Nutritional and chemical composition of black velvet tamarind ( Dialium guineense Willd ) and its in ﬂ uence on animal production: A review

: The world population is expected to double by 2050, creating a greater demand for animal protein sources. This calls for an increase in the supply of animal - derived foods in order to avoid food scarcity in the future. Non - ruminants, which would have been a panacea to this increased demand for animal - derived foods, have su ﬀ ered from the rising costs of feed ingredient world - wide. The increase in the prices of conventional feedstu ﬀ s have attracted the attention of researchers on the potentials of lesser - known tropical medicinal plants like Dialium gui - neense Willd ( black velvet tamarind ) as a feed resource for non - ruminants. The increase in prices of conventional feedstu ﬀ s have attracted the attention of researchers on the potentials of lesser - known tropical medicinal plants. One such lesser - known tropical plant that can be used in chicken and livestock production is black velvet tamarind. The plant is found in many parts of the world. It belongs to the family Fabaceae, has high forage yield, resistant to drought and insects, and does not require annual cultiva - tion. The whole seed is moderate in crude protein and ﬁ bre but high in carbohydrates. The leaf, on the other hand, is low in carbohydrate, moderate in crude protein, and high in ash. D. guineense stem bark and fruit are excellent source of essential oils, and rich source of dietary ﬁ bre, minerals, and vitamins for monogastrics. The plant is endowed with bene ﬁ cial phytochemicals such as alka - loids, tannins, phenols, ﬂ avonoids which may serve as natural alternatives to in - feed antibiotics. D. guineense has bene ﬁ cial pharmacological e ﬀ ects including antioxi - dant and antimicrobial properties. The purpose of this study was to review the body of knowledge on the nutri - tional and chemical composition of D. guineense and its impact in animal production.


Introduction
Animal production plays an important role in the socioeconomic wellbeing of smallholder farmers in developing nations and is one of the sources of protein for the teeming human population.However, animal products are still considered as luxury by the majority of people in developing countries.This is largely due to the high cost of products, which is controlled mainly by the high cost of feed, and this has burdened researchers on how to find cheaper feed ingredients to reduce cost of feed.Availability of several close substitute feedstuff could ameliorate the high cost of feed for animals by reducing the problem of scarcity.Alternative cheaper and available feedstuff is the only means of reducing the production cost of animal products, and this could be achieved through the use of lesser-known tropical medicinal plants.Tropical medicinal plants are abundant in nutrients and beneficial phytochemicals, making it suitable as feed [1][2][3][4][5].Black velvet tamarind (Dialium guineense Willd) is a medicinal plant widely grown in many West African countries.It is also believed to have nutraceutical health benefits due to its richness in beneficial phytochemicals [6].Given the benefits of medicinal plants in animal production, the objectives of this review are to (i) provide information on the proximate and nutritional composition of different parts of black velvet tamarind to maximise their potential as feed; (ii) to summarize the phytochemical and pharmacological properties found in the different parts of black velvet tamarind, and (iii) to discuss the origin, botany, and taxonomy of black velvet tamarind.

Literature search and study selection
Five online databases (Scopus, PubMed, Web of Science, Google scholar, and Google search engine) were searched for published studies that evaluated the nutritional and chemical composition of black velvet tamarind and its influence on animal production.Literature search was not restricted by date and the search terms used were origin, taxonomy, botanical description, proximate composition, nutrient values, phytochemistry and pharmacological properties, and farm animals.Authors also searched the reference lists of retrieved papers for related studies.
Papers included in the study met the following conditions: (i) described the origin, taxonomy, botany, and uses of black velvet tamarind, (ii) assessed the proximate, nutrient, phytochemical, and pharmacological properties of different parts of black velvet tamarind, and (iii) evaluated the effect of different parts of black velvet tamarind plants on the performance indices of farm animals.The requirements for inclusion in the dataset were satisfied by 32 studies out of the 183 papers retrieved from the search performed on the five online databases.Eighty-five publications were excluded after reading the titles and abstracts because they were duplicates across two or more databases.Sixty-three articles were also excluded from the study for being reported in small laboratory animal models.Three studies published in language other than English were also excluded.The tree grows up to 30 m height.D. guineense Willd is a woody plant that thrives well in the rainforest region of West Africa [6].D. guineense Willd has a densely hairy leafy crown, smooth grey bark, and whitish flowers that bear the thick black fruits.Each fruit has a single hard, flat, round, brown seed measuring 7-8 mm long and 3 mm thick.The seeds are shiny and coated with a small layer of starch.

Uses of Dialium guineense Willd
The barks, leaves, and fruits of D. guineense Willd are utilised in traditional medicines [6,7].Several parts of the plant are used in folk medicine to cure fever, prenatal pains, and oedema; the fruit is used to treat diarrhoea [7].The stem bark is used for the treatment of cough, toothache, and bronchitis [7].The pulp is edible and may be eaten raw or soaked in water and consumed as a beverage.The bitter leaves are ingredients in a Ghanaian dish called domoda.Its wood is hard and is used for construction, firewood, and charcoal production.The spermatogenic and anti-ulcerogenic effect of the D. guineense leaf meal in animal models has been reported [8,9].There are claims among women in south-eastern Nigeria that black velvet tamarind fruit pulps improve lactation.
5 Chemical composition of D. guineense

Proximate composition
Proximate analysis results are widely used in nutritional assessment and in the industry for quick evaluation of the nutrient value of plant materials.While such analyses may not give the true pictures of the nutrient content of the test material, they do provide clues for further analysis.Velvet tamarind has received a lot of attention in animal production because of its numerous nutritional benefits [10,11].The results of the nutrient content of D. guineense leaves and stem barks as reported by several authors are summarized in Tables 1 and 2. The chemical composition indicates that D. guineense leaves and stem bark are good sources of essential nutrients [12,13].Results suggest that D. guineense fruit pulp is low in protein and may not serve as a source of protein in animal feed [14].D. guineense leaves and whole seeds are moderate in protein suggesting that it could be a moderate protein supplement for non-ruminant feeding [13,17].The higher digestible fibre content, on the other hand, suggests that D. guineense leaves may be added to feed to aid nutrient digestibility.The total ash content of D. guineense leaves and pulp is higher than that of the whole seeds.Most minerals used in animal feed are found in inorganic form which means they may not be readily bioavailable to the animals, owing to mineral antagonism resulting in decreased nutrient absorption [18].The high ash content of D. guineense leaves and pulp, which is a reflection of mineral content of a feedstuff, suggest that this plant may be high in beneficial minerals.As indicated in Table 1, black velvet tamarind, particularly the fruit pulp, is high in complex carbohydrates.The fruit pulp is also rich in vitamins and minerals, as well as carbohydrates [15].All these properties may have given D. guineense its therapeutic potential.In addition, D. guineense stem bark has high amounts of B vitamins such as B1 (thiamine), B2 (riboflavin), and B3 (niacin) [15].

Nutrient composition and essential oil (EO)
D. guineense pulp is abundant in minerals (Table 3), whereas the seeds and the fruit coats are poor in potassium and sodium, respectively [19].The mineral content of D. guineense pulp reported by Ofosu et al. [19] was higher than the value reported by Adepoju [20], which could be attributed to the difference in the analytical methods and soil types.Similarly, Gnansounou et al.
[21] and Osanaiye et al. [14] reported that D. guineense pulp is high in micronutrients, particularly iron, magnesium, and calcium.The various parts of D. guineense are high in total ash suggesting that the plant is a source of mineral in animal feed.The leaf and fruit pulp of D. guineense are high in vitamin C [22], which may explain why it is known as the vitamin C plant.Therefore, the relatively high ascorbic acid content indicates that it could be  Nutrient value of Dialium guineense in animal production  3 used as a source of dietary antioxidant.The plant is a source of EOs (Table 4) which may explain why it is used in management of several diseases in traditional medicine [24].EOs are complex mixtures of hydrocarbons and oxygenated hydrocarbons derived from the isoprenoid pathways, with monoterpenes and sesquiterpenes making up the majority.D. guineense EOs are dominated by aliphatic alkanes and aldehyde [25,26].A study by Essien et al. [24] found that D. guineense leaf oil is high in precocene I followed by β-caryophyllene, valencene, and cadalene, whereas Pelissier et al. [26] discovered that the fruit pulp is high in EOs such as nonanal, nonanoic acid, and dodecanoic acid.The presence of high concentrations of precocene I in D. guineense leaf oil reinforces the antimicrobial properties of the plant [10].

Phytochemical composition
Phytochemical assay is one of ,the steps in the isolation of novel substances with promising bioactivities.The plant is high in organic acids (malic, ascorbic, tartaric and citric) and sugars [22,27,28], which may serve as cheap and readily available eco-friendly feed additives in non-ruminant nutrition.Table 5 indicates that black velvet tamarind contains tannins, alkaloids, saponins, flavonoids, steroids, and cardiac glycosides [15,16,[29][30][31][32]. Phytochemicals are abundant in D. guineense stem bark [33].Similarly, D. guineense leaves are endowed with important phytochemicals [31,34].The presence of several bioactive compounds in D. guineense may be linked to its medicinal properties [35].Flavonoids present in D. guineense have been confirmed to have anti-ulcerogenic and cytoprotective effects [36].Phenolics are the most abundant group of phytochemicals and are abundant in D. guineense, accounting for most of the plant antioxidant activity.The ability of plant to scavenge free radicals increases as its phenol content increases [37].Abu and Onoagbe [16] and Abu et al. [38] found that D. guineense contains high levels of phenols, flavonoids, saponins, and tannins, which may explain why it is used in traditional medicine.Flavonoid is one of the most potent antioxidants found in tropical plants [39].This finding is consistent with others who reported that tropical medicinal plants are high in phenolics [39].
Jolayemi et al. [40] put the flavonoid content of D. guineense fruit pulp at 860.63 mg/100 g which is higher than the value of 305-661 mg/100 g recorded for Dacryodes edulis and Irvingia gabonensis in Nigeria.Saponins exhibits hypocholesterolemic property by blocking the re-absorption of cholesterol in body, implying that D. guineense can be added to animal feed to improve the cholesterol content of the meat, egg, and milk.D. guineense may accomplish this by inhibiting the activity of hydroxy methylglutaryl CoA reductase, a key enzyme in cholesterol production.Its stem-extract contains alkaloids which possess several pharmacological effects like antihypertensive, molluscicidal, antiarrhythmic, anticancer, and antimicrobial [20].
Pathogens have negative impact on animal production as well as animal product quality [41][42][43].Antibiotics were added to feed to suppress the bacterial activity and improve the performance until the development of antibiotic-resistant bacteria [44][45][46].The use of herbal products to tackle this problem is becoming more common [47][48][49][50].The reported antimicrobial activity of D. guineense [51] suggests that D. guineense could replace antibiotics in animal feed with a minimal chance of development of antimicrobial resistance as well as growth promoters.Abu et al. [52] found that D. guineense stem bark is high in saponins and tannins.The use of tannins in the management of coccidal infection in poultry has been demonstrated [39].

Antioxidant activity
Phytochemical constituents are the naturally occurring antioxidant in plants [53] and exhibit redox properties [53,54].Studies have shown that D. guineense leaf extract exhibits in vitro antioxidant activity [10,55] and may be used as a source of antioxidants in livestock and chicken diets.Antioxidant-rich diets have gained prominence in animal nutrition because of assumed positive impacts on animal health, growth, and productivity.It is expected that this high levels of phenols and flavonoids in various parts of D. guineense plant contributed to the rich antioxidant potentials of the plant.Gideon et al. [55] investigated the in vitro antioxidant activity of D. guineense leaf and discovered a dose-related increase in 2,2-diphenyl-1picrylhydrazyl (DPPH) radical scavenging activity when compared to ascorbic and gallic acids.The antioxidant activities of methanolic D. guineense pulp, seed, leaf, and stem bark extracts have been reported [30].The methanolic extracts of black velvet tamarind pulp, seed, leaf, and stem bark show a concentration-dependent increase in DPPH radical scavenging and nitric oxide scavenging activities (Table 6).These findings supported Gideon et al. [55], who reported the antioxidant property of D. guineense (Willd) leaf extract.The dose-related increase in DPPH radical scavenging properties of black velvet tamarind plant demonstrates its ability to scavenge free radicals.This might be attributed to the high concentrations of phenolics in black velvet tamarind extracts (Table 6).Similarly, Oluwole-Banjo [30] reported that D. guineense pulp, seed, stem bark, and leaf extracts have high total antioxidant capacity which aids in the neutralization and absorption of free radicals as well as quenching singlet and triplet oxygen [56].

Antimicrobial activity
The principal mechanism of action of natural feed additives is the modulation of gut beneficial microbes by exclusion of pathogens.Previous studies have shown antimicrobial activities of phytochemical constituents from different parts of D. guineense [10,55,57].Besong et al. [10] investigated the antimicrobial activities of leaf extract of D. guineense following the methods of Rath et al. [58].The authors found that gram-positive bacteria were more susceptible to the extracts than fungi and gram-negative bacteria, suggesting that the extract could be used in animal production to manage bacterial infections.In contrast, Nnadi et al. [59] found that D. guineense fruit coat extract had stronger effect on gram-negative bacteria than grampositive.However, the disparity may be attributed to the part of the plant used which has been reported to influence the phytochemical content of plants [10].D. guineense leaf extract has demonstrated broad antimicrobial activity 8 Impact of adding Dialium guineense to animal feed Research by Iwuji et al. [9] demonstrated that incorporation of D. guineense leaf meal (DGLM) at 3 g/kg feed improved the reproductive traits in males when compared to those fed with diet without D. guineense leaf meal.This implies that incorporation of up to 30% of D. guineense leaf meal in rabbit diet supported spermatogenesis.The diets containing DGLM as a protein supplement enhance the growth performance characteristics in aquatic animals [62].Studies have shown linear decrease in total volatile fatty acid levels in the rumen when the supplementation levels of D. guineense leaves increased from 25 to 50% in sheep diet [15].The authors also reported that supplementation of D. guineense leaves at 25 and 50% in sheep rations reduced ruminal ammonia concentration which they attributed to the high condensed tannin content of the leaf.This suggests that D. guineense leaf is not a good protein supplement for small ruminant, despite its high crude protein content.The high condensed tannin content in D. guineense leaves [15] implies that it may need some level of treatment before it could be used as a protein supplement in small ruminant feeding.

Conclusion
This review showed that Dialium guineense leaves and fruits are high in essential nutrients and may be added to livestock feed to boost growth and productivity.The plant is also a rich source of phytochemicals and EOs that are responsible for its high antioxidant and antimicrobial properties.In the light of the ban on the use of antibiotics as growth promoter, the bioactive compounds in various parts of Dialium guineense plant could be harnessed and used as eco-friendly growth promoters in animal production.There is scanty information on the effect of Dialium guineense on the performance and product quality of livestock and poultry in the literature.Therefore, the use of D. guineense leaf, stem bark, and fruits, and their bioactive constituents as natural feedstuffs and additives in livestock and chicken is recommended.In addition, studies on the safety of Dialium guineense leaf, stem bark, and fruits as well as the mechanisms of action of Dialium guineense are also recommended.

Table 6 :
[59] inhibition and scavenging of nitric oxide by the methanolic extract of D. guineense plant Adapted from Oluwole-Banjo[30]; IC 50half maximal inhibitory concentration.Nutrient value of Dialium guineense in animal production  5 suggesting their wide application in animal production for the treatment of bacterial infection[55].In a similar study using stem bark extract, Olajubu et al.[57]found potent action of D. guineense stem bark extract against Salmonella typhi and S. aureus.These reports are in agreement with Besong et al.[10]who found antimicrobial activities of D. guineense leaf extract.A recent study by Ololade et al.[60]revealed that methanol D. guineense leaf extract had greater antimicrobial activity than gentamicin.The antibacterial activity of D. guineense is attributed to the hydrophobicity of its bioactive compounds, which permeate the bacterial cell membrane resulting in disintegration of cell membrane, leakage of ions, and ultimately cell death[61].Olajubu et al.[57]also found that Candida albicans was inhibited by D. guineense stem bark extract.Olajubu et al.[57] and Nnadi et al.[59]found that methanol D. guineense fruit coat extract had stronger antibacterial property than ethanol D. guineense stem bark extract.