Sorghum (Sorghum bicolor) is one of the most important traditional cereal crops of the hotter and drier regions of the tropics and subtropics. In areas with uncertain and erratic rainfall, sorghum is a preferred crop.
Sorghum has many attributes that make it withstand drought conditions. These are:
it has an extensive root system than other cereal crops
has less leaf surface area than maize and the leaves are covered with wax and this helps reduce water loss through evapotranspiration
can stay dormant when conditions are not favourable
There are two different types of sorghum. One type with tropical adaptation for high rainfall areas (1000-1400mm). These sorghums should be semi- to photoperiod sensitive and must possess high resistance to soil acidity and anthracnose. The second group should be sub-tropical in adaptation (300 – 700mm). These varieties must be photo-period insensitive with good levels of resistance to moisture-stress, heat and several diseases i.e. leaf blight, sooty stripe, downy mildew.
Both open pollinated varieties and commercial hybrids are now available at Zambia Seed Company premises and outlets. Hybrids give higher and more stable yields across seasons, locations and management levels, as they can face adverse growing conditions better than open pollinated varieties.
Traditional varieties of sorghum require a long growing season, have low yield potential, and are tall and non-responsive to improved management. Improved sorghums, however, are high yielding, input responsive, and far more resistant to drought.
Several varieties that are high yielding (4.5 – 8 tons/ha) and ideal for various end-uses (Forage, food and brewing types) have been released to the different categories of farmers. The maturity of these varieties ranges from 110 – 130 days.
This is an early, short, white grain variety with tan plants and with excellent milling properties. It is widely adapted to low rainfall areas with good resistance to most diseases in Zambia, except for anthracnose. Height: 140-170 cm; Maturity: 100-115 days. High yield potential 3 – 5 tons/ha. The variety has been evaluated in the neighbouring countries with successful results.
A medium – tall, medium-late, tan plants, white grain variety with juicy and sweet stalks for dual purposes – grain and forage; it is well adapted to all three agro-climatic regions of Zambia and moderately resistant to diseases. Height: 210-230 cm; maturity: 110-120 days. Potential yield: 3 – 6 tons/ha. This variety also does well in the neighbouring countries.
A widely adapted, white-grained variety, excellent grain quality and good levels of resistance to all diseases – an improvement over Kuyuma variety for the drier areas. Potential yield 3 – 5 tons/ha. Height: 140 – 160; maturity 100 – 112 days.
ZSV – 16
The variety has white-grains and is open-pollinated, with short plant height, juicy stems and an enhanced stay-green trait. The stems are not as sweet as Sima. Plant height: 140-200cm; Maturity: 115 – 120 days; Potential yield: 5-6 tons/ha. The stay green trait in sorghum is associated with resistance to several diseases as well as to termites, lodging and to grain moulds. This is due to plants remaining green and active even after physiological maturity of grain.
ZSV – 17
This is a short white grain sorghum variety with tan plant. It has long semi-compact cylindrical panicles with medium-size hard-grain. The variety is juicy and remains green (stay Green) even after physiological maturity conferring resistance to a lot of diseases and lodging. Plant height: 100 – 140cm; Maturity: 112 – 118 days; Potential grain yield: 4 – 5 tons/ha.
ZSV – 36R
ZSV 36 R is an early maturing open pollinated variety and of medium height with red grains. It is suitable for food (nshima/ugali) and brewing. It is widely adapted to low rainfall areas. Height: 130 – 170cm; Maturity: 110 – 120days. Potential grain yield 2 – 5 tons/ha. Yield losses due to bird damage are minimal. Birds shun the grain because of the bitter tannins in the seed coat.
Height: 210-240 cm; Maturity: 110-120 days. Widely adapted in Zambia and the region, high potential, brown grain hybrid with moderate levels of tannin and good resistance to all diseases of Zambia except for Downy mildew. 4 – 8 tons/ha. Recommended for the entire country.
Height: 210-240 cm; Maturity: 110 – 116 days. A widely adapted in Zambia and the neighbouring countries. High yield potential and brown grain hybrid with higher tannin content and excellent malting properties; resistant to most diseases. 5 – 10 tons/ha. This hybrid is recommended for the entire country and some neighbouring countries as well.
A tan, early, medium-height, white grain hybrid with good grain quality and high yielding; widely adapted to dry areas, resistant to most diseases except for anthracnose. Potential grain yield 3 – 6 tons/ha.
This is a short hybrid (130 – 140cm) that matures in 100 – 115 days. It is red seeded with a thin pericarp without a testa. Not easily attacked by birds. Fairly resistant to downy mildew, leaf blight and gray leaf spot. Not easily attacked by birds. Exhibits stay green trait. Potential yield of 4 – 6 tons per hectare.
Soils – Water and Soil Requirements
Sorghum does well in semi-arid tropics. It is tolerant to harsh weather conditions better than other cereal crops. It grows well at temperatures above 10OC. Some sorghum varieties are sensitive to day length, especially the local types. The soil pH should not be allowed to fall below 4.5 on sandy soils and 4.7 on clay soils. Sorghum is adapted to a wide range of soil types but sandy loam soils with good drainage and with organic matter are the best. The crop can also withstand some water logging giving desirable results.
Moisture conservation is critical in areas where sorghum is grown. To prevent soil evaporation, the soil surface must be tilled with a harrow to form a fine surface. Weeds use a large quantity of water and should be removed. In areas with sandy soils it is advisable to use a ripper instead of a plough. Minimum tillage practices should be employed to conserve water and the soil structure.
Sorghum needs a fine seedbed for planting, as the seeds are small. A fine seed bed is critical where the seed must be broadcast. However, plant seeds 3-5 cm deep. Deeper plantings may lead to poor germination. Weeds should be removed so that seeds are planted in a clean field.
For improved varieties, first to third week of December in low rainfall areas is best for high grain yields, but planting can be done up to mid-January in high rainfall areas. In general planting should be done when there is adequate moisture in the soil. Inadequate soil moisture leads to poor germination.
7-10 kg/ha. When planting is done with precision planters a low seed rate is recommended (7.0 kg/ha). When planted by hand-hoe, germination is seldom complete due to the uneven planting depth. Therefore, a higher seed rate (10 kg/ha) is recommended. The aim is to achieve a plant population of 120,000 to 180,000 plants/ha. For forage sorghums the seed rate is higher (15-20 kg/ha).
Planting can be done either in rows or in stations (in case of hand-hoe farmers). Spacing of 60 – 90 cm between rows and 15 – 30 cm in the row is recommended. In case of hand-hoe farmers, a spacing of 75 x 50 cm with 8-10 seeds per station is suggested. 3 – 4 plants per station should be retained after thinning. In very dry areas, 2 – 3 plants per station should be retained.
Sorghum needs fertile and well drained soils. Ideally, liming and fertilization of grain sorghum should be based on soil test recommendations. One of the most important points to remember is that some varieties of sorghum are not tolerant of soil acidity. In order to get maximum benefits from the improved varieties some fertilisers are necessary. For a good crop 200 kg/ha of ‘D’ compound as basal and 100 kg/ha of urea as top dressing is recommended.
Weeds can be removed mechanically, using manual labour or implements. It is advisable to keep the field free of weeds in the early stages of growth. Sorghum plants are more sensitive to herbicides than, for example, maize. “Atrazine” in low doses is effective during the early growth period. Commercial farmers could apply “Atrazine” at the rate of 3 litres per hectare as a pre-emergence application. The use of safeners such as “Concep” on the seed could be considered with proper recommendations from the chemical providers.
Witchweed – Striga or witch weed can be a serious weed of sorghum and mainly occurs under low-input farming conditions. It is a parasitic weed, and its roots penetrate the roots of sorghum. The weed feed on nutrients of its host and growth is therefore affected. Farmers should be advised to rotate their sorghum fields with other non-host crops such as legumes. Where it is observed, farmers should be advised not to allow the plant to flower as this reduces its spread.
Birds are undoubtedly the most serious pests of sorghum and yield losses due to bird damage can be high. If possible, avoid small hectarages and swampy areas that are breeding places for the birds. Bird scaring measures from the soft dough stage onwards are considered essential. The brown seeded varieties should be grown in areas where birds are a serious problem.
Although many diseases are known to attack sorghum, very few of them cause economic losses. Here below are the common ones found in Zambia.
Smuts cause various symptoms affecting the inflorescence and occasionally the foliage. Sori replace part or all the panicle or form on rachis branches of the panicle. In dry environments, the smuts are of much concern especially if the seed was not treated with a fungicide. Cultural practices such as uprooting and burning infected parts could help
Sorghum downy mildew occurs as either systemic or localized infection. The systemic form is induced when the pathogen colonizes the meristematic foliar tissues. Systematically infected seedlings are chlorotic and stunted and may die prematurely. This disease is more prevalent in medium rainfall areas. Farmers are advised to use seed that has been treated with fungicides. Most of the improved varieties are resistant to this disease.
Anthracnose is the most important leaf spot disease in high rainfall areas. To avoid this disease, use the recommended varieties for each region and follow the planting recommendations.
Integrated pest management
Integrated pest management is a system whereby various methods are applied to protect the crop by suppressing insect populations and limiting damage. These measures include the following: chemical control, biological control, plant resistance and cultural control.
Insects may attack grain sorghum from the seedling stage through maturity and even in storage. The extent of damage by insects in grain sorghum is often related to the planting date. When sorghum is planted late, more severe insect problems are likely to occur. In addition to early planting, control practices include the use of recommended insecticides when damaging populations of insects are detected.
Shootfly attacks sorghum plants when they are still very young. This is a common problem where there is a differential planting. Usually the second planting is attacked. The larvae feed on growing parts of the plant and causes them to die. If possible, plant at about the same time.
Armoured crickets are sporadic in nature and cause serious damage to many crops in drier parts of the region, the valleys. The green coloured young crickets (nymphs) start appearing at knee-high stage and, if not controlled, damage the heads and grain. It attacks a range of crops and migrates from one crop to another. IPM strategies such as timely weeding, and the use of “Karate” have been found to be effective in controlling armoured crickets. Poison bait (maize meal and “Karate”) around and inside the field is also effective.
Stem borers are the other pests that deserve attention. It is necessary to closely observe the occurrences of these pests and control them timely. Control measures concentrate on crop hygiene and crop rotation. A prophylactic spray with long lasting broad-spectrum insecticide like ‘Fastac’ is recommended at knee-high stage to protect the crop from insect-pests.
Traditionally, crop residue is left far too long in the fields after harvest. This practice harbours diseases and insect-pests like stalk borers. It is advisable to either remove the crop residue or incorporate it into the soil by winter ploughing.
Crop rotation as part of crop management can be helpful to farmers and has many advantages as follows:
Control of pests and diseases can be achieved when crops susceptible to certain diseases and pests are alternated with those that are either resistant or non-host.
Efficient utilisation of soil nutrients by alternating crops which differ in their relative requirements of certain nutrients or those that differ in depths of rooting systems.
Weed control can be achieved by alternating with crops that differ in canopy cover. Certain weeds are associated with certain crops e.g. witch weed.
Improving soil fertility by alternating crops that tend to exhaust the soil with those that enrich it e.g. cereal legume rotation.
The grain reaches physiological maturity when the spot where the grain attaches to the inflorescence turns from green to black. Where birds are a serious problem, heads can be harvested at this stage and dried on floors or racks. A combine harvester can be used to harvest the sorghum, or it can be done manually. Hand harvested heads can be threshed by a stationary combine or hand beating on the layers of sorghum heads.
Dried grain must be stored in closed bins that are airtight. Sorghum should be stored in the form of threshed grain rather than whole heads because the crop is more at risk to storage insects and rodents when the latter is done. Hard grain varieties store better and longer than soft-grained varieties. It is advisable to consume soft-grain types first and store hard grain types for later use.