Evaluation of the nutritive value of muiumba (Baikiaea plurijuga) seeds: chemical composition, in vitro organic matter digestibility and in vitro gas production

The nutritive value and production cycle of natural pastures depend on prevailing climate and soil conditions. Ruminant production in semiarid regions, especially small ruminants, is mainly extensive and highly dependent on rangeland resources including crop residues and their by-products (Ben Salem and Smith 2008). These feeds are characterized by high levels of fibre and low levels of energy, protein, minerals and vitamins, promoting low levels of voluntary intake and digestibility, which leads to an intake below the animal’s requirements during long periods of the year (Sarwatt et al. 2002). In fact, nutritional deficiencies are accepted to be one of the major technical limitations to increase productivity of ruminant livestock in the tropics (Camero et al. 2001). Several alternatives have been proposed to overcome feed limitations in such extensive production systems. Among these, shrubs and fodder trees (leaves and seeds) are considered to have high potential value as source of essential nutrients. This is due to their high nutritive value, in terms of protein and mineral content, but also to its tolerance to a wide range of management practices, high longevity and capacity to produce fodder when other species become dormant to avoid adverse climatic conditions (Paterson et al. 1998; Camero et al. 2001; Sarwatt et al. 2002; Ben Salem et al. 2004).

Although many studies have been conducted to evaluate the nutritive value of multipurpose trees and shrub legumes, results from tree seeds are scarce on literature. The existing studies show that these seeds can be an interesting nutrient source (Salawu et al. 1999; Smith et al. 2005; Mlambo et al. 2008). Native trees such as muiumba (Baikiaea plurijuga) are important sources of browse for ruminant production within the areas of Miombo woodlands of Africa, namely in the Cunene region. Small ruminants produced in this region exhibit, in particular during the dry season (from May to August), a natural tendency to ingest the young leaves and seeds of Baikiaea plurijuga as a complement to their limited availability and low nutritive herbaceous diet. In spite of this well-known behaviour, the nutritive value of these seeds cannot be found in the literature. The objective of this study was to evaluate the nutritive value of muiumba seeds and to compare it with conventional ruminant feedstuffs to verify its potential as an alternative feed in ruminant extensive production systems.

The chemical composition of muiumba seeds showed that this feedstuff could only supply moderate concentrations of protein making unviable its utilization as a protein supplement. Generally, it is assumed that a feed above 200 g/kg DM crude protein and less than 180 g/kg DM crude fibre (Harris et al. 1968, 1980) could be considered a protein supplement. The low protein value for muiumba may come as a surprise since it is a legume tree and thus a higher protein value was expected. In fact, although tree seeds can be used as protein sources, even for humans (Adewusi et al. 2003), CP values reported herein are lower than those reported for seeds from other types of fodder trees. Data concerning seeds from different species of Acacia trees show an average CP content around 300 g/kg DM (Mahala and Elseed 2007). Other multipurpose trees with a wide agricultural use in Africa, including human consumption and animal feed, are Moringa species. Results presented by Melesse et al. (2009) for seeds of Moringa stenopetala show higher CP values, around 400 g/kg DM. Generalizations maybe somewhat misleading as chemical composition is subjected to wide fluctuations depending on soil and climate characteristics. A recent study from Soliva et al. (2008) confirms the importance of multipurpose tree seeds as protein sources, showing that fruits and seeds medium CP concentration of ten different plants was around 300 g/kg DM.

Although its starch content is practically inexistent, the reduced NDF concentration together with the relatively high content in LMW sugars and NSP indicate that muiumba seeds could have potential feeding value as a dietary component for ruminants in areas where one of the most important factors limiting productivity is limited feed supply. The high glucose content as well as the xylose and galactose concentration of NSP (Table 3) suggests that muiumba seeds are rich in glucans, xyloglucans and galactans. This is not surprising since legume seeds are usually rich in these compounds (Buckeridge et al. 1995; 2000; Srivastava and Kapoor 2005). Although xyloglucan, one of the most abundant hemicelluloses, can generally be found in the plant cell wall structure it can also be a storage polysaccharide in some tree seeds. Like starch, galactan-containing polysaccharides are also considered to be storage carbohydrates and thus easily fermentable by the rumen microbial population. Furthermore, as lignin concentration is very similar to the NDF content, the majority of the fibrous fraction of the muiumba seeds is basically lignin and a small amount of cellulose. These concentrations mainly derive from the hulk, which in these seeds is firmly coated making its removal quite complicated. In this way, it could be argued that the fibrous content of muiumba seeds would not affect the digestibility of other compounds. This is confirmed by the values of IVOMD presented in Table 4. In fact, assuming that the NDF content of muiumba seeds (235 g/kg DM), mainly present in the hulk, is totally indigestible then the remaining fraction is practically totally digestible, as the IVOMD of muiumba averaged 0.770. This feature would rank muiumba seeds along the feedstuffs (Table 4) with higher digestibility in case the hulk could be removed.

Data for most of the seeds from other types of fodder trees also show that one of the main constraints on its utilization as animal feed is the inhibitory effect of lignin, limiting intake and digestibility of nutrients, as well as the concentrations of other anti-nutritional compounds. As previously discussed, this is not the case for muiumba seeds. Additionally, although values reported in the literature are quite variable within fodder tree species, muiumba seeds presented higher IVOMD when compared to the results reported by Melesse et al. (2009) for organic matter digestibility of Moringa stenopetala (523 g/kg OM) and Salawu et al. (1999) for in vitro DM degradation of Calliandra calothyrsus (471 g/kg DM) seeds. Data presented by Aganga and Mosase (2001) also point out to low dry matter digestibility values for the seeds of indigenous trees species of southern Africa, with a value around 400 g/kg DM. Tree seeds from the reported species generally present quite variable digestibility values. This feature is expected since it depends on its chemical composition. The phenolic content of fodder trees and shrubs is usually considered as one of the main factors that negatively affects its digestibility, and several authors have reported this fact (; Smith et al. 2005; Ben Salem and Smith 2008; Blache et al. 2008). A large number of non-conventional feedstuffs, such as tree seeds, may contain secondary compounds that are potentially toxic, due to their structure and rate of intake (Blache et al. 2008). Whether muiumba is rich on phenolic compounds will have to be evaluated in future trials. Nevertheless, the IVOMD value found in our study indicates that whatever the concentration of phenolic compounds in muiumba seeds its nutritive value is high.

The in vitro gas production data are in agreement with the IVOMD results, and confirm the muiumba seeds potential as an animal feed. The estimated asymptotic gas production (A) ranked it as similar to other commonly used supplements. Furthermore, its maximum gas production rate (RmaxG) and its fractional fermentation rate (R) showed that muiumba seeds are characterized by a highly fermentable fraction. By applying the equation suggested by Menke and Steingass (1988), for metabolizable energy calculations, Muimba seeds presented values of 9.1 MJ/kg DM. This feature mainly reflects the close association between the chemical composition on low molecular weight sugars and non-starch polysaccharides and fermentation rates, meaning that both soluble and the majority of non-soluble components have high rumen fermentability. The description of gas production profiles can be attributed to the fermentation of several fractions (Cone et al. 1997); a sub-curve outlining the water-soluble fraction, another outlining the non-soluble fraction and an additional one describing the microbial turnover. In a study comparing 20 feed samples Van Gelder et al. (2005) verified that gas production of the soluble and insoluble fractions could be estimated from the gas produced by each substrate at 3 h and 20 h incubation, respectively. These data support the findings of Cone et al. (1997) that for different types of substrate also pointed out that the gas production sub-curves could be estimated using this approach. In this way, sugars and the majority of proteins are considered to ferment within the first 3 hours of fermentation while starch and NSP are fermented between 3 and 20 hours of fermentation, depending on the substrates chemical composition. In fact, fibrous feedstuffs will tend to show lower fermentation rates and the period of fermentation of the non-soluble fraction will be more extensive due to the higher concentration of cell wall components. The gas production observed for the different feedstuffs used in this study (Figure 1) indicate that after 5 h of fermentation muiumba seeds gas production had a significant contribution of the non-soluble fraction, confirming that the NSP fraction is highly fermentable.

This is the first study presenting data on the nutritional value of muiumba seeds. Our results suggest that although muiumba seeds have a moderate concentration of crude protein and vestigial starch contents, its high digestibility and fermentation patterns suggest that its carbohydrate fraction is easily fermented. Nevertheless, further studies should be performed in order to evaluate the presence of anti-nutritional constituents such as tannins and other phenolic compounds.