Previous Page Table of Contents Next Page

Problems posed by the evaluation of the browse potential of the Sahel zone


(*) B. Peyre de Fabrègues, Agrostologist - I.E.M.V.T.

Exploitation of this vegetation by animals
Value of the grazing lands


As soon as the rains end, the herbaceous vegetation alone ceases to maintain cattle. Observations prove that the animals compensate for this by making use of woody shrubs whose qualitative value is well known.

The quantitative value of this browse is practically unknown and difficult to measure.

The importance of the contribution provided by the ligneous components of the forage makes the constitution of experimental protocols necessary to estimate the productivity of the shrubby stratum. This will result in suggestions for improving productivity.

The Sahelian zone has a rainfall averaging from 200 up to 400 millimeters during a rainy season lasting one to two months. As regards the vegetation, it is characterized by a vegetal cover consisting of two strata:

1) A herbaceous stratum, very largely dominated by annual grasses, with a height reaching 0.50 to 1 meter at maturity, producing 0.5 to 1.5 tons of dry matter per hectare, and having an active life between July and September only.

2) A ligneous stratum, dominated by thorn-bushes (Acacia spp.). These trees and bushes may be thick or sparse according to the soils, and their active period of growth, generally concentrated during the rains, may be displaced or even reversed (e.g. Acacia seyal blossoming in the cool season, Acacia albida defoliated during the rains).

Exploitation of this vegetation by animals

Owing to the sparsity of the precipitation and the long duration of the dry season, this zone is indubitably pastoral.

Observation of the way the animals behave when grazing shows that, according to the species, they use either one or the other stratum of vegetation more or less preferentially. So, goats and camels make more use of the ligneous stratum than do the sheep and cattle. But these preferences may vary according to the season.

At the beginning of the rains the animals find residual straw from the previous year (often in a poor state of preservation) mixed with young shoots of new plants (especially annuals) and new sprouts on the ligneous vegetation, sometimes fairly abundant.

Then the grasses rapidly reach the stages of active stem growth, flowering, and seed formation; and at the same time their amount of dry matter increases, whereas their protein value decreases. The shrubby stratum has a more regular production, and the qualitative value varies very little.

As soon as the dry season begins, the graminaceous layer yields no more than dry straw, with or without seed-heads, more usually already fallen, together with rare perennial grasses still growing. The trees reduce their production but still furnish some green vegetation.

As the dry season continues the situation deteriorates, the grass straw losing little by little its volume and value, more and more bushes shedding their leaves; it is only at the very end, before the rains, toward the month of May, that buds appear again on woody shrubs. The animals feed hungrily on them, whenever they have the opportunity to do so.


Observation of grazing bovids shows, on the one hand, that among the four domestic species they are the ones which make the greatest use of the herbaceous layer, but on the other hand they search for a part of their ration in the leaves, flowers and fruit of the woody shrubs, to a greater and greater extent as the dry season continues.

What is the proportion of the ration provided by forage produced by woody bush?

The participation of woody shrubs in making up the ration is, as a matter of fact, essential and indubitable, but measuring it is particularly difficult

A. - The nutrition needs of bovids

According to Boudet and Rivière (Rev. Elev. Med. Vet. Pays trop., 2, 1968) the reference animal is the T.L.U. (Tropical Livestock Unit). It is an adult bovid of live weight 250 kg. Its needs were evaluated as 2.3 F.U. (Forage Unit) and 125 g Digestible Protein per day for maintenance. The F.U. and D.P. must be contained in the ration of 6.25 kg dry matter (D.M.), because in order to have a satisfactory space-filling coefficient for digestion, the animal must ingest 2.5 kg D.M. per 100 kg of live weight.

Any F.U. or D.P. in the ration exceeding these values will be used for movement or production (meat or milk).

These needs for maintenance, movement and production (whether cumulated or not) can be expressed per kg D.M. of food given to the animal.

This ratio R.E. is the Ration Equivalent = total needs/6.25. It allows us to compare directly the nutritive value of food (theoretical value given by analysis) expressed per kilo D.M. with the requirements of the animal considered. The ration-equivalent will be:

- for maintenance of the T.L.U.: 0.37 F.U. and 20 g D.P./kg of D.M.
- for 7 km of movement per day: 0.64 F.U. and 4.6 g D.P./kg of D.M.
- for 100 g of weight gain: 0.053 F.U. and 4g D.P./kg of D.M.
- for 1 litre of milk: 0.061 F.U. and 9.6 g D.P./kg of D.M.

These data are summarized in the table below.

B. - The qualitative nutritive insufficiency of the herbaceous plants in the Sahel

In relation to the state of the herbaceous layer, whose production is the main source of " ballast " in the ration of the bovids throughout the year, four periods may be distinguished:

- Beginning of the rains: a period of young grass, before the period of active shoot growth (D.M. less than 50 %).
- End of the rains: a period of natural hay formation (D.M. greater than 50 %).
- Beginning of the dry season: very dry hay and straw (D.M. greater than 90 %).
- End of the dry season: very dry straw and damaged debris.

The analysis of the nutritive value of the herbaceous production during these periods shows that bovids cannot survive if they are supplied with this food source only.

Value of the grazing lands

The Animal Nutrition Laboratory of I.E.M.V.T. has carried out a great number of analyses of Sahelian forage plants, among which we selected the graminae which are the main component of the pastures used by bovids, by grouping them according to the above periods.

The weighted means of the results of these analyses give the following values:

Beginning of the rains

5 analyses of hydrophilic graminae, at 22 % D.M. average value: 0.69 F.U. and 122 g D.P./kg D.M.

18 analyses of graminae from sandy dry soils, at 33 % D.M. -average value: 0.53 F.U. and 47 g D.P./kg D.M.

End of the rains

14 analyses of graminae from sandy soils at 61 % D.M. -average value: 0.48 F.U. and 26 g D.P./kg D.M.

Beginning of the dry season

37 analyses of graminae from sandy soils at 95 % D.M. -average value: 0.39 F.U. and 1 g D.P./kg D.M.

End of the dry season

22 analyses of graminae from sandy soils at 95 % D.M. -average value: 0.35 F.U. and 1 g D.P./kg D.M.

The calculation of the energy value (F.U.) and the values of D.P. has been made using Dutch tables, according to the method proposed by Boudet and Rivière. For the D.P. the use of Demarquilly's formula (INRA 1974) leads to the following results: 108.44 grammes of D.P./kg D.M. (beginning of the rains), 13 gms/kg (end of the rains), and traces (dry season), respectively; that is to say, still lower values of D.P. than those calculated above. (Demarquilly's formula: D.P. (in g/kg/D.M.) = 9.29 D.P. (% D.M.) - 35.2).

Simple comparison of these values with the requirements of the animals shows that grasslands can assure production only at the youngest stages of growth; they are just sufficient for maintenance by the end of the rains and are certainly not a sufficient food during the greater part of the year, corresponding to the dry season.

On the other hand, it can be claimed that they are quantitatively sufficient because of their relative abundance (0.5 to 1.5 ton D.M. per hectare) and their easy grazing.


Short-range movements (rainy season)

Long-range movements (dry season)





Maintenance and Movement





M + M + 100 g weight gain





M + M + 200 g weight gain





M + M + 500 g weight gain





M + M + 0.5 1 milk





M + M + 2 1 milk





M + M + 5 1 milk





(F.U. expressed in units per kg D.M.)
(D.P. expressed in g/kg D.M.)

C. - Quantitative dietary insufficiency of bush grazing

Through the year, the vegetative state of the edible woody species is relatively varied, so that it may be considered that animals are always able to find some green, fresh leaves if there are some bushes within their reach. But the quantity of food they can make use of is itself very variable. Abundant during the rains, when it is very little used, for they prefer grass, this production becomes rare during the dry season, becoming at the same time more and more difficult to reach. Even during movements, which they often make for this purpose, cattle practically never have the opportunity of obtaining the 6.25 kg D.M. from the woody shrub that could form the ration.

On the other hand, the nutritive value of this foliage always (or nearly always) provides for the achievement of nutritive balance and a certain production. As examples, some analyses carried out by the Nutrition Laboratory of I.E.M.V.T. have been selected: they correspond to Sahelian woody shrubs among those most frequently chosen and accessible to cattle and regularly searched for by them (others, sometimes frequent, may be only casually used). It should be noted that the production of perennial herbaceous plants (grasses) should also be compared (value and period) with that of the woody shrubs, although they are very rare in the Sahel.

So, it appears that from a qualitative point of view, these food sources (except for the fallen dry leaves, which are akin to straw) are a food rich enough to allow for maintenance at all times and usually a certain production.

D. - Estimation of the forage productivity of the woody browse

Observation of cattle when grazing is sufficient to prove that the edible parts of the scrub are far from being negligible. But in an attempt to arrive at quantitative estimations of this food source, if it is at all possible to estimate the rather small quantities, we do not possess any experimental figures with which to make the calculations. In connection with certain agrostological studies we have obtained a very few values, as follows:

- pods of Acacia raddiana: 15 kg D.M. on a medium -sized bush (2 determinations - Niger),
- pods of Acacia albida: 50-150 kg D.M. on a medium -sized tree (3 determinations - Niger),
- green leaves of Cadaba farinosa: 1 kg D.M. On a medium - sized bush (1 determination - Chad),
- green leaves of Ziziphus mauritiana,
- green leaves of Bauhinia rufescens,
- green leaves of Piliostigma reticulatum,
- green leaves of Mitragina inermis,
- green leaves of Crataeva religiosa,

about 1 kg D.M., this being accessible to cattle on the lowest branches of each species (determination November 1974, Chad).

Hence one is led to ask "How do we evaluate, with acceptable precision, the average forage productivity per hectare in a well-defined vegetal structure? " Or alternatively, " In what proportions are the food sources produced by woody bush a contribution to the make-up of the ration of cattle?"

E. - Necessity of the experimental determination

The estimations so far available remain approximate or even theoretical. It appears necessary now to perfect a method of determination involving directly the cattle affected. Simultaneous measurements of the vegetation consumed and the performance of the animals should enable us to derive a way of estimating the forage potential of the ligneous stratum.

Such an experimental approach should include:

- the search for homogeneous vegetal formations, in areas as large as possible, so that control areas or reference zones can be constituted, numerous enough and comparable, if not identical in value and vegetal composition.

- a very precise estimation of the forage yield of the herbaceous and ligneous strata, taken separately, before and after the animals have passed.

- the constitution of exploited reference zones, with and without the herbaceous stratum (eliminated by cutting or burning).

- the controlled exploitation of the pastures, with a high stocking rate by homogeneous groups of animals remaining for short periods on each reference zone, to reduce the errors resulting from the possible re-growth of the ligneous food sources.

- control of the cattle (weighing) when entering and leaving every experimental zone.

- the concomitant representative sampling of all edible foliage, etc. for nutritive value analysis.

Nature of the sample

% D.M.

No. of analyses

F.U./kg D.M.

D.P./kg D.M.

Fruits of woody shrubs and bushes

Dry pods of Acacia albida





Dry pods of Acacia raddiana





Dry pods of Acacia Senegal





Green pods of Acacia raddiana





Green pods of Acacia seyal





Fruit of Balanites aegyptiaca





Leaves of woody shrubs and bushes

Leaves of Acacia seyal





Leaves of Acacia Senegal





Leaves of Balanites aegyptiaca





Leaves of Maerua crassifolia





Leaves of Ziziphus mauritiana





Leaves of Cadaba farinosa





Leaves of Cordia rothii





Leaves of Salvadora persica





Leaves of Boscia senegalensis





Dry leaves of Balanites aegyptiaca





F. - Conclusion

Observation of bovids grazing in the vegetation of the Sahelian shrubby steppe with an annual herbaceous layer shows that there is a necessary complementarity between the shrubby and herbaceous strata. A good natural pasture will be constituted of vegetation including both strata.

Though knowing the value and productivity of the herbaceous layers is relatively easy, the forage potential of the ligneous stratum is less known and less easy to measure, although it is much used by the animals.

The problem concerns the method usable to make these measurements.

By means of a number of experimental protocols it will be possible to succeed in evaluating the productivity and the forage value of the scrubby stratum. Also, we will be able to define the means of exploitation and pastoral management which, taking into account the use really made of the total edible vegetation, will lead to an improvement in the productivity of the Sahelian pastures.

Previous Page Top of Page Next Page