1In Yemen, as in many developing countries, grain production, grain import and grain storage play significant roles in food security. Underground storage pits (local name: madāfin) have been in use since ancient times and are still an attractive option for many. Storage pits offer the advantages of relative easy construction, safety from theft, good thermal insulation, and protection from rodent attack and insect infestation. Sorghum is the main crop traditionally grown in many parts of Yemen, especially on terraces in mountainous areas. Sorghum was commonly stored in pits, as were wheat, barley and pulses. The main aim of this paper is to highlight the use of this storage method, its construction and the basic principles of its effect on pest control, as well as its possible significance for the future.

Pit storage

2Farmers traditionally store grain for food or seed in pits excavated inside or outside their houses. Some farmers store their grain in compact baskets made of date‑palm leaves with a capacity not exceeding 150 kg.1 Such baskets are then placed in underground pits covered with soil. Grain stored by this method remains in good condition for a long time, provided that their moisture content is low and the soil is dry.2 There are several types of pits, most of them flask-shaped and covered with sticks, cow dung and mud, or closed by a large stone embedded in soft mud. In some cases, pits are conical and very limited in size, especially when dug into rock or beneath a house.

Structure

3A pit is dug either through the ground floor of the farmer’s house, or into a heavy clay soil or in rocks outside the house. Generally, storage pits have a few cubic metres capacity. In many cases, the pit is flask-shaped, having a narrow neck opening towards the ground surface and a wider storage chamber below (Figure 1). Pits used by farmers are on average 3.5-5.0 m deep and 2-4 m across at the widest point, and those used by government organisations are up to 15 m deep and 3-4 m across.3

4The inner surface has a very gentle slope to prevent the structure from collapsing. The walls may be plastered with clay mud reinforced with fibre and dung, to make them relatively impervious to moisture and gas losses. The top of the pit is sealed by using a flat stone and mud or cow dung to ensure an air-tight structure and to prevent the entry of water and pests. If pits are dug into soil, the soil must be compact and hard to avoid as far as possible the infiltration of water.

Basic principles of pit storage and its effect on pest control

Hermetically sealed atmosphere and biological action

5In hermetic storage the controlled atmosphere required for grain preservation is produced through biological respiration. Pit storage is a form of airtight storage that works mainly on the principle of O₂ depletion and the C0₂ enrichment to levels lethal to pests. Some traditional procedures are of the type that would produce, intentionally or otherwise, rapid moulding in a localised portion of the storage container. These procedures include the practice of lining a pit with damp straw or green plant material before loading.4 Such materials produce conditions that promote rapid reduction of O₂ and quick elimination insect infestation.

6The atmospheric composition in hermetic storage, such as adequately sealed underground pits, depends on the following factors: 5

• grain condition,
• storage temperature,
• the prevailing humidity or grain moisture content,
• leakage of the storage enclosure,
• type and number of organisms in pit,
• period of storage.

7In warm climates the influence of temperature on insect respiration is more effective than in temperate climates, since insect intake of O₂ in warm climates is very intensive.

8Grain with moisture content greater than 13% is in practice always associated with micro-organisms such as moulds, which can respire and grow under such moist conditions. At the lower moisture levels of 12-13%, cereals like sorghum undergo little change when stored under airtight conditions, and the grain can still be used for seed. At the higher moisture content of 15-16%, anaerobic fermentation takes place after the metabolism of insects and moulds have depleted O₂ levels in the storage container.6 Anaerobic conditions have an adverse effect on the viability of grain, which loses its ability to germinate. Under wet storage conditions accompanied by anaerobic respiration, CO₂ levels exceeds 40% with very little residual oxygen; under dry storage conditions with less than 70% relative humidity and a high level of insects infestation, the oxygen level may fall to less than 2%.7 The rate of oxygen depletion increases infestation levels, temperature, and/or humidity increase.

9The sealed enclosure provides, among other effects, some protection against external pest attack. Under completely sealed conditions with a high level of infestation, less than 2% oxygen can be achieved in less than two days.8 With some leakage, high levels of infestation may reduce the oxygen concentration to such an extent that complete mortality occurs. At intermediate infestation levels, the oxygen concentration may oscillate as successive infestations build up and crash (see Figure 2).

Problems associated with pit storage

10The main problems associated with pit storage are accessibility of pits, loading and unloading, maintenance of the seal in storage structures, and safety of workers.

Accessibility to the stored products

11In underground pits, one of the main problems to be solved is access. It is necessary to load the storage rapidly and seal it quickly. For unloading, it is necessary to empty the pit at one time. If only part of the grain is removed, the remainder must use up the air that entered during handling. This may also permit further mould growth and even insect damage to the grain.

12The problems of grain handling greatly constrain the use of hermetic pit storage for either long‑term storage of dry grain or for the storage of higher moisture grain under temperate conditions where facilities do not exist rapidly to handle the grain.9 To overcome the problem of storage in pits by small producers, the use of barrels for grain storage has now become a rule all over the country.

Maintenance of the pits

13Unless the pit is adequately sealed, the stored grain may significantly deteriorate through the activities of pests and moulds. High rainfall results in caking, especially near the pit walls, and the level of grain damage may be very high; this circumstance may also discolour the grain and damage their bright colour and natural odour.

14To avoid spoilage of grains due to ground water infiltration, cracks in the pit walls should be plastered and that the walls should be lined with thick polyethylene welded sheets. 10

Safety

15The atmosphere generated during hermetic storage quickly becomes lethal to humans. Deaths have occurred after entry into newly opened pits containing moulding grain, due to very low oxygen and high C0₂ levels. Thus if a hermetic storage facility must be entered, it must first be well ventilated. A storage pit traditionally was considered safe to enter when its atmosphere would support combustion. For instance, a lighted candle or paraffin lamp lowered into a pit should not be extinguished. Sometimes it may be necessary to wait for several days after opening before a storage pit is safe to enter. Unstable walls can also be a problem for the workers.

Discussion and future of pit storage

16Pit storage is a residue‑free storage method that ensures long‑term preservation of grain without expensive control measures or energy input, and protects the farmer against seasonal fluctuations in grain prices. These are advantageous aspects for grain storage in Yemen, especially at the level of individual farms. The main advantages to the use of storage pits in Yemen are the simplicity of their construction and the irrelevance of insecticide treatment.

17Storage pits, if well sealed at ground level, do not appear to be at risk from insect attack. The lack of O₂ ensures that once atmospheric changes have eliminated pests, reinfestation is improbable. The main approach to achieving lower levels of O₂ and higher accumulations of CO₂ is fitting the pits with plastic liners.11 The more widespread adoption of the system is hindered by the inconveniences discussed above. With engineering development, it may be possible to overcome those difficulties.12

Figure 1. Diagrammatic structure of narrow necked pits (after Kamel, op. cit. 1980, and Gillman, G. and R. Boxall, The storage of food grain in traditional underground bins. Trop. Stored Prod. Inf. 28: 19-38, modified).

Figure 2. Variation of oxygen content with time at three different initial infestation levels of Sitophilus granarius adults sealed in a slightly leaky container (20°C, 70% r. h.) [From Banks, op. cit., 1981, after Oxley and Wickenden, op. cit., 1963].