H.N. Le Houérou
The present study is dedicated to those who sweated the data out of the bush, thus making this synthesis possible. In particular I should mention: J. Audru, R. Bartha, G. Boudet, A.K. Diallo, J.F. Ellenberger, G. Fotius, A. Gaston, J.P. Nebout, B. Peyre de Fabrégues, J. Piot, G. Rippstein, R. Rose-Innes, B. Toutain, J. Valenza and others to whom the scientific community is indebted for our present knowledge on this important subject.
The aim of this paper is to bring together and compare data scattered over some 50 sources, most of which have had limited circulation, in order to draw some general conclusions as to the present state of knowledge, identify gaps and suggest priorities for future research on the chemical composition and nutritive value of browse in West Africa.
From the literature we gathered some 540 analyses concerning 105 species of browse belonging to 72 genera and 30 families. Ninety-seven species are indigenous and 8 are exotic but widely represented in West-Africa; the latter are indicated with an asterisk * in the tables.
The geographical area concerned covers the Sahelian, Sudanian and Guinean ecological zones north of the equator, from Mauritania and Senegal in the west to the Sudan in the east, and from Mali to the Central African Republic. Eastern, Central and Southern Africa were excluded as these areas are being or have already been treated in other similar studies.
Since the different parts of woody species namely: leaves, twigs, bark, flowers, fruits and seeds are consumed by different domestic and wild animals, reference to the part concerned is made for each analysis reported. Country and date are also given whenever possible (see section 2).
The basic chemical analyses are:
a)crude protein (C.P.): Kjeldahl nitrogen × 6.25;b))crude fibre (C.F.): residue after reaction of a triacid concentrate solution (acetic, trichloracetic, nitric) maintained at boiling point for 30 minutes;
c) total minerals: ash residue after calcination in furnace at 600°C;
d) total fat (or Crude fat): ether extract under heat;
e) silica: portion of ashes insoluble in HCl;
f) macroelements: P, Ca, Mg, K, Na: these were generally measured by either complexometry or spectrophotometry.
g) Nitrogen-free extract (NFE) (mainly carbohydrates): equals 100(CP + CF + Fat + Mins).
All figures are expressed in percentage of dry matter (DM) unless otherwise specified.
Feed value is computed from these chemical data in the following way:
Gross Energy (GE) = 17.5 MJ/kg DM
Metabolisable Energy = 14.0 MJ/kg DM
Net Energy = 6.9 MJ/kg DM
Net energy is a function of the crude fibre and ash contents; it is computed from Dijkstra's regressions; the values are found in tables (see Boudet et Riviére, 1968; Boudet,1975; Riviére,1977; Jarrige et al 1978). These tables are expressed either in Starch Equivalents (SE) or in Feed Units (FU). However, in order to comply with the recommendations of IBP, ICSU and other international scientific bodies, the results are given here in megajoules (MJ) per kg of DM. One MJ= 239 Kcal= 0.145 FU= 0.101 SE;
The above estimates of animal needs in energy, protein and minerals are based on the assumption of a daily intake of 2.5 kg of DM per 100 kg of liveweight (Boudet et Riviére, 1968; Boudet, 1975). However this may prove slightly conservative where zebu cattle are concerned1; there are indications (although no hard evidence, to my knowledge at least) suggesting that intake for zebus is about 3 kg. In any case 2.5 kg is certainly too low a figure for sheep, as recognized by Boudet and Riviére themselves (op.cit.), while goats can consume over twice as much DM per unit of liveweight as cattle do (French, 1970). As a consequence, the above requirement figures are likely to be a little high for cattle, and much so for small stock.
______________________________________
1 Rose-Innes and Mabey (1964) found a daily intake of 3.3 kg DM per 100 kg lwt (40% browse, 60% grass) using W. African shorthorn cattle (B. taurus) in Ghana.
Table A. Estimation of forage quality with respect to NE and DP (Boudet et Riviére, 1968)
Quality |
Net Energy (MJ/kg DM) |
Digest. Prot. in % DM |
Nutrit. Ratio in g DP/FU/kg DM |
Poor |
NE<3.10 |
DP < 2.5 |
NR < 55 |
Fair |
3.10<NE<3.45 |
2.5<DP<3.4 |
55<NR <68 |
Good |
3.45<NE<4.15 |
3.4<DP<5.3 |
68<NR <88 |
Excellent |
4.15<NE |
5.3<DP |
88 <NR |
Table B. Estimation of maintenance needs in minerals (Boudet, 1975)
Macroelements |
Content of forage in % of DM |
Animal needs in g per 100 kg lwt |
Na |
0.08 |
0.5 |
NaCl |
0.20 |
5.0 |
Ca |
0.20 |
5.0 |
P |
0.12 |
3.0 |
Mg |
0.20 |
5.0 |
K |
0.40 |
10.0 |
Table C. Trace elements in mg/kg DM or PPM (Boudet, 1975)
Al |
<400 |
Fe |
8.0 |
MO |
20-30 |
Co |
0.10 |
Ia |
70-140 X 10-3 |
Se |
30-40 |
Cu |
10.0 |
Mn |
50.0 |
Zn |
50.0 |
a In microgrammes
Compared with tropical grasses, browse appears to be richer in protein and minerals (except for protein for young grasses) as already shown by Rose-Innes some 15 years ago (1964, 1966, 1967) and later by Boudet (1975), Piot (1969), Le Houérou (1965, 1966, 1972, 1978) and others. In addition, the proportion of silica is two to three times lower in browse (see Table 5). But the most interesting point to consider is the comparison between grass and browse during the dry season, when browse is most used by livestock.
Dry mature grasses (straw and chaff) have a very low- and often a zero-digestible protein content (see Table IV), practically no carotene and a very low level of phosphorus (0.02 to 0.10% i.e. an average of 0.05% versus a minimum required maintenance level of 0.12%) (see Table 4). According to Demarquilly and Weiss (1970), DP is nil when CP content is 3.8% or less (0.6% of N). In terms of energy browse contains double the amount in dry grass (owing to its low content in crude fibre, as compared to grass) Table 5.
Given the above, dry mature grass cannot ensure livestock maintenance without an additional minimum of 0.1 g of DP and 0.07 MJ per kg of liveweight per day, or 25 g and 1.7 MJ per TLU/day during the dry season, in the absence of other forms of supplementation. These 25 g of DP are provided by 330 g/day of browse DM whereas, to meet its phosphorus needs, the average TLU should consume some 4 kg of browse DM. In fact the consumption is usually less than the latter figure because by selection of browse, livestock are able to balance their diet (Blancou et al 1977).
The same applies to ß carotene and vitamin A, (see Granier, 1977, and also Provost et al); but here we have only indirect evidence in the form of pathological observations, as the carotene content in West African browse has hardly ever been measured. However, we know there is no carotene in grass straw and chaff. Moreover, it seems that livestock can store vitamin A in the liver far shorter periods than is sometimes thought, probably a matter of weeks rather than months, as shown by several authors in the US (Dietz, 1972).
There are, however, a few exceptions to the above statements as regards browse richness in protein and energy and its low content in silica. Some species are poor in protein such as: Stereospermum kunthianum (Bignoniaceae) Maytenus senegalensis (Celastraceae), Hyphaene thebaica (Palmae), Gardenia erubescens (Rubiaceae), Sterculia setigera (Sterculiaceae), Gmelina arborea (Verbenaceae), Vitex cuneata (Verbenaceae).
These species, having usually less than 2.5% DP, do not seem able to provide the maintenance needs of livestock in this nutriment.
Some species have a high content in silica (over 10% of the DM), viz. Stereospermum kunthianum (Bignoniaceae), Cadaba glandulosa (Capparidaceae), Oxytenanthera abyssinica (bamboo) (Graminaceae), Celtis integrifolia (Ulmaceae).
However according to the data available, and the method of estimate used, all browse species seem able, in all their phenological stages, to meet the energy requirements of livestock at maintenance level and often well above. According to generally accepted criteria, West-African Browse may be classified as excellent fodder, with the very few exceptions mentioned above.
Contrary to what is commonly believed, the best average forage quality in browse is not found in legumes as a family, but in the Capparidaceae as shown in Table 3 (this may explain why most of them are so heavily over-browsed:
e.g Boscia spp., Cadaba spp., Maerua spp. etc.). This is a fact common to both West and East Africa. Capparidaceae have on average 25% mote protein than legumes, twice as high a silica-free mineral content and nearly as much energy (96.5%). The nutritional ratio in Capparidaceae is thus 180, as against 145 for legumes and 100 for West African browse as a whole. Capparidaceae browse DM is actually comparable to many local concentrate feedstuffs from agro-industrial by-products such as copra cake, palmcake, cereal bran, cottonseeds, brewer's grains; Capparidaceae DM is, however, less rich than groundnut cake both in energy and protein (see Mongodin et Riviére, 1965).
The mineral content of browse is, on the whole, adequate in terms of phosphorus (0.15%) and magnesium (0.60%), but it is a little high in calcium (1.7%) and potassium (1.5%). However, the calcium/phosphorus ratio is usually much too high: 11 (as against the optimum figure of 1-2). The calcium/magnesium ratio is about adequate: 2.8. There is a general and regrettable lack of information on sodium and trace elements, although the latter are a little better documented than the former. From the scarce data available, it would seem that livestock's needs in iron, cobalt and manganese are generally covered in browse whereas deficiencies in copper and zinc may occur rather currently; it is quite possible that the Capparidaceae, which are very rich in minerals, as we have seen, may play a balancing role; but here we enter the domain of speculation!
However there are, as usual, some exceptions to the above "rules". Combretaceae, for instance are high in phosphorus (0.4%) with a Ca/P ratio which is reasonably favorable (2.6), while legumes are reasonably high in phosphorus (0.25%) but with a Ca/P ratio which is too high (5). Incidentally, contrary to a common belief, leaves of legumes are usually richer than pods (but poorer than seeds). Capparidaceae although high in minerals are rather poor in phosphorus (0.11%) with a very poor Ca/P ratio (14.5%). Some species such as Salvadora persica and Cadaba spp. are extremely rich in minerals (15-30%) and probably also in trace elements. Salvadora, rich in Na C1, constitutes a true "salt cure" for livestock on its own. One should remark incidentally that the role of the "salt cure" in the Sahel and Sudan zones seems to be far from fully understood.
When browse in West and East Africa is compared, there is a reasonably good agreement between the data (Table 3); albeit there is also a striking difference: the content in crude fibre is consistently and significantly higher in East Africa (60%). To a large extent this can be explained by the different aims of the studies concerned and the resulting field sampling procedures used. The studies in East Africa were mainly concerned with wildlife nutrition (Dougall et al, 1958, 1963, 1964 a and b) so that the material studied included samples of bark, stems, branches, roots and other fibrous materials. On the other hand these were not sampled in West Africa, where studies were solely concerned with livestock feeding (especially cattle). This explanation is supported by the figures given by Wilson and Bredon (1963) where crude fibre averages 20.8% as against 29.4% in Dougall et al (op.cit.). Wilson and Bredon's studies were also concerned with cattle feeding. Nevertheless Wilson and Bredon's figures are consistently higher than those reported here (20.8% versus 18.3%) which may not prove to be highly significant. However a slight difference due to the methodologies used in analysis procedures cannot be ruled out, in my opinion. The protein, calcium and potassium contents are slightly higher in East Africa (6%, 10% and 13% respectively) but phosphorus is much higher. According to Dougall et al (1964a) the average phosphorus content of browse in Kenya is 0.20% (average of 234 figures) whereas Wilson and Bredon's figures (1963) average 1.99% P 205, i.e. 0.87 P (average of 18 figures). Combining the two figures we arrive at the following:
(0.87 × 18) + (0.20 × 234) = 0.247 = 0.15, versus 0.25 for West
252
252
Africa. The phosphorus content of browse in thus 67% higher in East Africa than in West Africa, which is a very considerable difference (incidentally on the same order of magnitude as the. difference in crude fibre).
The difference in protein between East and West (6%) may not be significant; but it is certainly not so in the case of phosphorus. In the latter case the difference can probably be ascribed to the respective soil qualities in the two zones. West African soils are overwhelmingly deficient in phosphorus, as everybody knows; but it is not so in East Africa where many basalt derived soils are more than adequately provided with this element. However, if the case of legume browse alone is taken and the figures of Dougall and Wilson are combined (64 and 7 samples respectively), the figure reached is 0.26% which is the. same as the West African figure of 0.25% for legume browse.
The figures are not significantly different in the two zones for Ca, K, Ca/P and total mineral. However, the silica content is almost double in West Africa (2.2%-versus 1.2%). The mineral content of Capparidaceae is higher in West Africa by nearly 40%. Digestibility. According to Mabey and Rose-Innes (20, 21, 22, 23, 36, 37, 38, 39), apparent digestibility (DM content in forage minus DM content in faeces) of the four species studied in Ghana (using local West African shorthorn cattle) is high:
Dry matter 64% (5369)
Organic matter 65% (5470)
Crude protein 77% (7881)
Crude fibre 46% (3859)
Nitrogen free extract 72% (5881)
Crude fat 33% (1363)
Ash 45% (3655)
However from preliminary experiments (using goat and sheep) carried out by Dicko-Touré (unpubl. pers. comm.) in 197879 in the Sahel zone of Mali, it would appear that the digestion coefficient of dry matter for both leaves and pods of Acacia albida, A. seyal, Bauhinia rufescens and the leaves only of Ziziphus mauritania would be in the range of 4954%, which is considerably lower than the values found by Mabey and Rose-Innes (op. cit.) and closer to the figures found for browse in non-tropical conditions (Geri et Sottini, 1970; El Hamrouni et Sarson,1974; Wilson, 1977; Ghadaki, Van Soest et al,' 1974; Dietz, 1972). However it should be pointed out that all these experiments were carried out under different climates, using different browse species, different species and breeds of animals; so that no conclusion can be drawn, except that browse digestibility trials should be a high priority research subject in animal nutrition in tropical Africa.
The feed value of browse will not be known with any accuracy until extensive studies are made on digestibility both in vivo 0and in vitro, at least for the most common and important species. For instance Geri and Sottini (op. cit.), using in vitro techniques on Mediterranean browse in Sardinia, found the regression:
Y = 57.49 0.232X 0.725Z where Y = true in vitro digestibility of dry matter
× = Crude fibre
Z = Crude fat
If one applies this regression to West African browse one would find, for instance, that the energy content would be less than half the figures obtained from Dijkstra's tables which have been used in the present study. The obvious conclusion is that we are in the dark as to what is the real feed value of browse; as a consequence, to repeat, the study of browse digestibility should be treated as "the" top priority subject in African livestock nutrition research today.
The data available and the methods used show that West African browse in most cases constitutes an excellent forage which is high in protein (82 g DP per kg of DM on average) and energy (6.0 MJ or 0.87 F.U. per kg of DM), the nutritional ratio DP/FU is of 100 for browse as a whole, 145 for legumes and 180 for the Capparidaceae.
The silica free mineral content is also higher than in most classical fodders (8.7%), with an adequate supply in phosphorus (0.15%) and magnesium (0.6%); a high content in calcium (1.6Mo) and potassium (1.5%). The Ca/P ratio, however is much too high (11.2) although some species, in particular legumes, show a better calcium-phosphorus balance (with a ratio of 5). The Capparidaceae are particularly rich in minerals (14%).
West African browse appears to be broadly similar to East African although clearly lower in phosphorus and crude fibre, but somewhat higher in energy.
It is safe to say that browse constitutes a necessary and adequate supplement to grasses in the dry season, as dry-season grasses are extremely deficient in protein, phosphorus and carotene cannot, alone, meet livestock maintenance requirements.
Calculations from available data show that an adequate maintenance diet would require a minimum of 20% browse in livestock diets during the dry season. This statement is supported by experimental data on livestock behaviour on the range, including, among other techniques, the use of fistulated animals, as well as by the practices of herdsmen.
However there is a general scarcity of data on the sodium, trace elements and carotene contents of browse. There are still fewer data on the digestibility so that the feeding value of West African browse cannot be assessed with undisputable accuracy. We suspect and we do have every reason to believe, indeed, that the role of browse is extremely important, but we still lack decisive arguments to prove it. It is therefore felt that priority for research should first be given to intake and digestibility and second to the content in sodium, trace elements and carotene.
Table 3. chemical composition and nutritive value of browse
Family |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
Averages all browse |
||||||||||||||||
West Africa |
540 |
12.5 |
18.3 |
4.2 |
53.2 |
8.2 |
6.0 |
100 |
10.9 |
2.2 |
0.15 |
1.68 |
0.60 |
1.47 |
11.2 |
|
East Africa |
290 |
13.3 |
29.3 |
|
44.4 |
|
|
|
10.9 |
1.2 |
0.20 |
1.82 |
|
1.66 |
9.18 |
47/53 |
Legumes W.A. |
261 |
16.8 |
22.7 |
3.1 |
48.2 |
12.1 |
5.8 |
145 |
6.8 |
0.9 |
0.25 |
1.29 |
0.33 |
1.35 |
5.1 |
|
Legumes E.A. |
76 |
14.8 |
28.8 |
|
45.2 |
|
|
|
8.1 |
0.6 |
0.26 |
1.82 |
|
1.19 |
7.0 |
47/53 |
Capparidaceae W.A |
56 |
20.7 |
17.4 |
2.7 |
45.4 |
15.1 |
5.6 |
180 |
13.9 |
2.7 |
0.11 |
1.6 |
0.7 |
2.0 |
14.5 |
|
Capparidaceae E.A. |
21 |
20.7 |
23.7 |
2.2 |
42.9 |
|
|
|
10.5 |
1.1 |
0.17 |
0.7 |
|
2.0 |
4.1 |
47/53 |
Table 4. chemical composition and nutritive value of browse
Family Species |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
Dry season grasses |
81.7 |
3.1 |
39.8 |
tr |
2.8 |
7.7 |
0.25 |
0.05 |
5.2 |
||
Sahel/Sudan zones |
|||||||||||
1. Annuals |
95.1 |
2.7 |
41.2 |
tr |
2.5 |
7.7 |
0.23 |
0.05 |
4.6 |
||
Aristida mutabilis |
10-02 |
95 |
3.9 |
39.1 |
1.0 |
3.0 |
7.9 |
0.15 |
0.03 |
5.8 |
07/44 |
Aristida mutabilis |
03-06 |
95 |
2.1 |
41.0 |
0.0 |
2.5 |
9.0 |
0.14 |
0.02 |
7.0 |
07/44 |
Aristida funicolata |
10-02 |
95 |
3.1 |
42.0 |
0.0 |
2.4 |
8.3 |
|
|
|
07/44 |
Aristida funiculata |
03-06 |
96 |
1.9 |
40.5 |
0.0 |
2.0 |
13.1 |
|
|
|
07/44 |
Cenchrus biflorus |
10-02 |
94 |
3.1 |
38.8 |
0.0 |
2.9 |
9.0 |
0.27 |
0.10 |
2.7 |
07/44 |
Ctenium elegans |
03-06 |
95 |
2.8 |
44.0 |
0.0 |
2.4 |
3.8 |
0.26 |
0.04 |
6.5 |
07/44 |
Diheteropogon hagerupii |
10-02 |
95 |
1.5 |
45.3 |
0.0 |
2.2 |
4.2 |
0.34 |
0.03 |
11.3 |
07/44 |
Eragrostis tremula |
12 |
97 |
3.4 |
38.7 |
tr |
3.4 |
5.8 |
0.34 |
0.17 |
2.0 |
07/44 |
Pennisetum pedicellatum |
10-02 |
95 |
2.8 |
44.0 |
0.0 |
1.9 |
7.8 |
0.17 |
0.04 |
4.2 |
07/44 |
Schoenefeldia gracilis |
10.02 |
95 |
3.4 |
40.0 |
tr |
2.9 |
7.2 |
0.22 |
0.03 |
7.3 |
07/44 |
Schoenefeldia gracilis |
03-06 |
94 |
1.8 |
40.4 |
0.0 |
2.7 |
8.4 |
0.19 |
0.03 |
6.3 |
07/44 |
2. Perennials |
60.8 |
3.8 |
37.5 |
0.3 |
3.3 |
7.7 |
0.35 |
0.55 |
8 |
||
Andropogon gayanus |
10 |
45 |
4.3 |
35.1 |
0.5 |
4.0 |
6.9 |
|
|
|
07/44 |
Andropogon gayanus |
02 |
95 |
2.8 |
39.0 |
0.0 |
3.1 |
6.8 |
|
|
|
07/44 |
Echinochloa stagnina |
04 |
92 |
2.9 |
37.9 |
0.0 |
3.4 |
7.4 |
0.35 |
0.03 |
11.6 |
07/44 |
Hyperthelia dissoluta |
12 |
37 |
4.3 |
35.1 |
0.5 |
3.4 |
11.3 |
0.36 |
0.08 |
4.5 |
07/44 |
Panicum thurgidum |
10 |
59 |
4.9 |
36.4 |
1.0 |
3.5 |
8.7 |
|
|
|
07/44 |
Panicum thurgidum |
12 |
49 |
4.4 |
38.9 |
0.6 |
3.1 |
7.0 |
|
|
|
07/44 |
Panicum thurgidum |
02 |
49 |
3.6 |
40.2 |
0.1 |
3.0 |
5.6 |
|
|
|
07/44 |
Table 5. chemical composition and nutritive value of browse
1 |
2 |
3 |
4 |
5 |
6 |
7 | |
Dry Grass/Browse ratios |
0.25 |
2.17 |
0.0 |
0.46 |
0.70 |
||
Silica/Ash (Grasses) |
0.45 |
47/52 | |||||
Silica (browse)/Ash |
1.12EA 0.20WA |
47/52 47/52 |
EA = East Africa
WA = West Africa
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