This chapter has voluntarily been limited to the most common mechanized operations such as transport, tillage, secondary cultivation and sowing. Nevertheless, animal power also can be used for water-lifting or harvesting (e.g., groundnut lifting). For further details on the mechanization of such operations, see the many manuals available [1–4, 6, 13].
Equipment for Transport
In rural and urban areas, the daily transport of people and goods is a highly important activity. Various means are used: riding, pack and cart transport (two-wheeled carts or four-wheeled trailers), and sledges.
Riding donkeys, horses and camels is common throughout the Mediterranean and sub-Saharan areas. Riding donkeys without any harnessing system is predominant be- cause of their high hardiness. Oxen also may be employed, as in Mauritania and Chad.
Camels and horses are more prestigious. They are harnessed with specific saddles locally manufactured.
Carrying loads on the back of animals is very common throughout all the tropi- cal areas. Loads vary between 80 and 100 kg in weight for donkeys, to 300 kg for dromedaries. Three methods can be applied: bulky loads are directly placed over the animal’s back and held in place with ropes; rigid materials (stones, firewood, water containers) are transported on pack saddles with a wooden frame fitting on protective paddings; and in-bulk products are placed in symmetrical pannier baskets over the ani- mal’s back (Fig. 1.15).
In certain areas of southern and eastern Africa and Madagascar, artisan-made wooden sledges are used. Their main advantage is that they are cheap, being easy to make and maintain. They are narrower than carts, have a low center of gravity, and can be used on steep, wet, or unbearing ground. Nevertheless they cannot be recommended on tracks because the tractive force required is higher than for a cart, and repeated passages accelerate erosion. That is why they have been banned in some hilly countries such as Lesotho and Zimbabwe. Using a sledge, a pair of oxen can carry a load of about 200 kg, at 0.8 m per second, over several kms.
In the tropics, animal-drawn carts are the most widely used equipment for transport.
In rural areas, farmers, artisans and traders employ carts for domestic needs (water and firewood), agriculture (seeds, fertilizers, manure, harvest), trade and social purposes.
Figure 1.15. Using horses and donkeys for carrying a load or carting. Source: [11].
The load capacity of a cart (the load a cart can carry across country without distortion or breaking) is 500 kg with a donkey and 1,000 kg with a pair of oxen. The tractive force Tr required to move a cart is the product of the total weight P of the load (load capacity+dead weight), by the rolling coefficient K (which varies with the soil surface state) and the slope i (slope coefficient in percent), that is: T r =P x(K+i ), where T r
and P are in kg. A braking system is required in hilly areas. Carts may be fitted with wooden or steel wheels. But wheels fitted with pneumatic tires–similar to those on light power-driven vehicles–are becoming increasingly common.
In the past, wheels with wooden spokes and metal rims were used on all carts. Made today in Northern Africa, Egypt and Madagascar, this type of wheel is rather uncommon in other countries.
Metal wheels are being abandoned in many areas. They have been introduced for transport purposes in the sub-Saharan wet areas (having high ground clearance, well- suited to negotiate ruts and holes), and are still used in the sandy areas where spiky bushes can cause problems of punctures with pneumatic tires.
Two-wheeled carts with pneumatic tires are the best suited to satisfy the varied needs and situations. Tired wheels require a minimum tractive force as their rolling coefficient is optimum–above 35% on dry track and up to 64% on muddy ground.
Tip-carts with tip-up boxes facilitate the unloading of materials such as earth or manure but are little used because of their high manufacturing cost.
Four-wheeled carts (trailers) can be found in southern African countries, in urban areas and near cities (Bamako, Cairo). They also are used on some plantations. They allow heavy loads up to 3 tons because the animals only support a very small part of the load weight.
Equipment for Tillage Under Dry Conditions
Under dry conditions tillage objectives are as follows:
• To loosen the soil for creating conditions conducive to in-depth aeration and water movement, which promotes root growth, facilitates rain water infiltration, and achieves convenient conditions for germination.
• To control weeds by uprooting, burying, or even promoting weed emergence for later destruction.
• To plow in fertilizers and soil improvers and mix them with soil.
Choosing a tillage method depends on the type of soil, ground moisture, and on the cultivation system applied. For cultivation in flat ground, tined implements are suitable on dry sandy soil, while plowing is essential on wet, silty soils. For bedding or ridge cultivation, the use of a plow or a ridger is essential.
There are four main categories of operational sequences to be performed prior to planting:
• Tine tillage on dry soil (subsoiling) which can be done before surface tillage and plowing operations. Currently tested in tropical areas.
• Tine tillage on the row.
• Surface tillage on dry or wet soil, in a single or several passes, with tined or rotary implements.
• Plowing on wet soil with or without subsequent cultivations.
Subsoiling under dry conditions consists of breaking up the surface layer of soil and penetrating to an average depth of 7–10 cm with a rigid tine (Coutrier) developed by
32
Figure 1.16. (Continued)
Centre d’Etudes et d’Exp´erimentation du Machinisme Agricole Tropical (CEEMAT) as to favor infiltration of the water of the first rains. This technique is particularly suitable for semi-arid areas where soil sealing and the crusting of silty soils prevent the water of the first rains from infiltrating, and prevent traditional tools with cutting edges from penetrating. The draft force required for the operation can be supplied by a yoke of oxen.
This results in a cloddy profile with a pass every 50 cm (10–12 hours per ha) to a depth between 8 cm in crusting soils, and 20 cm in sandy soils. On the other hand, the technique makes it possible to stagger the operations of land preparation before sowing.
Ripping [1] consists of working on the row with a subsoiling tine mounted on a plow frame or a toolbar (Fig. 1.16). Compared with plowing, this technique results in valuable
time savings because of highly reduced working times and also because it makes it possible to work earlier at the beginning of the rainy season. Combined with a seeder, such equipment may be used for direct seeding in light soils.
The main advantage of soil preparations by surface tillage without soil inversion is that they make early sowing possible because of the limited working time required. The implements used loosen the soil up to 8–10 cm in depth by breaking it up into small clods without excessive fine soil. As they do not turn the soil over, they do not bury the organic matter, but mix the surface layer and so destroy the weeds. They can be used prior to seeding for direct soil preparation or to complete plowing, or after seeding for weeding operations.
Three main types of surface tillage implements are available: with a point (ard plows);
with tines (cultivators and harrows); of the rotary type (disk harrows).
Ards, dating back to antiquity, are still widely used in North Africa, Ethiopia, Central America and some regions in Asia. In ancient times, they were entirely made of wood.
Today, steel is used for some working parts.
Tined implements include harrows, cultivators and their derivatives (e.g., scarifiers, weeders, hoes).
Harrows consist of a steel or wooden frame mounted with rigid tines. Rarely used among African farmers, harrows are used to complete the work made with a plow. They crush the clods, level the loosened surface layer and provoke light compression on the disturbed layer. Light harrows (25–30 kg) comprise 20 tines for a 1m working width.
Working times are between 8 hours and 15 hours per hectare at a working depth between 3 cm and 7 cm.
While harrows have wide shares, cultivator shares are generally narrow. According to the species and number of draft animals used, they can be differentiated by their width (determined by the type and number of tines) and their working depth. Cultivation tools are commonly mounted on multipurpose toolbars used for tilling, weeding and ridging purposes (Fig. 1.16).
Rolling implements (disk harrows) are rarely used with draft animals.
Soil preparations with soil inversion (plowing, ridging) consist of turning over the topsoil, between 10 cm and 25 cm in depth, plowing in plant residues, and destroying the weeds. Soil inversion results in an irregular and slumpy soil, which must be further prepared carefully with other implements such as harrows or cultivators to obtain a convenient seedbed.
Plowing generally is done with a simple moldboard plow, and ridging with a ridger or ridging plow (Fig. 1.16). There are various designs of plows in service. They mainly differ by the species and number of draft animals used. A donkey can pull a plow between 25 kg and 30 kg in weight with shares between 15–18 cm in size. A pair of oxen of 600–
800 kg can pull a plow weighting more than 35 kg, with shares between 25–30 cm in size.
Working times are about 20–40 hours per hectare.
In certain regions, ridging substitutes for plowing. Ridging consists of combining two inverted furrow slices to make a backfurrow, or ridge, and an open furrow. The center of
the backfurrow is not loosened and this can later interfere with root growth. The ridging time with a simple plow or a ridging plow is about 10–15 hours per hectare.
Under Irrigated Conditions
In all irrigated areas, rice cultivation prevails because rice offers good tolerance to flooding. Soil preparation of rice fields depends on the water control level (zero control, partial control, or full control), and also on the planting method applied (dry sowing, wet sowing, wet transplanting). For dry sowing, the tillage methods described earlier in Section 1.1.7 also apply. For wet sowing or transplanting with water control, soil preparation generally consists of a plowing operation with subsequent cultivations using harrows or other implements on flooded rice fields to obtain a smooth and level mud layer for efficient destruction of weeds.
Where clay soils prevail, plowing with a moldboard plow–very difficult with draft animals under dry conditions–is done under wet conditions but on dried swamps. For plowing in flooded fields, light plows of Asian design (described as reversible turn-wrest type) can be used. They have a triangular share and a slatted moldboard with slats that can move separately. Their working width is between 20 and 30 cm (Fig. 1.17).
The final seedbed preparation can be done in dry soil with harrows. Nevertheless the draft effort required is high in clay soils found in rice cultivation. Working times are about 12–15 hours per hectare.
Puddling on flooded soil requires specific equipment. Comb and rotary harrows are used widely in Asia but little in Africa (Fig. 1.17). In heavy and sticky soils, rotary harrows can be particularly effective because their working parts are rotating. Their working time is about 8–10 hours per hectare.
Sowing
Sowing takes place in the same period as soil preparation, or just after. Conversely to hand sowing, mechanical sowing makes it possible to achieve a good regularity of seed distribution. The objective is to place seeds in optimal conditions to use natural resources (light, water, minerals), and achieve a uniform distribution of seeds.
Sowing density and pattern depend on the varieties and species concerned and on environmental conditions.
Sowing effectiveness varies with the conditions of seedbed preparation (importance of tillage), and use of sowing implements (depth adjustment, row and seed spacing, appropriate choice of the feed system).
Using animal-drawn seeders may be impossible, for instance, in ridge cultivation be- cause of the lack of appropriate equipment. It proves also poorly suitable for mixed crop- ping (different seed sizes and sowing densities). Among the various sowing techniques applied with cultivation in flat ground (the most common method of soil preparation), some can be performed with animal-drawn implements as dibbling and row seeding (or drilling).
Broadcasting is a manual or power-mechanized operation.
Dibbling, i.e., single or several seeds dropped together and buried in a same pocket, may be done manually or with animal-drawn single seeders.
Figure 1.17. Some implements for tilling on flooded soils.
Drilling can be carried out with multiple-row seeders (3 to 9 rows). With such equipment, it is possible to control row spacing, seed rate and covering. These seed- ers are of little use in sub-Saharan Africa because of their high cost and also because soil preparation and seeds are poor in quality. For a seeder with a 1–1.5 m working width, pulled by a pair of oxen, the working time is between 3 hours and 5 hours per hectare.
Precision seeding assumes evenness in seed or pocket spacing in the row. In sub- Saharan Africa, one-row seeders pulled by a single animal are the most widely used implements for dibbling grain crops (maize, millet, sorghum), and also with single seeders for groundnut and cowpea (Fig. 1.18). Light, simple, robust and low in cost, these seeders require good working conditions: conveniently loosened soil, without weeds or
Figure 1.18. Animal-drawn sowing equipment.
plant residues; well adjusted equipment, and appropriate feed system; clean, sorted and graded seeds; and a 0.8–1.1 m per second working speed. Pulled by a donkey or a horse, their working time is between 4 hours and 6 hours per hectare. With several draft animals, two or more seeders can be combined.
Direct seeding consists of dropping the seeds into a furrow open with a coulter or a disk, without previous seedbed preparation (Fig. 1.16). Although rarely used in ani- mal traction, the technique is increasingly appreciated in light soils by cotton compa- nies in southern African countries (e.g., Zambia, Zimbabwe, Tanzania), and in Brazil as well. Because good weed control is required, herbicides are used in high rainfall areas.
Secondary Tillage Operations
The less suitable the soil preparation and sowing, and the later the weeding operations, the faster weeds invade crops.
To delay the emergence of weeds and limit their spread, farmers must control weeds as soon as possible at the various stages when effective weed control means can apply:
• At seedbed preparation, prior to seeding, by uprooting and burying weeds (see Section 1.1.7).
• At sowing, by using seed densities resulting in plant populations hindering the growth of weeds (see Section 1.1.7).
• At young plant level, by destroying weeds.
Using animal-drawn equipment for weeding row crops is an easy and economic method available to many farmers.
During growth of the sown crop, two main techniques can be applied: harrowing at an early date (early weeding) and interrow weeding and cultivation.
Early weeding makes it possible to limit the development of the young weeds sig- nificantly and makes them easy to remove. Spring tine harrows are used to perform this shallow cultivation (1–2 cm deep).
Secondary tillage operations must be repeated to loosen the part of the topsoil that promotes root development, and to control weeds. Interrow weeding, which consists of
breaking up the topsoil crust (3–5 cm), favors infiltration of rain water and limits surface evaporation. Weeding operations consist of cutting weeds at a small depth with cutting working parts. In fact, both operations are often combined in a single implement. Several passes are required.
Under dry conditions, mechanized secondary tillage operations make it essential that regular interrow spacings are wide enough (more than 0.5 m) for the passage of an animal. The work performed, most frequently limited to a single interrow, corresponds to a working width between 0.3 m and 0.8 m suitable for the draft force of a single animal in light soils, or for a pair of animals in heavier soils. Interrow cultivators or weeding tines attached to multipurpose toolbars are used (Fig. 1.16). The types of tines and shares depend on the operation to be done and soil conditions:
• In light soils, straight rigid tines with wide shares are used for weeding purposes, with narrower shares for hoeing.
• In clay soils, spring tines with duckfoot shares, assuming high penetration, are preferred.
• In sandy clay and gravelly soils, spring loaded tines with narrow shares are recom- mended for hoeing operations, with wider shares for weeding.
Interrow working times are between 10 hours and 15 hours per hectare, according to animals and crops.
Additional cultivations such as earthing-up, which may be applied to strengthen the bottom of plants with soil, also have a weeding effect on interrows as they cover weeds to some extent.