Session 2 Uses

Invited Paper

The Fodder Role of Acacia albida Del.: Extent of Knowledge and Prospects for Future Research

M.I Cissé¹ and A.R. Koné²

Abstract

The current knowledge presented in this paper on the fodder role of Acacia albida is drawn from an extensive compilation of studies carried out in West Africa. The discussion focuses on the fodder production, chemical composition, and nutritive value of A. albida. Placing future research within a multidisciplinary program aimed at evaluating the agroforestry and agropastoral contribution of A. albida is stressed.

Introduction

In the semi-arid régions of West Africa, trees play an essential role in the life of rural populations and in traditional agrarian systems. In Sudanian and Sahelo-Sudanian zones, Acacia albida is regularly associated with the most intensively cultivated lands ("Soforo" in the Bamanan dialect; "Lara" in Dogon). According to Pélissier (1979), A. albida stands are critical to the lives of farmers dependent on rainfed cereal agriculture and breeding of domestic animals. A. albida is at the foundation of this agropastoral system. Seignebos (1978) indicates the importance of this species:

"Thanks to its reverse [phenology], [A. albida provides] a...microclimate favorable to crops. By its litter, it enriches the soil and permits increased millet and sorghum yields [without requiring] fallow periods. The nutritive value of its fodder is as important as its [fodder role in the dry season]. [And despite intensive exploitation], it tolerates delimbing."

In other respects, the seeds, gum, bark, and wood are utilized for many purposes—food, traditional medicine, construction, furniture, canoes, and other domestic uses—making A. albida the "miracle" tree of the Sahel.

This paper summarizes the knowledge compiled on the fodder role of A. albida while focusing on the close relationships that exist between this species and domestic animals. Because of its highly nutritional leaves and fruit, A. albida provides domestic animals with excellent fodder in the dry season; in return, cattle assure its regeneration by pretreating the seeds in their digestive tract and then acting as disseminators.

Problems in Evaluating the Fodder Potential of Leguminous Species: The Case of Acacia albida

In West Africa, references to A. albida as a fodder tree are numerous (Curasson 1953 and 1958; Gillet 1960; Adam 1966; Audru 1966; Boudet 1970 and 1972; Diallo 1968; Mosnier 1961 and 1967; Peyre de Fabrègues 1963; Naegelé 1971; Touzeau 1973; Giffard 1974; Le Houérou 1980b). All of these authors agree that A. albida leaves and pods serve as fodder for cattle, sheep, goats, and camels; however, they make no mention of the production of this species.

Interest in evaluating fodder production of tree species is recent (Bille 1977 and Poupon 1980 in Sénégal; Nebout and Toutain 1978 in Burkina Faso; and Cissé 1980 in Mali). Such evaluation involves problems linked to:

•  traits distinct to tree species: notably, longevity, production cycles, and size, which limit the trees accessibility for domestic animals and make it necessary to use particular forms of exploitation (pruning, delimbing, and coppicing);

•  defining fodder production of A. albida. Except for the wood, all canopy material is edible.

Besides the pods, which have obvious fodder value, a number of other tree components may warrant attention. The fodder value of tree bark, which is generally of little or no interest to domestic animals, has rarely been evaluated, and never for A. albida. The branches, usually evaluated with the leaves, may have distinct fodder potential and therefore deserve a separate evaluation. The flowers of A. albida do not appear to be much sought after by domestic animals; however, the fruit has fodder potential. The production of A. albida fruit was evaluated in Niger (Lemaitre 1954), Sénégal (Jung 1967) and Sudan (Wickens 1969). Tree leaves, generally the most browsed part of the tree, are an important fodder source. Cissé (1980) evaluated the foliar production of A. albida in Mali, and many other similar studies have been done on other species.

Due to the relative abundance of research done on the fodder potential of A. albida leaves and pods, the following discussion will be limited to these two components.

Pod Production of A. albida

A. albida bears fruit between January and April, a critical period in terms of fodder availability. Fallen fruit are consumed and, as this period progresses, they are knocked down and then collected, and fed to animals or sold in markets.

Pods of A. albida mature slowly. In Niger, Bau-mer (1983) found that, under very exceptional conditions, pod development begins between the 2nd and 4th years but in general, fruit-bearing commences around the 15th year. In Sénégal, Nongonierma (cited by Felker 1978) observed that the first pods appear during the eighth year; in Niger, Lemaitre (1954) observed that they appear between the 8th and the 12th year.

Reports of A. albida pod production are sometimes unreliable. Based on an average established on twenty 30-year old trees, Lemaître (1954) estimated a production of 6 to 8 kg tree^-1 around Zinder in Niger. In Bambey, Sénégal, Jung (1967) estimated fruit production of one tree (crown surface area = 230 m²) at 125 kg. Wickens (1969) harvested 135 kg from a mature tree in Sudan.

Evaluating these diverse results, Felker (1978) concluded that "because of marked differences between Lemaître's results and those of others, the absence of information on the method used by Wickens and the serious limitations of Jung's method, we should consider the average pod production per tree and per hectare as unknown." Thus, these data do not have indicative value.

The evaluation of pod production, therefore, requires the adoption of a viable method. One method may be to determine whether correlations can be established between pod production and physical parameters, for example, crown size (Lemaitre 1954). Referring to evaluations done on Acacia tortilis in Kenya, Ciss6 (1983), noted positive correlations between pod production and height (r = 0.72), trunk circumference (r = 0.92) and crown area (r = 0.85). Applying these allometric relationships to inventory data would allow evaluation of stand production.

To establish allometric relationships, individual tree production was evaluated in the following order: aerial harvesting of pods; estimation of areas over which the pods were gathered; evaluation of collected pod density by counting over an area of 12 x 0.25 m; estimation of pods remaining on the tree in relation to those collected; and evaluation of mass of pods sampled relative to the total number of pods.

According to Bille (1977) and Poupon (1980), fruit production fluctuates from year to year among Sahelian tree species. To allow for these variations, the evaluation of pod production should take into account only trees bearing fruit at the time of sampling.

Foliar Production of A. albida

 Seasonal Distribution

Cissé (1980) compared foliar biomass at peak production (B) using data based on five trees per trunk-circumference class (10-cm classes) with the following parameters:

•  the trunk circumference (C) measured at 40 cm from the base on single-trunk individuals and at the base of immature trees;

•  total height (H);

•  crown surface area (S) (the average of two perpendicular diameter measurements).

The most significant physical parameter linked to foliar biomass was analyzed using a log-log linear model on data derived from 50 subjects. The correlation matrices (Table 1) indicate strong positive correlations between the physical parameters as well as between each of the physical parameters and foliar biomass. The linear regression is acceptable (0.90 eR² <0.96), and the resulting adjustment curves indicate an exponential relationship (Table 2).

Table 1. Matrix of correlations between log of physical parameters and log of foliar biomass of A. albida.

Table 2. Ratios between foliar biomass (B) (g DMr and physical parameters2 of A. albida. (Source Cissé 1980.)

Large seasonal fluctuations of fodder production among tree species (Cissé 1982) necessitate production evaluation by season. Changes in foliation of A. albida were monitored over the course of 1 year on 20 heavily exploited and 20 unexploited individuals. Samples of standard-size twigs (diameter 20 mm at the base) were taken from each tree every month. The biomass of the twigs, which were chosen to represent the phenological state of the subject, constituted an index of foliation. The ratio (σ) of the foliation index in a given month (bt) to the foliation index at its maximum (bo) expresses variations in foliar biomass over the course of a year. These ratios, expressed as a percentage, were used to form foliation patterns.

In contrast to other deciduous species, the maximum foliation of A. albida occurs in January during the cold period of the dry season. In the rainy season (July-August), the tree is totally defoliated. Botanists become lost in conjecture and speculation when considering the "aberrant" phenology of this species (Portères 1957; Lebrun 1968; Trochain 1969). However, A. albida owes its importance to this very phenology and provides green fodder during periods of fodder scarcity. Fodder exploitation extends the foliating period of A. albida and enhances foliation in the rainy season. This may be detrimental to crops and provide little benefit to domestic animals, which prefer highly nutritious green grasses during this period.

The foliation curves indicate foliage availability on the stump as well. The difference between maximum availability and availability on the stump reflects the extent of defoliation.

Foliar Exploitation in Mali

Foliar production figures represent foliar biomass of a tree as a function of its physical characteristics. They are utilized in descriptive forest inventories to estimate foliar biomass of a stand.

In Mali, A. albida is found in pure stands or in stands associated with Vitellaria paradoxa, Borassus aethiopium, Sclerocarya birrea, and Lannea microcarpa. These occur in the natural stands of Goudo-Mondoro, the Bandiagara-Houbou plateau, in the agroecological zones of the Bas Plateau Bobo, the Koutiala Falo plateau in Falo région, and in the dry Central Delta zone of the Niger River.

Forty samples were taken in chosen stands; locations (distances from villages) and soil types were noted. The density, cover, and foliar biomass of the A. albida population in each of these stands were estimated. For every stand, the biomass of A. albida was calculated by summing the contribution of all the clumps from one of the allometric relationships.

On the average, A. albida represented 48% of stems and 64% of total cover of these stands and annually produced 300 kg ha^-1 of foliar dry matter. These averages, however, mask important differences between stands. As hypothesized, the location of the stand and the type of soil on which it developed also had an impact on foliar biomass production (Table 3).

Table 3. Influence of the location of the stand and soil types on total foliar production (kg DM ha-^-1) of A. albida in different stands in Mali.

Using maximal biomass estimates, which were derived using allometric relationships, the monthly foliar availability was evaluated by multiplying by the factor σ(σ = bt/bo). As an example, Table 4 shows the seasonal changes and extremes in foliar availability of A. albida by stand and (A. albida + Vittellaria) A/V, which demonstrates the extreme foliar biomass.

Effect of Lopping on Production of A. albida

Only a small part of foliar production is directly accessible to animals and herders who delimb A. albida. In the short run, this affects fodder production. The frequency and intensity of lopping has a marked im-pact on the foliar production of A. albida (Cissé 1984).

For unlopped individuals, repeated pruning during periods of peak average biomass stimulates foliar production. Resulting regrowth is especially vigorous in the first year, but it decreases as exploitation continues. Pruning in January produces more leaves than pruning done in June; however, pruning in June extends the foliation season. A. albida tolerates two prunings per year, but the trees show signs of stress at the end of 6 years.

Delimbing was shown to decrease pod production. This is critical not only because of the decrease in fodder potential but also because the pods constitute a commercial commodity for farmers and herders. Rational management of fodder production of A. albida must take into account both pod and leaf production.

Fodder Value of A. albida

The interest in tree species as a food source for domestic animals lies in the fact that they offer green fodder rich in protein in a period when animal feed is scarce. Behavior studies on cattle and sheep/goat herds in millet agropastoral systems around Niono in southern Sahelian zones of Mali (Dicko 1981) showed that all three types of animal consume browse: about 87% of the goat diet, 36% of the sheep diet, and 11% of the cattle diet are browsed material.

The fodder trees are distinguished from forage grasses by their high total protein (TP) content, which ranges from 60 to 250 g kg^-1 of dry matter (DM). Some samples have contained up to 330 g kg^-1of DM in certain young sprouts and leaves (Rivière 1977; Diagayeté 1981; Koné 1987). Tree fodders are high in lignin content, comprising up to 20% of the mass (Koné 1987).

The leaves of A. albida contain an average of 200 g TP kg^-1 dry matter (Fall 1978; Le Houérou 1980a; Diagayeté 1981; Koné 1987), the pods contain an average of 150 g, and the seeds an average of 240 to 280 g (A.R. Koné, CRZ personal communication; Wickens 1969, cited by Felker 1978). Following ingestion of A. albida pods, however, only 34% of seeds (which contain a large percentage of TP) are digested (Wickens 1969, cited by Felker 1978). The seed:pod ratio, ranging between 1:2 and 1:8, explains the great variability observed in the chemical composition of these types of fodder (Table 5). The TP includes diverse protein and nonprotein matter which is not discernible in a simple portion of TP. The protein content that is apparently nondigestible can be very high.

Table 4. Estimation of the seasonal distribution of foliar biomass (kg DM ha->) of A. albida in two stands in Mali.

Table 5. Total protein content, residual, and parietal contents of pods and leaves of A. albida1.

Feed Value of A. albida

It is difficult to determine the feed value of A. albida just by determining dry-matter, protein, and crude cellulose contents. Estimate of crude cellulose and TP content allows us to evaluate fairly accurately the digestibility of forage grasses with less lignin. Tree species require more detailed analyses of parietal content (method of Van Soest (1983)) and protein content (Koné 1987). Preliminary studies have shown a good correlation between the degradability of TP and the nondegradable protein contents in the acid detergent fiber (ADF) residue (Koné 1987). Measuring the rate of enzymatic breakdown in sacco (in the rumen) of the organic matter and/or proteins is another way of determining digestibility. The results can be compared with those obtained in vitro.

The proteins are degraded in the rumen into more soluble products. The extent of degradability varies greatly with the nature of the food and the type of animal. Fall (1978) found that in sheep, solubilization and degradation rate of A. albida pod proteins were higher than those of leaves (Table 6). In goats, however, the intraruminal breakdown of leaf proteins was higher than that of pods (Koné 1987). The dry matter digestibility of A. albida leaves ranges from 14 to 53%. Total protein digestibility varies from 67 to 72%. Dry matter digestibility of whole pods ranges from 50 to 57%; total protein digestibility can reach 73% (Dicko 1979; Fall 1978; Diagayate 1981).

Table 6. Intraruminal degradation (%) of A. albida fodder in small ruminants. (Source: Fall 1978.)

The reduced digestibility of protein is linked to two complementary factors. The first is the inhibiting effect of tannins on protein breakdown in the rumen and in vitro (Palo 1985, Barry and Marley 1986). The pods of A. albida contain 36.5% of phenolic substances and 0.27 A. 550 of insoluble proanthocyanidians (Tanner 1988). Tannins affect the entire enzymatic system (Milic 1972) and have a negative impact on ingestion and, as a result, on animal growth. The second is interspecific variation of protein distribution between the cellular contents and the paries. The non-digestible proteins are to a large extent linked to the acid detergent fiber (ADF) (Mason 1969; Van Soest 1983) because of high lignin content.

Fodder Efficacy

Despite the presence of tannins, A. albida—notably the pods—may be used as a supplement in animal diets because it is rich in protein and digestible energy (77%). Such an energy value would explain the results obtained in Mali (Dicko 1979), where incorporating pods into a sheep feed of sorghum leaves gave an increase in the quantity of dry matter ingested from 40.4 to 61.6 g kg^-0.75 in sheep weighing an average of 37.5 kg. The ingestion of leaves by goats was only 41.3 g kg^-0.75 per day.

In Sénégal, Fall (1978) tested the quality of pods and leaves of A. albida on sheep (weighing on average 22 kg) which received as much rice straw as desired, 100 g of bean cake day^-1 per head and bone meal. In addition, one lot of animals received 100 g of A. albida leaves, another 100 g pods, and a third 200 g pods per head. After 92 days, the results were as follows:

• the rice straw had a refusal rate of 10% and a consumption level of 43 g kg^-0.75 (lot 1), 41 g kg^-0.75 (lot 2), and 36 g kg^-0.75 (lot 3). Large quantities of pods lowered the level of consumption of rice straw.

• pod supplements resulted in slight growth in sheep. The level of 100 g day^-1 per head appeared to be more profitable than that of 200 g (GMQ = 18.5 for lot 2 and 20.6 for lot 3). Lot 1 lost weight (GMQ = -27) because of poor digestive utilization of leaves.

This suggests that incorporating pods of A. albida into a low-quality feed enhances digestion without reducing the digestibility of the ration.

Future Research

Much remains to be learned about the fodder role of A. albida. Because of the multiple-use potential of A. albida, research on its fodder production must take the form of a multi-disciplinary program to improve the contribution of this species to agroforestry systems.

In the A. albida-soil-animal association, the mutual benefits can be generally illustrated; however, the returns—whether they be in the form of nutrition, fertilization, or growth—have yet to be fully understood, let alone fully evaluated. To determine the role of fodder in this association, future research should:

• evaluate fruit production of A. albida using a viable method;

• evaluate total and seasonal fodder production (leaves and fruit) as a function of diverse ecological factors such as rainfall, soil type, and frequency and intensity of exploitation;

• determine the chemical composition of fodder as a function of its phenological stage (including dating samples), focusing in particular on parietal content (Van Soest 1983) and content of anti-nutritional substances such as tannins;

• study ingestion and digestibility of A. albida fodder to create models to forecast food value; and

• evaluate performance resulting from rations containing and/or rations composed purely of A. albida components on different types of animals (sheep, cattle, goats, and camels).

Conclusion

A species more Sudanian than Sahelian, A. albida derives its importance in fodder production from the fact that it drops its leaves in the rainy season and refoliates in the dry season. Hence, it benefits domestic animals by providing fodder in the form of leaves and pods.

Although a method has been developed to evaluate foliar biomass and its seasonal changes as well as the impact of exploitation on foliar production, fruit production has not received the same attention.

Feed-value analyses have revealed that pods and leaves are rich in parietal content. Leaves, pods, and seeds contain respectively 200, 150, and 260 g TP kg^-1 of dry matter. Antinutritional substances such as tannins limit the digestibility of the abundant protein; however, in spite of the presence of tannins, A. albida components—particularly pods—can be incorporated into low-quality fodder to enhance ingestion without reducing digestibility.

Future research on the role of A. albida as a fodder needs to be done as part of a multidisciplinary research effort that evaluates the contribution of this species to agroforestry systems.

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Footnote__________

1. Direction de recherche forestiere et hydrobiologique (DRFH)/Institut d'economie rurale (IER), B.P 1704, Bamako, Mali.

2. Centre des recherches zootechniques (CRZ), Sotuba, Bamako, Mali.

Cissé, M.I., and Koné, A.R. 1992. The fodder role of Acacia albida Del.: extent of knowledge and prospects for future research. Pages 29-37 in Faidherbia albida in the West African semi-arid tropics: proceedings of a workshop, 22-26 Apr 1991, Niamey, Niger (Vandenbeldt, R.J., ed.). Patancheru. A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics; and Nairobi, Kenya: International Centre for Research in Agroforestry.