditional high yield areas because of the build up of inoculum. Striga asiatica increased following the spread of the sorghum hybrid CSH-5 in India. Viruses are not a serious problem in Zambia, but an exotic sorghum variety released in 1982 was super susceptible to the virus and became a potential spreader of the disease. It was quickly replaced. These serve as examples of unexpected problems that the research group should be able to respond to quickly and effectively.
The number of traits that a breeder can manage at one time is generally considered to be 3 or 4. As the number of traits increases, the gain per cycle of selection for each trait decreases. This emphasizes the problem of prioritization of traits. At ICRISAT, several years ago, consideration was given to this concern. As the situation dictates, it is probably expedient to first introduce simply inherited traits, such as grain colour, plant colour, plant height, maturity, disease resistance, etc. Breeding can then focus on more difficult traits such as yield and insect resistance, where heritabilities are lower and the traits polygenic in nature. In this process, care must be taken to ensure that deterioration in other traits do not occur. It is necessary to monitor traits of importance in the region to ensure that newly developed cultivars are not more susceptible than those already in farmer use. The role of biotechnology in simplifying and hastening this process of trait introduction is potentially of great value.
Some traits, such as resistance to stem borers and S.triga. have components. For example, with stem borers, eggs are laid usually on the underside of leaves. The adaxial trichomes, when present, disorient the young larvae, slowing the rate of migration down the leaf. The waxy cuticle can retard their motion and the vertical angle of the leaf can contribute to disorienting the larva. Rapidly elongating internodes contribute to resistance. These traits help keep the young borer subjected to a desiccating environment and exposed to predators. Again at ICRISAT, resistance to Striga asiatica was found to be associated with low seed germination stimulant from host roots and a difference in the rate of thickening of root endodermal cell walls, and deposition of silica crystals in those cells interfering with haustorial penetration by this parasite (Vasudeva Rao, 1985; Ramaiah et al., 1991). Ejeta et al. (1992) at Purdue. USA, are actively searching for a series of mechanisms that contribute resistance to Striga so that they can be identified, selected, and combined. This process of identifying component traits, their characterization, selection, and breeding while retaining other valuable traits in the cultivar is relatively tedious. As is being done for Striga, the use of biotechnology may make this type of study much more practical than it is now.
Sorghum has a number of traditional as well as commercial uses. People become accustomed to a certain grain and it becomes prelerred in terms of food. It is nearly impossible to develop new varieties and hybrids with exactly the same characteristics. Much can be done to select cultivars close enough to local standards to be acceptable. In Africa, sorghum is frequently made into a beer where malting and colour represent desirable traits. In Southern Africa, the variety Red Swazi is not visually impressive, but it withstands drought, has a satisfactory diastatic power, and has a red coloured grain making a good local opaque beer.
Sorghum flour is blended with that of wheat, maize, and grain legumes. The flours of some varieties are better than those of others for blending, and the characteristic is a selectable trait, The range of uses led to consideration of the idea of breeding for end use. When blending flours, a programme generally considers only the crop that it is working with. Ideally. the characteristics of both crops should be considered. For example. adjustments might be made between the gelatinization temperatures of the starch of both crops.
Sorghum is known to take more energy and time to cook than wheat or rice. It has also been demonstrated to have a lower food value. Traditionally, fermentation has been one technique used to improve food value. Recently a highly digestible type has been found, nearly as good as wheat (Haymaker, 1995). Tannins are antimetabolites useful to reduce grain moulding and to repel birds, but they also reduce food and feed value. A white seeded sorghum was recently found that has resistance to bird depredation without the tannins that would have adverse effects on food quality. Details of its use are being studied (L. Butler, personal communication). With forage, it is possible to breed for higher protein, lower fiber and, via the brown mid-rib gene, lower lignin content resulting in higher animal weight gain (Cherney et al., 1986). Some traits such as resistance to Striga asiatica (low stimulant production) and midge, as well as many traits of the grain and brown mid-rib in the plant, are simply inherited and effectively used in breeding. Other traits such as resistance to stem borers and shootflies are polygenic and. at best. only partially effective m pest control. Little resistance has been found lotpests such as head bugs (Calocoris angustatus) in India. When resistance has limited contribution to pest control, it needs to be coupled with other practices such as clean cultivation. destruction of crop residue and use of chemicals. These practices are aspects of Integrated Pest Management (IPM).
Shootfly is a serious problem in many of the sorghum growing areas of India and West Africa. The maggot of the fly feeds on and desti'oyb the growing point of seedlings. Some wild sorghums are immune as well as some other crops such as sugarcane. Efforts to bring this resistance into sorghum have not worked by traditional techniques. Procedures from biotechnology might be effective.
Sorghum is a crop of world-wide importance. Initially, the grain was used primarily for food and still is in many developing countries. Its use as a feed now exceeds its food use in developed countries. There is great variability in sorghum, making selection possible for most traits of economic importance. Germplasm, in the form of breeding stock, collection accessions. and converted tropical cultivars, now moves readily world wide and has contributed significantly to the crop's improvement in terms of yield, resistances and utilization/quality traits. In recent years, there has been exciting research accomplishments to which organizations such as INTSORMIL and ICRISAT have contributed. A greater effort is needed to see these developments from research into the farming and user communities. There are still substantial areas using traditional cultivars with traditional management. It is difficult for these farmers to res. pond to market demands. There is, therefore, need for policy to encourage market participation but also to resolve the problems of extending technology where it can be used. For many countries, financial resources are becoming increasingly difficult to realize. Shared responsibility. with focus on particular problems between countries of a region. with international organizations and with universities. should make research more efficient. Clearly sorghum is one of the world's important crops and, while improvements have been significant, there is a tremendous challenge for sorghum to continue to expand in local and international importance.
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Copyright 1995 African Crop Science Society