Threshing is the action of separating the grain, for example of cereals, grain legumes or grasses, from the stalk of the plant. One traditional method of threshing is that of beating the plants with a wooden stick or with a hinged wooden flail, with the heads of the plant inside a sack, or with the unenclosed heads lying on a mud or concrete threshing floor. Another traditional method is to drive cattle repeatedly over the plants so that threshing occurs by the action of the hooves. An improvement on that technique is to use an aniaml-drawn threshing sledge or threshing rollers. The main problems of these types of animal-powered threshing are fouling, burying or cracking of the grain. These human and animal-powered methods of threshing are still widely used.
In the 18th Century, rotating threshing mechanisms were effectively used for the first time (Meikle), and are nowadays commonly found as part of combine harvesters, which perform the functions of cutting, feeding, threshing, winnowing, cleaning, grading, conveying and bagging. In most so-called Developed Countries, high agricultural wages render less mechanised techniques uncompetitive when compared with combine harvesting. Despite this, a few European countries still produce power-driven threshers, for use on experimental plots and for export. Combines are economically and technically effective when operating in large well-cleared fields of monoculture, with reasonably high yields per hectare, and with plants of low-to-medium height. These conditions often do not exist in present-day tropical farms.
Though not of immediate relevance, it must be mentioned that in recent years in some Developed Countries, adverse weather conditions during harvesting have led to some interest in merely harvesting plant heads in the fields, possibly with a tractor-driven flail-type forage harvester, and then performing threshing and subsequent operations at the farmstead.
In Tanzania, maize, cassava, and sorghum and millet are the principal staple foods. Wheat and rice are grown mainly for urban consumption, and some sorghum and millets are also grown for the production of local beers. Combine harvesters are used on large-scale state and private farms- mainly for maize at Iringa and for wheat at Arusha, both these areas being fertile with relatively high yields per hectare. Combines are not usually used for sorghum and millet, except on seed schemes, because yields are usually relatively low and because most production is by villagers and small-holders on relatively small and uncleared plots. One reason why yields are relatively low is that sorghum and millets are drought-resistant crops which are usually grown on land with average rainfall too low to justify the growing of maize. Moreover, despite crop-breeding work on high-yielding dwarf varieties, most sorghum and millets grown in Tanzania are traditional low-yielding tall local varieties.
It was brought to the attention of the Writer, by crop production and agricultural extension personnel, that there was a need for low-cost threshing equipment for village use, particularly for sorghum and millets, which have a high specific energy requirement for threshing when compares with such crops as maize or cowpeas.
This report details one particular design of man-powered thresher, which evolved after 4 years of occasional work on man-powered and engine-powered threshers.
The thresher described incorporates a grain mill, and thus it is necessary to provide some background information on milling.
In most parts of Tanzania, and in many other countries, cereals are the staple food and they are usually consumed as porridge made from fine flour. The traditional technique for obtaining this flour is for one or several women to pound and re-pound the grain with what is effectively a large wooden mortar and pestle. This is often done on a day-to-day basis, mainly because flour is much more susceptible to attack from insects and dampness, but also because pounding is rather hard work.
The modern technique of milling uses a co-operatively or privately-owned diesel- or electrically-driven hammer mill, whose steel beaters travel at an angular velocity of typically 3000 rev/minute and a milling velocity of around 70 metre/second. These beaters shatter the grain on impact. Hammer mills have been manufactured on a small scale in Tanzania for some years no, and it is probable that half of the population lives within 10 kilometres of such a power mill. Current charges for power milling are in the region of US$ 0.040 per kg, compared with a grain value for maize or sorghum of around US$ 0.220 per kg (1976 prices). People walk or cycle long distances to get their grain milled at a power mill, partly because of the fine flour obtained from the hammer mill and partly because of the heavy labor required for pounding.
"Plate" Mills or "Burr" Mills, both hand and engine-powered, have apparently been used in West Africa for some years, but have not become popular in East Africa.