A. Kader DIALLO *
(*) A. Kader Diallo: Agrostologist, Doctor of Veterinary Medicine, National Laboratory for Livestock and Veterinary Research (I.S.R.A. - Dakar-Hann).
The agrostological studies carried out in Senegal have allowed the drawing up of natural rangeland maps of almost all cattle breeding areas.
However, the methods used for the mapping of those rangelands have not afforded results which can be used in the elaboration of development projects in the areas studied.
In fact, the vegetation modifications resulting essentially from the action of various ecological factors varying from year to year, and the difficulty in estimating the food value of tropical pastures and in reading correctly the photograph screens at the botanical level, create problems in the establishment of the validity of the information given.
It is therefore very important to up-date the data already obtained and also to carry out further studies concerning the physiological aspects of animal nutrition.
Agrostological studies carried out since 1963 in Senegal have made it possible to draw up maps of more than 78,000 km²). of natural rangeland, representing a little over one third of the area of the country, which is estimated at 210,000 km²). Work currently being carried out will increase the area of rangeland studied to 96,000 km²). by 1977, at the end of the fourth four-year economic and social Plan.
Without doubt these studies constitute an impressive achievement of observation carried out by technicians who have a thorough knowledge of rangeland problems in the tropical zone.
However, while the data obtained represent documentation of undoubted scientific value, they cannot in their present form be used for the preparation of development projects in the areas studied. Moreover, they are not suited to the needs of field workers.
The methods used by the agrostologists in the study of the rangeland lead only to theoretical and often incomplete data, as a result of the short time (two years) allowed to the agrostologists for the execution of the work.
Well-known methods were used at all times in the inventory of the vegetation. The method most often used in Senegal and other African countries is based on the use of abundance/dominance factors, which are derived from the scale proposed by Professor Emberger in 1955 (2).
This scale runs from + to 5, as follows:
+: species present in isolated and rare instances,
1: species present in isolated but well distributed instances,
2: physiognomically abundant species, but occurring in less than 5 per cent of the area surveyed,
3: abundant species occurring in 50 to 75 per cent of the area surveyed,
4: dominant species occurring in 50 to 75 per cent of the area surveyed,
5: dominant species occurring in 75 to 100 per cent of the area surveyed.
Factors +, 1, and 2 indicate an abundance of minority species, while the others indicate the apparent cover of the dominant species.
Since this method is essentially subjective, it may be easily appreciated that the value of the results obtainable from it will depend on the conditions in which it is used by research workers. In other words, two research workers who have used the same method on the same type of vegetation will rarely arrive at the same phytosociological conclusions in their survey. This may make it impossible to recognise in the field the different types of vegetation which have been defined.
Also, the qualitative and quantitative composition of the vegetation are closely tied to the ecological factors of rainfall, man, and animals, particularly in the Sahelian and northern Sudanian zones.
In respect of rainfall, it has been noted that the length of the rainy season, and the quantity and distribution of the rain in time and space, have a marked effect on the herbaceous vegetation that forms the basis of feed for the animals.
These rainfall factors vary not only from one year to another, but also within the same year, from one zone to another. One might therefore agree with Mosnier (4) that " for the principal species, over the years and for a single rangeland type, the abundance/dominance factors are relatively different".
Man's impact on the vegetation also varies; it is manifested not only in the clearing of land for cultivation but also, most importantly, by the burning of the brush. Such fires impose changes on the vegetation, changes which vary in importance from one year to another, and according to the time they occur.
Following tests carried out at the Data Centre for Zootechnical Research in the Sudano-Sahelian zone J. Valenza (5) noted that " on rangeland consisting of Diheteropogon hagerupii and Zornia diphyla, early fires have little impact on the vegetation as a whole. At most they encourage the leguminous species, which are very sensitive to rainfall. Fires occurring late on the other hand, cause from the first year onwards an increase in late grasses, a sharp decrease in leguminous species, whose ratio quickly stabilises, and a progressive decrease in early grasses and other species ".
A combination of the impact of factors of climate and brush fires therefore causes a variation from one year to another in the floristic composition of the rangelands studied.
Animals also affect the vegetation. This may be observed near boreholes and encampments, as well as on the transhumance routes, where, depending on the soil type and the degree of concentration of animals, it is possible to witness the invasion of the herbaceous stratum by such species as Cenchrus biflorus, Trianthema portulacastrum, Cassia tora, Cassia occidentalis, Tribulus terrestris, Zornia diphyla, etc. The movements of encampments cause changes in the cattle routes, and may be the cause of a significant modification in the physiognomy of the vegetation of a zone which has already been studied.
This constant evolution of the vegetation, under the influence of ecological factors which at the moment are difficult to overcome, has made it impossible to put into practical use the rangeland maps that were drawn up in the conditions described above and which relate to the duration of the agrostological studies, which have so far been limited to two years.
Moreover, as many research workers have emphasised, calculation of the carrying capacity of natural rangeland comes up against the problem of lack of data relating to the exploitation of the forage produced on these rangelands by the animals.
The calculation of the forage value and digestible nitrogenous matter content of the species of vegetation, made using Dutch tables, often leads to incorrect results and to conclusions which observation in the field does not always bear out.
This is particularly true in the evaluation of the carrying capacity of natural rangeland in the dry season. At that time of year these grazing lands are made up for the most part of straw whose nutritive value, according to the Dutch tables, is almost nil, so that the grazing land ought not to be sufficient for the essential needs of the animals. Nevertheless, it can be observed that although the animals lose a large part of the weight gained during the winter season, they manage to survive and even to reproduce.
We may therefore say that our rangelands certainly do not have the same characteristics as the rangelands of temperate countries, and that our animals have different powers of assimilation from those of European cattle. This difference is seen particularly in the utilization of cellulose. It is also necessary to point out that it is impossible at our present state of knowledge to evaluate correctly the quantitative contribution of the lignaceous species which make up part of the animals' nutrition.
The adaptation of zebu cattle to the severe climatic conditions of the Sahel relates without doubt to special physiological characteristics about which little is yet known.
With regard to cartography, mention should be made of the fact that because of a lack of recent aerial photographs, maps were drawn up using old documents which in many cases did not show the state of the vegetation at the time of the study. This makes work difficult for the agrostologist in the field.
In addition, the choice of shots is subjective, and there will be differences depending on the interpreter. Finally, it sometimes happens that photographic screens which appear identical correspond to different types of vegetation.
It is clear from the above that in the preparation of development projects for the zones studied, it is essential to check the data obtained upon completion of studies already carried out and to up-date them if need be.
At all events, in-depth studies should be undertaken in order to increase our knowledge of the floristic composition of our rangelands and the evolution of these rangelands under the influence of different ecological factors.
In addition, research work should be intensified so as to increase our knowledge of the special physiological characteristics of our animals and to draw up tables of food values, without which I feel it is impossible to estimate the nutritive value of our forage.
1. BOUDET, G. and BAEYENS, F. - A method for the study and mapping of tropical rangeland. Rev. Elev. Med. vet. Pays trop., 1963, 16 (2): 191-219.
2. DIALLO, A.K. - The natural rangelands of Ferlo-sud (Republic of Senegal). Maisons-Alfort, I.E.M.V.T., 1968. 173 p. (Etude agrostologique n° 23).
3. MAINGUY, P. - Tropical grasslands - Synoptic review of principles of methods of study - Application of vegetation sampling. Rev. Elev. Med. vet. Pays trop., 1958, 11 (3): 305-338.
4. MOSNIER, M. - The natural rangelands of the Gallayel region (Republic of Senegal). Maisons-Alfort, I.E.M.V.T.. 1967. 133 p. 1 map. (Etude agrostologique n° 18).
5. VALENZA, J. - Dynamic study of different types of natural rangelands in the Republic of Senegal. 11th Int. Congress on Rangelands, Surfers Paradise, Queensland, Australia, 13-25 April 1970.
6. VALENZA, J. and DIALLO, A.K. - Study of the natural rangelands of north Senegal (Republic of Senegal). Maisons-Alfort, I.E.M.V.T., 1972. 311 p. (Etude agrostologique n° 34).