|Animal Traction in Rainfed Agriculture in Africa and South America (GTZ, 1991, 311 p.)|
|E. Aspects of implement use|
A central problem for the utilization of implements, especially in the humid climatic zones, is the huge amount of organic material lying on the fields at the start of the cropping cycle. Fallow, both short and winter fallow that has a high weed infestation, or undecomposed harvest residues are the cause of this condition. This frequently leads to the practice of burning. Thereby not only nutrients but especially the organic matter content is reduced, which the cation exchange capacity and the stability of the aggregate maintains. To prepare the fields for subsequent soil preparation and further work operations the following processes are necessary, depending upon the climate and land-use intensity:
- clearing and removal of tree stumps and roots,
- chopping of vegetation from short fallow,
- management of harvest residues,
- working in or mulching of green manure.
In order to reduce the workload and to prevent burning animal-drawn implements are partially utilized. However, to date very few techniques exist at the level of animal traction to clear the fields.
Simple implements have been developed for the removal of tree stumps. For mulching weed material or green manure crops, especially in humid areas, only mowing bars, which to date have not been accepted in the regions investigated, and knife rollers exist, which are also suitable for processing harvest residues. The disc harrow (section E 5.3) can also be used for working in harvest residues. In South Brazil the knife roller is commonly found, both on motor-mechanized farms which practice no-tillage as well as on smallholdings with draft animals. Further experience has been made in Northeast Brazil, Tanzania and Cameroon, where they are however not widespread.
The knife roller consists of a roller made of wood or metal, upon which flat metal knives are mounted on the circumference. The principle of operation is that the knives of the roller bend over or chop off the stems of plants as it passes over them. The expended pressure depends upon the number of knives and their spacing, which in turn is determined by the circumference. If the number of knives is increased with the same circumference, then the pressure is reduced and the risk of clogging in the space between the knives increases. Further factors determining the efficiency of the roller are: working weight, construction material for the knives and mounting angle of the knives. The quality of the work is determined by the fiber and moisture content of the plants.
In South Brazil to date the knife roller has primarily been used
to chop residues of fallow. In the meantime, it is also recommended for the
mulching of green manure and chopping of harvest residues (maize stalks).
However, it cannot process all types of green manure, for example black oats. On
slopes of over 20 % and on stony ground the knife roller can hardly by utilized.
David (1988) states that the speed of oxen is too slow. This implement is
manufactured by the farmers
or by local artisans.
Trials have been made at IAPAR to improve the knife roller. The roller material (tree trunks, oil drums), working weight, method of applying weights, number of knives, circumference and cutting angle of the knives were tested. In the TIRDEP project in Tanzania the knife roller has also been tested for weed control, especially after fallow and in terrain having many roots. The roller proved to be essentially well suited for work in grass up to 3 m high following fallow and for weed control in permanent crops. Usually two working runs were sufficient.
For weed control after seeding, which would be feasible with a smaller implement in principle, the implement was less appropriate. With weed control in maize the spacing between the knives remained too high, also at a maximum number of knives, so that the weeds could not be destroyed at an early stage of growth. Furthermore, during this season the wet soil clogged the knives. For draft-animal use one model made of wood with assymetric application of draft power has proven to be suitable; it is arranged so that the animals need not walk directly through high plant growth. For the protection of the animals, especially on slopes, either the roller is covered or the attachment of a drawbar is necessary. The advantages with the drawbar are improved maneuverability, smaller headland (6 m instead of 10 m with chain) and easier reversing in the case of obstacles (Becker, 1987).
A knife roller developed in Northeast Brazil that weighed 70 kg proved to be too light. The recommended weight should be between 250 and 400 kg, depending upon the draft animals and the existing vegetation. It is advantageous to be able to adjust the weight. However, no moveable fill material should be used, e.g. water. This would reduce the quality of the work, leave tracks and cause an extra load on the animals. As an optimal solution Becker (1987) investigated an implement in Tanzania of 1 m diameter with 80 cm-long knives and 8 -10 knives made of tempered and sharpened steel -used leaf springs can be applied for this purpose - with a spacing of 25 cm at an mounting angle of 0 degrees. A sub-divided or flexible knife roller would be a more useful implement, but this would raise the cost considerably. (Figueiredo, 1988; Becker, 1987; compare Bertol and Wagner, 1987) The labour saved with a 3-year growth of grass (organic mass = 10 t/ha) as opposed to manual work is substantial: with the knife roller 6 days/ha vs. 70 days/ha by hand. (Becker, 1987).