Jean VALENZA *
(*) J. Valenza, Vétérinaire Agrostologue, Laboratoire d'Elevage, B.P. 2057, Dakar, Sénégal.
It can be considered that during the twenty years since the development of the " sylvopastoral " area of Northern Senegal (Ferlo) through the establishment of permanent watering places, the Sahelian/Sahelo-Sudanian natural grasslands have undergone on the whole few changes, if those due to rainfall are excepted.
Only the interior zones within a 3- or 4- kilometre radius around the watering points have been transformed or even improved, but not destroyed. There occurred a replacement of one pasture type by another at least equivalent in quality and value.
Since the new, grazed ecosystem is potentially unstable, the permanent study and the supervision in time and space of its evolution must be carried out with care.
Until about 1954/55, the sylvopastoral region of Senegal, sometimes called the " Ferlo Desert ", was visited by men and livestock only during the rainy season and at the very beginning of the dry season (July to November). The absence of permanent water points was the principal if not the only reason for their departure towards more favourable areas.
It can therefore be assumed that the vegetation was in equilibrium with the various components of the environment. When crossing this zone in 1953, Adam wrote: " It is remarkable in Senegal where groundnut cultivation has transformed the natural vegetation on sandy soils, to note, even at a distance from the water points, a climax, subclimax vegetation which has not been disrupted by cultivation, and hardly changed by grazing, since this has occurred only, until recent years, during the four summer months, where the vegetation is dense and close to standing water."
From 1954/55 onwards, deep boreholes into the upper Senonian stratum were drilled and put into operation in Ferlo, and the permanent availability of water for the animals permitted the year-round exploitation of the natural pasturelands of the Sahelian/Sahelo-Sudanian type. These boreholes attracted an increasing number of animals, causing a relative sedenterisation of populations and herds.
Better feeding conditions and strengthened preventive health measures led to an increase in the livestock population. The land was thus transformed, mostly around the boreholes; the approach to the boreholes is marked, from the beginning of the dry season, by progressive destruction of the herbaceous stratum, which becomes total at the edges of the holes. Adam wrote in 1957, " The development of the land will, in future years, disrupt the flora and vegetation groups... but it is very possible, for example, that contrary to what has been thought, the edges of the boreholes, instead of deteriorating because of trampling, will become a compact seedbed produced by the seeds contained in the dejecta of the livestock".
As a general rule, the exploitation of the pasturelands of Ferlo is currently as follows: the encampments of graziers are situated about 6 or 7 km at least from the boreholes; the animals for the most part are watered at pools of standing water, generally small ones, which form during the rainy season; they then graze the surrounding pasture. A very small number continue to come to the borehole. At the end of the rainy season, as the pools dry up, the animals arrive in increasingly large numbers at the borehole. From November/December they all come, and very soon watering only takes place every two days because of the distance between the grazing area and the watering place. It can thus be considered that the natural pastureland is exploited to a considerable and increasing extent from the month of October, to a radius of several kilometres around a borehole. In such conditions it is rapidly destroyed, giving the impression when one travels in Ferlo during the middle of the dry season that it has deteriorated. During the rainy season, on the other hand, the impression gained from general observation of the spontaneous vegetation within the same circle is the reverse; the herbaceous stratum appears to be more abundant, of better botanical composition, and of greater forage value. Progressively, as distance from the borehole increases, the pastureland is less and less heavily exploited, until exploitation practically ceases at about 15 kilometres. The vegetation can then be classed as climax vegetation.
When carrying out the "Study of Natural Pastureland in Northern Senegal" between 1969 and 1972, Dr. A.K. Diallo and I, using for the purpose of preparing a distribution map aerial coverage dating from 1953 - i.e. before the development of the area - felt that the vegetation characterizing each of the types of pastureland represented was then in balance with the various components of the new pastoral ecosystem, especially as the boreholes were on average 25-30 km apart; the influence of each on the vegetation as a result of heavy exploitation at the beginning of the dry season was limited to a circle of 5 km radius.
In his "Study of a Sub-desert Ecosystem" between the boreholes of Mbidi and Tatqui, Bille felt that " 10 kilometres should be regarded as a real limit and the only point at which it is possible to speak of quite modest deterioration of the plant formation if it exists".
Referring to the Labgal borehole, Naegele thought that "the deterioration of the natural vegetation of the edges of the borehole is not substantial. The deteriorated area forms a circle around the water point whose radius does not reach or exceed one kilometre ".
In September 1974, after the drought years which affected the Sahelian zone in Senegal to a greater or lesser extent, a more accurate study of the herbaceous stratum of four of the more widespread and best types of pastureland, focused on three boreholes, was undertaken. Below is a brief summary of the initial observations.
Regardless of the rainfall of the area, the type of pastureland and the type of subsoil, one may observe from the watering points, point 0:
- The presence of essentially nitrophilous vegetation, practically unconsumed by the livestock over a radius of 200-250 m
- A regular increase in the percentage of Graminaceae together with a fall in the percentage of leguminous plants up to 2 or 2.5 kilometres;
- A decline in the density of the vegetation up to the same distance;
- Farther on, a tendency on the part of the Graminaceae to fall in number in general, while the leguminous plants increase, each rapidly reaching the levels which characterize the type of pastureland;
- Farther on, too, the density rises, then stabilizes;
- Among the leguminous plants, there is variation, greater or smaller depending on the case, in the percentages of the two most important, Zornia glochidiata and Alysicarpus ovalifolius (the latter dominant over the first few hundred metres);
- Among the Graminaceae, the dominant species are replaced as distance from point 0 increases. Thus Cenchrus biflorus or Dactyloctenium aegyptium broadly dominate over a kilometre, gradually replaced, depending on the type, by Chloris prieurii, Eragrotis tremula, Aristida mutabilis and/or Schonefeldia gracilis; beyond 3-3.5 km those characterizing a type vary little. From 3.5-4 km, the few differences of botanical composition that are noted are most often due to the proximity of a camp of herdsmen or to topographical variations.
No systematic sampling of vegetation was carried out to study productivity and food value, but the few bromatological analyses which were made now show a higher total nitrogenous matter content in the dry matter gathered within a radius of 0.5-1 km than beyond it, reflecting well the fact that the standing leguminous plants are better-grown, even if they are fewer.
Such a situation is certainly the result of substantial and growing organic fertilization in the first few dozen centimetres of soil depth by the various animal dejecta. The effect is perhaps even more marked in dry years.
Is it then possible to speak of deterioration of the pasture, when the dominant species which make it up (principally Graminaceae) are replaced by others equally well consumed by the livestock, and when one finds a considerable proportion of apparently more productive leguminous plants? Transformation, modification of the pasture or replacement of one type by another are terms which fit better. The term deterioration should be kept for the area within a circle of 200-250 m around the watering points, which makes up only an infinitesimal part of Ferlo.
Yet it is certain that this facies of transformation, if not improvement, of the natural pasture will be destroyed very quickly at the end of the rains as a result of the rapid increase in the number of animals coming to the borehole and the rising intensity of trampling. Although it is difficult to quantify the percentage of this herbaceous biomass used by the livestock, it does not seem that it could exceed 1015 per cent. (In normal conditions, Bille estimates the fraction exploitable by the animal at 35-40 per cent at most.)
It is therefore essential that a rational method of exploiting this substantial stock of green matter of good or excellent forage value be instituted rather than lose it; cutting and tedding of this " improved " pasture represent a solution which seems applicable. This would be an excellent means of protecting if not recovering a substantial amount of the meat produced during the rainy season.
Accordingly, it appears that 20 years after the start of a pastoral water supply programme which is being added to each year, the natural pasturelands of the sylvopastoral zone in Senegal, and particularly those in the western or " sandy " part of Ferlo have undergone only slight modifications in their botanical composition, which are more likely to be the result of variations in rainfall (principally rainfall distribution) than of continuous exploitation by increasingly numerous livestock. Only within a radius of a few kilometres around the permanent watering points can the influence of the development be noted, and this influence may be regarded as beneficial for the herbaceous stratum alone.
We reject the term deterioration of pasture, and prefer to use the term transformation, replacement of one type by another, and perhaps even improvement.
But clearly it is not possible to be certain that this new ecosystem is in balance, and- it must always be feared that it will develop unfavourably, especially if no change is made in the methods of exploitation and management of the herds planned to be increased. It is absolutely essential to maintain a balance between the productivity of the pasture and the livestock it feeds. This productivity varies from year to year depending on rainfall, and any undue increase in the stocking rate, though praiseworthy, can only lead to imbalance which, if continued, could result in deterioration.
The drought of recent years has demonstrated the precariousness of this balance, this new ecosystem. It is thus important to monitor and supervise, before it is too late in whatever manner, variations and trends over time and space, in order to ensure that it is at least maintained.
1. ADAM, J.G. - Eléments de politique sylvo-pastorale au Sahel sénégalais - Contribution à l'étude floristique des pâturages du Sénégal. Service des Eaux et Forêts. Dakar, 1957.
2. BILLE, J.C. - Etude d'un écosystème subdésertique. L'écosystème sahélien de FETE-OLE: Essai de bilan au niveau de la production primaire nette annuelle (Etude ORSTOM - DAKAR-HANN) Mai 1973.
3. NAEGELE, A. - Etude et amélioration de la zone pastorale du nord Sénégal. F.A.O., 1971.
4. VALENZA, J. et DIALLO, A.K. - Etude des pâturages naturels du nord Sénégal. Rapport et cartes (Etude agrostologique I.E.M.V.T. n° 34), juin 1972.