Sorghum

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Sorghum

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General Information

Grain sorghum (Sorghum bicolor) is the most important cereal crop grown after wheat, rice, corn and barley. Main producers of sorghum are Nigeria, Sudan, Burkina, Ethiopia, Mali and Egypt.

Sorghum is a vigorous perennial grass, mostly cultivated as an annual crop. It grows up to 4mt high and shows considerable tolerance to difficult growing conditions. Sorghum has the ability to go dormant during drought, and then to reawaken after a period of rain. It also has an efficient root system, which makes it more drought resistant than most other cereal crops (the exception being millet). Sorghum is grown in marginalized cereal production areas. The growing interest in sorghum in Africa is largely due to its drought tolerance.

Crop residues of sorghum grain are also a valuable livestock feed. Forage types of sorghum are considered one of the best crops for silage because of their high yields and high sugar content as well as the juiciness of the stalks. In frost-free areas, sorghum will continue to tiller and will produce new green leaves for grazing as long as there is some moisture in the soil.

In Africa, where sorghum is grown for subsistence, yields are generally low, ranging from 500 to 900kg/ha, which is far below the potential of the crop. The low yields are due to inappropriate production practices, damage by insects and diseases, the Striga parasitic weed and drought.

Sorghum is well suited for subsistence or local market farming. However, there is hardly any international market for human consumption of sorghum. For most farmers, the possibilities for improvements through technological means are very limited. Nevertheless, knowledge of the factors limiting production of the crop and of methods in line with the principles of organic farming is important for successful organic cultivation.

Diversification strategies

Under arid conditions, sorghum tends to be intercropped, rather than grown in rotation, as a response to water shortage. Under higher rainfall conditions or irrigation, rotation with other crops is common. Simultaneous cultivation of different cultivars, even with different maturity periods, is widespread.

Rotation of sorghum with other non-host crops reduces the abundance of insect pests, soil borne diseases and weeds (Striga). A planned rotation also enhances soil fertility. Combining different complementary crops and diverse varieties within one crop improves yield security in case of rain failure and increases returns from the land.

Crop rotation

Repeated cultivation of sorghum on the same field is not recommended for most arable crops with a high nutrient demand. This practice increases the risk of a build-up of pests and diseases. Rotation (with cotton and soybean) reduces the abundance of soil borne pests, such as wireworm and white grubs as well as some cutworms; as they all depend on a grass type crop, have a long life cycle and are soil-bound due to their underground larval stage.

Sorghum thrives when planted after a legume and in rotation with a broad-leaf or tap-rooted crop (cotton or soybean) that does not host the same pests or diseases. Other plants in the grass family should not be planted in the same rotation. If Sorghum is grown after a pulse crop, a midge resistant variety must be used. Sorghum should only be grown in the same place every 2 to 3 years.

Crop rotation is of major relevance in Striga control. The spread of this parasitic weed is enhanced by consecutive cultivation of cereals and the abandonment of fallow which leads to a reduction of soil fertility. The Striga weed attacks maize, millet and rice; whereas cotton, soybean, pigeon pea, bambara bean and groundnut are trap crops (in decreasing order of efficiency) as are sunflowers, field peas, cowpeas, lucerne, sunnhemp, sesame, linseed and castor beans. The trap crops induce germination of Striga, but do not serve as hosts. As a result, the weed dies and the seed bank is reduced. Thus, rotating sorghum with Striga trap crops (ideally with legumes to improve soil fertility) is an important preventive, as well as curative control measure against Striga.

Sorghum is rotated with cotton, groundnuts, sunflowers or sugarcane. Recommended rotations are:

  • Groundnut – sorghum – pigeon pea/cowpea;
  • Finger millet – field bean;
  • Amaranth – castor – sorghum – chillies.

Intercropping

Commercially produced sorghum is usually grown as a pure stand. Intercropping in rows is more common in farming crop production. Sorghum can be intercropped with maize, millet, groundnut or cowpea. In rain-fed conditions, sorghum is used for separating or fencing groundnuts or cotton plots.

Damage due to sorghum midge is reduced when sorghum is intercropped with leguminous crops. Intercropping of sorghum with trap crops of Striga can contribute to reducing the level of infestation (the higher the density of non-host plants the more effective the Striga control).

When intercropped with pigeon pea, both crops are planted at the beginning of the rainy season. Sorghum is harvested after 100 days, while pigeon pea is left to use the remaining soil moisture and nutrients until harvesting after 160 days. If grown for forage, sorghum can be intercropped with other legumes such as cowpea to improve the nutritional value of the fodder.

Selection of cultivars

The selection of appropriate cultivars is essential. Selected cultivars are adapted to the local growing conditions, show tolerance or resistance to major insect pests and diseases, are resistant to lodging and reduce vulnerability to pests, merit of their open panicles.

Growing cultivars with good resistance to insect pests and diseases can be vital. Resistance can be based on panicle form, early and uniform maturity, and good adaptation to local growing conditions. Varieties also exist that are tolerant of and resistant to pests and diseases such as Striga, mildew and other leaf diseases, sorghum midge, greenbug, stem borers and panicle feeding bugs.

Cultivars with open panicles are generally less sensitive to attack by the larvae of pests that feed on the developing kernels and they are more tolerant to weathering than those with compact panicles.

Transmission of viruses is avoided by growing virus-resistant varieties. Awned cultivars are less prone to losses by grain eating birds. The cultivars should have a maturity time that is adapted to the local agro-ecological zone. Good tolerance to moisture stress is generally important in arid climates.

Varieties that mature early and uniformly may escape infestation by some pests. Under irrigation, however, cultivars with longer maturity yield best. Iron-tolerant sorghum varieties should be used in areas where iron deficiency is a problem. In addition to physiological criteria, tan-colored plants and seeds are much preferred by consumers (as colored plants stain the grains). Varieties that give high quality flour are of special interest. Recent breeding programs have also focused on the improvement of grain quality. Red and brown grains are preferred for animal feed and for brewing. In general the differences in the nutritional properties of different cultivars are less than the variability brought about by environmental factors. Forage or grass sorghums, such as Sudan grass, are best for grazing. There is a growing interest in sorghum varieties that give a good grain yield and also produce considerable amounts of leaves for animal feed. These are known as dual purpose varieties.

Hybrid varieties of sorghum are available. Hybrids tend to be more sensitive to low soil pH and low availability of phosphorus and potassium, and do require improved agronomic practices. On irrigated land they are more productive than other seeds. If hybrid seeds are used in a certified organic farm, close attention must be paid to avoid the use of seeds that have received chemical treatment.

Pest management

Cultivated sorghum is prone to attack from a wide range of pests and diseases. Some can cause considerable losses (also during storage). In the traditional farming context, direct control measures are rarely undertaken, as the crop is largely cultivated under low input conditions. Improved cultural practices (incorporation of infested residues after harvest), the use of tolerant or resistant cultivars and natural inputs can, however, reduce losses considerably. Organic farming always advocates preventive methods of crop protection. Curative or direct methods are recommended only as a last resort when preventive methods have proved ineffective.

Sorghum can act as a host for many fungi, bacteria, viruses and nematodes. Some diseases are very common. These include grain moulds and anthracnose on grains, foliar diseases such as anthracnose, leaf blight, leaf spot and tar spot, downy mildew and rusts. Other less common diseases include honeydew disease or ergot, as well as root and stalk rots.

Covered smut (Sporisorium sorghi): Sorghum can only be infested by covered smut when the seeds are infested by airborne spores at harvest. Infection of new plants occurs in the soil before the seedlings emerge (with the ideal conditions being soil temperatures of below 25°C and medium dry soil). Diseased plants show individual grains that are replaced by whitish to grey or brown smut sori. Heavily contaminated seeds may turn greyish-black, especially in white-seeded sorghums. The fungus rarely survives in the soil between cropping seasons. Covered smut has almost been eliminated where hybrid seeds are used, as they are usually chemically treated. The disease can still be serious where no seed treatment is used. Infested seeds can be treated with hot water with great success. The susceptibility of sorghum cultivars to covered smut varies.

Downy mildew (Peronosclerospora sorghi): The disease affects the plant at nearly all stages, resulting in vivid green and white stripes on the leaves and heads that are partially or completely sterile. The major sources for infection are spores that survive in the soil, and airborne spores coming from infected plants. High plant density and rainfall after planting encourages development of the disease. The disease is not transmitted by seeds provided that they are properly dried and stored. There are some resistant cultivars. Effective control is also possible through deep ploughing of infested plant residues. Proper crop rotation is more effective. A break of at least 3 years between cultivating two sorghum or maize crops prevents new spores from being added to the soil. Natural fungicide applied as seed treatment or foliar spray provides further effective control.

Ergot (Claviceps africana): This fungal disease occurs wherever sorghum is grown. It attacks the unfertilized ovaries and reduces them to a white fungal mass, which is visible between the glumes. Infected flowers exude sweet, sticky honeydew that will drip onto the leaves and soil and under moist conditions will produce a white, powdery mass, on which secondary wind-borne spores develop. The fungus produces alkaloids that may have negative impacts on animals if fed to them. Cold nights 2-3 weeks before flowering and cool, wet weather during the days after flowering promote the disease. Ergot disease is mainly a problem that occurs when cultivating hybrid seeds. Cultural measures such as early sowing, removal of infected panicles at harvest, a 3-year crop rotation and deep ploughing of field residues will reduce the severity of infection, but these measures will have little impact if applied individually. The resistance of some cultivars is due largely to rapid pollination and fertilization. Chemical seed treatment with fungicides is effective, but is not allowed in organic farming.

Rough leaf spot (Ascochyta sorghi): A widespread fungal disease of sorghum species which generally leads to only minor crop losses and little economic damage. The disease is often more severe in fields where sorghum or Sudan grass are grown in succession. Infection most probably spreads by spores in wet weather or when there is heavy dew. The fungus first makes small, reddish, discoloured spots on the leaves, which become larger and in which yellow-brown centers emerge. In the later stages of development, the affected areas feel rough when rubbed between the fingertips. Entire leaves may turn brown and die. One control measure is to avoid repeated cropping of sorghum or Sudan grass in the same field. Some cultivars are highly resistant to rough leaf spot. Spraying of Bordeaux mixture (copper) reduces disease intensity, but can also lead to a toxic reaction of the plants.

Most insect species that infest sorghum occur widely and attack not only sorghum, but a range of other natural, and cultivated plants. Most insects appear at a specific stage of the crop's development. Many insects feed on the leaves of seedlings; some bore into the stem giving a dead heart; many feed on the foliage during the vegetative stage and some suck the sap (also know as sap-sucking insects) developing within the glumes. The most common insect pests of sorghum are shoot fly, stem borers, sorghum midge and head bugs. Cultural measures, such as the use of appropriate cultivars, seedbed preparation, and seed treatment are generally sufficient for managing these pests. The direct control of insect pests is rarely practiced. Application of non-specific insecticides has also been shown to kill natural enemies and to result in resurgence of the target or other pests.

Sorghum is also susceptible to storage pests such as rice weevil (Sitophilus oryzae), flour beetle (Tibolium castaneum) and the grain moth (Silotroga cerealella).

Shoot fly (Atherigona soccata): Crop residues should be collected and destroyed before the beginning of monsoon rains. The use of tolerant or resistant cultivars is recommended in areas that are regularly damaged by the shoot fly and in cases where planting has been delayed. Inoculating seeds with the bacteria Azospirillum and Pseudomonas considerably reduces shoot fly damage. High plant density, intercropping (with legumes or especially garlic), ensuring the availability of sufficient moisture and nutrients in the soil, delayed thinning and careful weeding all reduce the damage of the shoot fly. Wild grass species can serve as trap crops. Fertilization with cattle manure may result in greater damage by shoot fly (and stem borer). Plants with shoot fly damage should be removed during thinning, and destroyed. Where plants are at risk of damage, spraying with neem can be found useful.

Stem borers (Busceola fusca, Eldan saccharina, Sesamia sp, Acigona ignefusalis, Chilo partellus): These insects prefer sorghum, but also attack other cereals and grasses such as sugarcane and maize. They can lead to major losses. The larvae feed on the growing points, leaves and stems of the plants at different growth stages. The symptoms are similar to those due to shoot fly, but occur later in crop development. Other symptoms are scarification of leaves in the vegetative stage (feeding in rolled leaves) and tunnelled stems in later stages of crop growth. Late attacks in the generative phase may result in chaffy heads and, in severe cases, the peduncles may snap. Stem borers pupate in the stems or between the stem and the leaf sheath. Depending on temperature, two or more generations develop per year. The insects survive from one season to the next as fully grown larvae in stems. Cultural practices to control stem borer populations include early sowing of sorghum, promotion of natural enemies, intercropping with millet (as adults do not lay eggs on millet stems) and destroying the residues after harvest to kill the caterpillars. Light-trapping of the adults, which are active at night, may give early warning of a possible infestation. Spraying pesticides for stem borer control is usually ineffective, as these products do not reach the larval stages that live inside the stem. Repeated application of Neem kernel powder mixed with sawdust or clay and placed into the funnel of young plants can, however, be used to control stem borers where major damage is expected. In some areas, extracts of the leguminous Fish bean plant (Tephrosia spp.), a widely used fallow, green manure or cover crop, are used as a general insecticide.

Biological control of stem borers is possible with the wasp Cotesia flaviceps Cameron. Controlling stem borers in sorghum have found to be useful by adopting the “push-pull” method.

Rows of napier or Sudan grass are planted around the sorghum field. They act as trap crops, attracting and killing the stem borers. Additionally, repelling crops such as Desmodium spp. and Melinis minutiflora can be sown between the rows of sorghum. These companion crops all have the added advantage of being useful fodder plants. Desmodium also supplies nitrogen to the soil and suppresses the parasitic Striga weed.

Sorghum midge (Contarinia sorghicola): Potentially the most destructive pest of sorghum grain, this pest occurs wherever the crop is grown. The adult is a tiny orange fly, which deposits small yellowish-white eggs in the spikelets of flowering heads a few hours after hatching from nearby spikelets in the morning. Extreme temperatures and very dry or wet conditions during flowering hinder the development of the insect. High infestations of sorghum midge occur among low density plant populations, or when there is a prolonged flowering period, due to staggered sowings and/or cultivation of cultivars with different maturities, and the presence of alternative (weed) hosts. Late-flowering crops are particularly susceptible to heavy losses, as the midge population builds up throughout the season. Natural enemies (parasitoids) do exist, but their populations only develop to significant numbers after the damage has been done.

If sorghum is sown early in the growing season, it usually escapes infestation. The use of (hybrid) resistant cultivars considerably reduces damage. Cultural practices such as appropriate rotation with non-host crops, and the intercropping of sorghum, help to reduce pest damage and to conserve natural enemies and environmental quality.

Spraying of insecticides is practiced in some countries, but it is costly, difficult to apply, must be well timed to coincide with the flight of the adults and is less effective than other measures. Insecticide applications are used in some areas, primarily to reduce losses in late plantings. The benefits of insecticide application are greater on midge-resistant cultivars than on susceptible cultivars. When farming organically, natural pyrethrum may be used.

Head bugs (Calocoris angustatus and other): Panicle-feeding bugs have become a major pest of sorghum. The head bugs feed on maturing grains, resulting in severe reductions of yield and quality. Improved varieties with compact panicles have been shown to be more susceptible to head bugs. When damaged by head bugs, the grains of early maturing cultivars are more likely to develop moulds; particularly those that mature under conditions of high humidity during the rainy season.

Birds: Losses due to bird attack during grain-filling are widespread. The cultivation of varieties with grains that have a purple seed undercoat containing tannin is an efficient control measure, as the birds ignore the bitter tasting seeds.

Harvest and post-harvest handling

Harvest

Sorghum is harvested at the end of the rainy season (for rain-fed crops), or when the grains are colored and begin to harden. Prompt harvest is important to avoid major losses by birds. For manual harvest, the grains should have less than 20% moisture. For combine harvesting, however, 13% moisture is considered best (otherwise the grain needs to be dried). The yield potential of sorghum under favourable conditions is about 7 tons/ha. However, the average yield of grain sorghum under rain-fed conditions in the tropics is below 1 ton per hectare. Yields can range from between 2, tons to as little as 200kg/ha. Irrigated sorghum may yield double or more.

Farmers harvest sorghum by hand. The heads are either cut off, or the whole plant is removed and the head is cut off later on.

Post-harvest

Sorghum grains are more difficult to store than other grains. Proper handling after harvest is of major importance, to avoid significant losses. Sorghum grains are very susceptible to storage pests, and moist grains encourage the development of moulds. Unfortunately, farmers negate the tremendous efforts they have made in producing the cereals by not complying with the basic rules of appropriate conditioning and storage.

Panicles must be dried properly, threshed to separate the grains from the panicles and placed in well-aerated bags for storage. The panicles are dried in the sun. Small amounts of sorghum grains are separated from panicles through pounding with a pestle after drying. As with wheat, the seeds separate easily from the floral brackets when threshed. After threshing, grains are winnowed.

Panicles are stored in granaries. Plastic bags should not be used as they retain moisture and will promote moulds development. To reduce infestation by fungi and insects, a layer of neem leaves can be laid out on the bottom of the granary. Cats and snakes can play a useful role in helping to control rats. As a general rule, sorghum stalks and re-growth from the stubble should be worked into the soil carefully, or be grazed or destroyed soon after harvest to prevent further development of insect pests. Burning of the stubble is not recommended in organic farming, as valuable organic matter in the topsoil is destroyed, and soil organisms are killed. Burning also causes soil erosion.

Sorghum forage is often dried and stacked. It can also be made into silage. The drying and ensilage of sorghum forage is an effective way of avoiding poisoning from prussic acid. If the intention is to use the fled for grazing (pasture) after the harvest, the sorghum will grow back better if 10 to 15cm stubble is left. Forage sorghum is usually only cut once after flowering under rain-fed conditions. Where water and nutrients are adequate, forage sorghums can be harvested several times.

In Africa, conventional intensive vegetable production is characterized by extremely high input of pesticides and fertilizers. In many cases, intensive vegetable production does not apply wide crop rotations and large regions are specialized only in a few vegetables. Single-crop production leads to an over-exploitation of the land and increasing pest and disease pressure. Intensive use of pesticides and fertilizers contaminates water, air and soil. After cultivating the same crop year after year on the same land, the insects and diseases become resistant to the common pesticides. Therefore, farmers become dependent on costly external inputs, and suffer yield depressions due to loss of soil-fertility. This mechanism has caused farmers from an entire region to shift to different crops or give up their production. Furthermore, growers have suffered health problems due to contamination by agro-chemicals.

Sources and References

Bibliography

Shank Robert, UNDP Emergencies Unit for Ethiopia (1996). Striga: The parasitic weed and its relation to poverty. [Accessed 12th June 2012]


FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Conversion to Organic Farming. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Crop Management. [Accessed 21 March 2012]


FiBL (2011): African Organic Agriculture Training Manual. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick. [Accessed 12 July 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

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