5.7 Losses caused by rodents

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Rodents cause food loss by consuming grains and also by contaminating far more than they consume. They also spread diseases which may be transmitted to humans.

Three species of rodents are major pests of stored products:

• Rattus rattus (black rat) and Rattus norvegicus (brown rat)
• Mus musculus (house mouse)
• Praomys natalensis (multi-mamma/e rat)

Rats become active after dark or when the premises have become quiet. Black rats and brown rats have a habit of following set routes as they move between the stored product, the source of water and their normal resting place. After some time these routes become marked by greasy traces and are easily identified. This habit also means that rats will avoid unfamiliar objects such as traps or poisoned food, particularly when they are first laid down.

Clues to the presence of rats include:

• droppings
• loose earth from burrowings
• footprints on dusty floors
• greasy marks on set routes of travel, e.g., on beams or along electrical wiring
• holed sacks with grain escaping
• gnawing damage to building fabric

5.7.1 Control methods. Eliminating a single rat from a domestic house is very different from controlling a large number in a group of storage warehouses. Knowledge of the habits of rats is important in establishing effective and economic control measures.

The most effective control is to prevent access of rodents to the store. This can be achieved by "rat-proofing", preferably as the warehouse is being constructed, but also as a feature to be added later.

The main methods for controlling an established rodent population fall into the categories of mechanical and chemical.

The principal mechanical method of control is by trapping. The cage trap is preferable for godowns and should be set on the rat run. After being placed in position and left open for several days, unbaited and unset to overcome the rats' shyness of new features, the trap should be baited with food attractive to the rat. This ensures maximum results.

Figure 5.6 Rat proofing storage warehouses

Figure 5.6 (cont.) Rat proofing

The chief chemical control is by poisoning, either through a single dose (acute poison) or a multiple dose (chronic poison).

Single dose. Zinc phosphide is the most widely used. Two stages are essential for effective control.

(a) Pre-baiting. The sites, baits and containers should be the same as those to be used for the poison at the next stage. The more attractive the bait, the more successful the control. Cooked rice, soaked wheat or maize, and flour mixed with syrup are attractive baits. Pre-baiting should continue for three to four days; freshly prepared bait should be provided each day.

(b) Baiting with poison. One part of zinc phosphide is mixed evenly with 20 to 40 parts of a bait similar to that used for pre-baiting. The special containers used for pre-baiting should then be furnished with the poisoned bait and left at sunset in the same positions as those of the pre-bait containers. The next morning the remaining poisoned bait should be removed from the containers and destroyed. The containers should then be replaced, after loading with pre-baiting (nonpoisonous) material. If this is eaten it indicates that further control measures are needed and the whole operation must be repeated. Dead rodents should be removed every day.

Figure 5.7 Mechanical control of rats

Multi-dose chronic poisons. These are generally blood anticoagulants causing death by internal bleeding. Their main advantages over single-dose poisons are as follows:

(a) rat colonies are not alarmed because deaths appear to be from natural causes, and they will continue to ingest the poisoned bait, ultimately giving a better overall control than single-dose poisons;

(b) such poisons do not give rise to bait-shyness, and no pre-baiting is necessary; and

(c) such poisons are used in very small quantities, and they are slow-acting, therefore presenting less risk of accidental ingestion by humans and domestic animals.

The manufacturers' instructions for anticoagulants should always be closely followed, and the bait containers located where only rodents have access. Rats are killed in about ten days, although for mice it may take 20 days. Affected rodents seek fresh air and water and therefore generally emerge from the store to die. Carcasses should be disposed of carefully because any anticoagulants remaining therein will affect scavenging animals.

6. Drying

6.1 Introduction

Cereals are annual crops grown for their edible starchy seeds. They are humans' main source of carbohydrate. The natural sequence of events is that the grains mature, ripen, become dormant and are then scattered on the ground where they eventually germinate and produce a new plant.

This sequence of events is successfully interrupted when the seeds are harvested, dried and stored. Their metabolic activity is thus reduced to such a low level that they do not deteriorate significantly; but the grains have to be protected against losses which may arise from other causes.

Temperature and moisture content are the two factors we have learned to use to control deterioration during storage. Combinations of these factors which have proved to be effective for safe storage against specified agents are shown in Figure 6.1. The total combined effect of these factors is shown in Figure 6.2.

In the tropics, temperature control is too costly, so that drying remains the most cost-effective method. Even at low moisture levels, however, stored grain is not safe from deterioration caused by insects when the temperature is above 15°C.

Many different methods and systems are used for drying and storing grain. To evaluate them for the farmer, village or cooperative, the basic principles involved in the exchange of moisture between the air and grains must be understood.

6.2 Air and water vapour: psychrometry

Atmospheric air consists of dry air and water vapour. For example, in Ibadan, Nigeria, at noon on a day in mid-October, 1 m³ of atmosphere contained 1 131.2 g dry air and 20.36 g water vapour. The dry air and the water vapour were evenly distributed throughout the 1-m³ space. The temperature was 30°C and the relative humidity was 66 percent.

The quantity of water vapour present is usually expressed as a proportion of the dry air present in the same volume as the water vapour, and is known as the specific humidity (s). In this case, the specific humidity was as follows:

Figure 6.1 Combinations of moisture content and temperature providing safe storage conditions for bulk-stored grains. Source: K.A. McClean. Drying and storing combinable crops. Farming Press Ltd. 1980.

Figure 6.2 Combinanons of moisture content and temperature providing safe storage of grain against the four main agencies. Note: Area at risk from mites is shown separately because mites are not always a problem

TABLE 2. Air and water vapour in 1 m³ of space

^a See Figure 6.4.

No rain fell on that day, and by 15.00 h the air temperature had risen to 35°C. The specific humidity was still 0.018 because there had been no change in the amount of water vapour, but the relative humidity had fallen to 49 percent. (The small change in density of the air/water vapour mixture caused by these changes is ignored.) Was it possible that the air was now drier?

In effect, the relative humidity (rh) measures the percentage saturation of the space by water vapour; and because warm air can coexist in the same space with far more water vapour than cold air (see Table 3), the rh (or percentage saturation) has fallen, although no water vapour has been removed from, or added to, the space. The temperature only had risen. The space is capable of holding far more water vapour at 35°C (see Table 3).

By 22.00 h the air temperature had fallen to 24°C and the relative humidity had risen to 95 percent, although the specific humidity remained at 0.018 kg water/kg dry air.

The relationships between temperature and humidity are complex but may be represented by mathematical equations, which allow the effects of change in any of the factors to be calculated, or by a psychrometric chart. This chart is in the form of a graph, the axes of which are temperature (horizontal) and specific humidity (vertical). Values of relative humidity are represented by a series of curved lines. A simplified form of the chart is shown in Figure 6.4 where the three points representing the situations at noon, 15.00 and 22.00 h are plotted, as already discussed. Using the chart, refer back to Section 6.2 and check the values of relative humidity, specific humidity and temperature at each point.

The fourth scale of figures on the chart represents the wet-bulb temperature and is used to determine the working point on the chart, because it is easier to measure temperature than the other factors involved.

TABLE 3. Water vapour carrying capacity of atmospheric apace at various temperatures and normal pressure

If the space is not saturated then it can, of course, accept more water vapour. If a piece of clean cotton cloth is made wet with distilled water and air is forced to move past it, some water will evaporate from the cloth. This evaporation requires energy which is absorbed from the wet cloth, resulting in a fall in its temperature. Equilibrium is soon reached when the temperature of the cloth remains steady. This is the wet-bulb temperature, a term derived from the practice of enclosing a thermometer bulb with the piece of wet cloth. Dry-bulb temperatures and wet-bulb temperatures are read from thermometers mounted side by side in a whirling hygrometer. From these two readings, all other characteristics of the air/water relationship may be established from the chart. The wet-bulb depression is the difference between wet-bulb and dry-bulb temperatures and may be used to find relative humidity from tables.

Figure 6.3 Air and water vapour in 1 m³

Figure 6.4 Psychrometric chart. Source: Chartered Institute of Building Services (London, UK)

TABLE 4. Weight of water lost from wet grain when dried (g/kg)

The important points are summarized as follows.

(a) Ambient air consists of dry air and water vapour.

(b) The ratio of water vapour to dry air is very small.

(c) If the actual water content of the space occupied by ambient air does not change, then relative humidity is reduced as temperature is raised; and is increased as temperature is lowered.

(d) These relationships can be represented by a graph known as the psychrometric chart.

6.3 Moisture content and relative humidity

The moisture content of cereal grains exposed to ambient air changes continuously in response to the relative humidity (rh) of the air. The greater the rh, the greater the moisture content of the grain. The changes take place relatively slowly, but given sufficient time a new value of relative humidity is maintained and a near-equilibrium value of moisture content is reached.

This characteristic of an equilibrium (equilibrium moisture content referring to grain and equilibrium relative humidity referring to the air surrounding the grain) is extremely useful because it can be applied in practice to adjust the moisture content of grain during drying and storage.

The equilibrium moisture contents for a wide range of grains have been measured, some of which are given in Table 5. These have been determined by exposing grains to atmospheres of different relative humidities and measuring the moisture content of the grain after several weeks of exposure. Obviously, many other factors are involved in determining a grain's equilibrium value, but these tabulated values serve as a very useful guide.

Figure 6.5 Water movement

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