Sorghum is fourth in importance among the world’s cereals, after wheat, rice and maize. It is grown throughout the tropics and sub-tropics and can be grown from sea level up to 2 700 m with different varieties which have been developed for different altitudes. Sorghum is one of the oldest crops known to have been grown for food and developed from wild sorghum in Ethiopia. This variety is thought to have been growing from 3 – 4 000 years B.C. Selection and improvement began there and the grain carried throughout Africa by migrating tribes.
Samples of grain dating back to 400 A.D. have been found during excavations in Kenya and Tanzania and later samples have been found at the Zimbabwe Ruins and Inyanga in Zimbabwe. Sorghum was carried to India, China and America by trading ships.
Sorghum is divided into forage varieties which are used for grazing, silage or hay and grain varieties which are harvested when dry. Sorghum is classed as a coarse grain, used for human consumption after being milled to form flour, stock-feed and in the brewing of beer, varieties with dark brown grain being used for this purpose. Specialised types of sorghum are sorgos which have a juicy sweet stem and used to produce syrup and sugar. Broom Corns have long, stiff seed heads which are turned into brushes and brooms. This type is grown mainly in the United States of America.
Sorghum is grown for sale to breweries, for stock-feed and human consumption. Sorghum has two great advantages. It is comparatively drought-resistant and can be grown in low-rainfall areas. It grows and matures very rapidly making it suitable for late sowing, even when it is sown following the early failure of a maize crop.
Varieties include hybrid, grain and forage varieties.
GRAIN SORGHUM – HYBRIDS
PAN 8909: This cultivar has a good milling quality and an excellent yield potential and has a uniform growth habit with a good ‘threshability’ and ‘standability’ with a general leaf disease resistance with good resistance to head smuts. This variety won the ARC and PANNAR trials over the past 2 years. It takes 79 – 81 days to flower and 135 – 142 days to harvest. It has a plant height of 110 – 115 cm.
PAN 8625: This is a bitter sorghum variety. It has a good yield performance level with excellent agronomic characteristics. It has a medium plant height with good standability rating and a good head smut resistance and is classified in the GH class. It takes 82 -85 days to flower and is harvested in 140 -145 days. Plant height is 120 – 130cm.
PAN 868: This is a late flowering variety by up to 20 – 30 days. It allows for a longer period in which one can use sorghum for feeding and produces vegetatively for longer, thus ensuring greater yields of green material or fodder. This cultivar reaches a height of 2.8 m and flowers at 130 days from planting.
PAN 888: This cultivar is used as a grazing sorghum and can be grazed two or three times in a season. It flowers about 75 days after planting.
Seed can be saved from the open-pollinated varieties and grown the following year but this should not be done with the hybrid varieties because the yield will be much lower. With these varieties, new seed should be purchased each year.
Sorghum will grow well in a variety of soils, but being a small seed the seedbed should be well cultivated, fine and free from clods. A rough seedbed will cause poor germination and an uneven crop.
Seed should be planted 25mm deep where there is sufficient moisture and on heavy soils the planting depth should not exceed 25mm. In drier conditions the seed can be planted deeper but should not be deeper than 50mm. On sandy soils the planting depth should not be deeper than 50mm. The row widths for sorghum with good, deep soils and high rainfall should have narrow widths of 0.91m. In areas of poor water holding soils and low rainfall row widths of 2.3m should be used and for in between areas a row width of 1.5m is suitable.
If a tractor is being used for cultivating the crop, the rows should have a strip of 2.3m within the narrow rows to allow the tractor wheels to pass up and down the crop. If weed control is being carried out by hand rows can be closer together.
Time of planting should be at the first rains but depends on soil temperature, frost fee areas and cultivar that are used. Planting can be from mid-October to Mid-December. Seed should be sown on a scale of 3 – 7 kg’s per hectare depending on the condition of the seedbed and distance between the rows. Seed can be sown with a maize planter as well as a wheat drill or planted by hand.
Table 1: Depending on the nutrient status of the soil, the following rates of fertilizer should be applied
|25 – 45kg/ha
|45 – 75kg/ha
|25 – 35kg/ha
|30 – 55kg/ha
|55 – 80kg/ha
|25 – 35kg/ha
|30 – 55kg/ha
|55 – 80kg/ha
The soil pH should be from 5.5 – 8.5 and lime should be applied to maintain the pH within these levels.
LEAF BLIGHT, SOOTY STRIPE AND DOWNY MILDEW
These are fungal diseases which are carried over in the soil or in crop residues and can be avoided by using crop rotations and the burning of all crop residues before ploughing.
This is a fungal disease carried on the seed. It attacks the grains in the seed heads causing the grain to be filled with a black, sooty mass of fungal spores. The disease can be prevented by using a suitable seed dressing on the seed before the crop is sown.
Figure 2: Covered smut
Is a similar disease which can be controlled by using sound crop rotation.
These can be controlled by spraying at 3 – 4 weeks after planting. All crop residues should be burned before ploughing. The economic threshold level for sorghum is 10% infested plants before one should spray the crop. To control one can spray with Beta-cyfluthrin at 70ml/ha, and should be done at the first signs of infestation and re-applied every 10 – 14 days if there are still signs of infestation.
Should be sprayed when the crop is in flower. Aphids can be controlled by applying Demeton-S-methyl with a low volume spray at a rate of 500ml/ha.
|Threshold: the level at which one starts to feel or react to something. Threshing: separate grain from, typically with a flail or by the action of a revolving mechanism.
Cutworms attack the young plant. They should be killed by the application of Trichlorfon mixed with bran and sugar which is, placed in small heaps on the soil before the crop is planted.
Sorghum can be harvested by hand by simply cutting off the seed heads. One man should be able to reap about 200 kg of heads per day. These heads are dried in heaps on the ground or a threshing floor. If the whole plant is cut it must be stacked and left to dry and mature in the field for 10 – 14 days and then threshed. The seed can only be stored at a moisture level of 12 – 13% or less. A combine harvester can be used but should be kept to the smaller dwarf varieties.
Where a large area is grown the crop can be combined and the more even the crop is in height the easier it is to combine without losing too many grains. When grown under good conditions, yields of dryland sorghum can be as much as 5t/ha of grain.
Sorghum can also be used for silage and this should be harvested when the stems reach a height of 80 – 120 cm tall and is suitable for hay, silage and green chop. The best stage is at the dough stage when the feeding value of sorghum is at maximum. Making silage or green chop out of sorghum is suited for mechanical harvesting.
The sorghum once harvested is graded. The grading is divided into four classes which are Class GM. This is malt sorghum that does not contain dark testa and is a listed GM cultivar. Class GL does not contain any dark testa and is of a GM cultivar but cannot be graded in the GM class or is from a GL variety. Class GH is a malt sorghum that contains dark testa and is from the GH cultivar list.
When brewing sorghum beer on a small scale, the following steps are:
- The grain is soaked in water and allowed to germinate. The various micro-organisms, fungi and bacteria multiply and produce a number of enzymes, together with an increase in Vitamin B. After a period of germination, the ‘malt’ as it is now called, is dried, ground and mixed with cold water
|Fermentation: the fermentation process involved in the making of beers, wines, and spirits, in which sugars are converted to ethyl alcohol.
- Some fresh grain is ground into flour and is steeped in boiling water;
- The cold water malt is added to the flour mixture and the enzymes in the malt turn the starch in the flour into sugar. Bacteria which are present then convert some of the sugar to lactic acid which gives the beer its sour taste and more Vitamin B is formed; and
- This mixture is left for 1 day and then heated, causing various fungal spores to germinate and produce more enzymes. Yeasts which were present on the original grains also multiply causing fermentation and the production of alcohol more Vitamin B and some new proteins. At the same time the bacteria continue to convert sugar into lactic acid. The brew is then left for 4 – 5 days, after which it can be drunk.
There are many modifications of this system and has been refined and developed into the large-scale brewing of opaque beer.
Figure 3: Sorghum growing well