Nutritive value of some important indigenous livestock browse species in semi-arid mixed Mopane bushveld, Botswana

L M Dambe, K Mogotsi, M Odubeng and O E Kgosikoma¹

Range and Pasture Research Program, Animal Production and Range Research Division,
Department of Agricultural Research, Ministry of Agriculture, P. O. Box 10275, Francistown, Botswana
kbmogotsi@yahoo.com     or     kbmogotsi@gov.bw
¹ Range and Pasture Research Program, Animal Production and Range Research Division,
Department of Agricultural Research, Ministry of Agriculture, Private Bag 0033, Gaborone, Botswana

Abstract

The utilization of natural browse as quality livestock feed during the late dry season can alleviate nutritional deficiencies and maintain livestock production, particularly in arid and semi-arid environments. This study investigated the nutritional value of browse species in eastern Botswana. Samples of fresh green leaves, old leaves from the previous year’s growth as well as the rarely-utilized barks were collected from a total of 16 browse species during the late dry season (September and October). Pods were unavailable and thus were not sampled.

The highest crude protein content was observed in fresh leaves, followed by old leaves and lastly barks with ranges of 6.08-23.9%, 6.09-13.4% and 4.62-11.5% respectively. Crude fiber was highest in barks with an average of 35.75%, followed by old leaves with 29.1% and fresh leaves with 27.8%. In vitro digestibility averaged 63.2% for old leaves, 61.8% for fresh leaves and 57.1% for barks. Mineral content of the browse species also varied according to the plant part sampled. Overall, these results indicate that browse species have high nutritional values hence can be used as sources of feeds for livestock to improve livestock production, while low phosphorus values can be corrected through phosphorus supplementation especially during the late dry season.

Keywords: anti-nutritive factors, bark stripping, browse quality, chemical composition, Colophospermum mopane, crude protein, late dry season, livestock productivity

Introduction

Browse, or the available parts of the tree or shrub plant such as leaves, twigs, flowers, fruits or pods is an important fodder resource for ruminant livestock in semi arid rangelands such as found in Botswana. This is more pronounced particularly during the dry season or during drought years, when both the quantity and quality of herbaceous cover significantly declines. Not only are the browse species generally higher in some minerals and protein during these periods, but they also offer a more stable feed reserve when compared to the large biomass variation exhibited by the grass layer. Despite not gaining inclusion into routine carrying capacity assessments, browse and the role it plays in livestock production has gathered widespread recognition and research impetus over the years (e.g. Wilson 1969; Le Houerou 1980; Walker 1980; Otsyina and McKell 1985; Tolsma et al 1987; Devendra 1992; Shelton 2004; Oppong et al 2008). According to Otsyina and McKell (1985), the use of browse as an available source of quality feed during the dry season is important to maintain seasonal and yearly stability in livestock production thus ensuring sustainable rural livelihoods. It is now generally acknowledged that browse fodder is a potential inexpensive locally-produced protein supplement for ruminants, and can correct N deficiency in herbaceous vegetation during the dry season (Le Houerou 1978; Van 2006).

Le Houerou (1980) reported that in North Africa, browse forms 60-70% of rangeland production and 40% of the total availability of animal feeds in the Sub Saharan region. Botswana’s natural rangelands consist of a lot of browse plants which are quite rich in proteins and minerals. Previous studies have documented the available or potential browse plant species in southern parts of the country such as Kweneng, South-East and Kgatleng and parts of the Central District (e.g. APRU 1980; Mosimanyana and Kiflewahid 1991; Macala et al 1992; Moleele 1998; Aganga et al 2000; Aganga and Tshwenyane 2003; Aganga et al 2005; Aganga and Mesho 2008) while very limited studies have been carried out up north in mixed Mopane bushveld found in the North East District. Livestock particularly cattle and goats play an important socioeconomic role in the area, as well as in other regions throughout the country. In addition to being the second smallest district in the country at ~5993km², a substantial portion of this area is occupied by fenced ranches, leaving only a small section reserved for livestock communal use. This scenario exposes the district to environmental ills such as rangeland degradation and calls for practical livestock feed resource interventions to safeguard the backbone of the rural economy under a changing climate. Perhaps the use of already-existing natural browse as livestock feed or as supplements during dry seasons and drought years could make economic sense vis-à-vis purchasing expensive protein supplements or at least augment current protein sources like Lablab purpureus forage legume (Madzonga and Mogotsi 2014). This study therefore set out to identify and profile some of the common browse species utilized by livestock in eastern Mopane bushveld to augment the existing pool of knowledge as well as provide a baseline for further research on the same browse species in Botswana and other similar environments in the region and beyond.

Materials and Methods

Study Site

The study was carried out in Impala Ranch (21°08’ - 21°11’ S, 21°35’ - 27°37’ E), 7 km East of Francistown, in the North East District of Botswana. The average rainfall amount is 630mm annually and it is both erratic in distribution and time. Thus the area is characterized as semi arid, with distinct hot wet (November - March), hot dry (September – November) and cool dry (April – August) seasons. The area is classified as mixed Mopane bushveld and major woody plant species include Colophospermum mopane, Dichrostachys cineria, Grewia flava and associations of Acacia species, as well as Combretum apicalatum. The soils are classified as haplic lixisol, a typical sodic type and characterized by clay (FAO 1991). The ranch is about 1687 hectares in which 1644.5ha are allocated for grazing by cattle, sheep and goats. Average rainfall amount for the years 2010 and 2011 are as shown in Figure 1.

Collection of Samples

Browse sampling was done in the late dry season (September and October 2011). The existing firebreaks around the ranch perimeter fence and within paddocks enabled unrestricted vehicle access for sampling purposes. Hand-clipped samples of available plant parts (fresh leaves, old leaves and bark) were taken from browse trees and shrubs utilized by livestock. Old brownish/yellowish leaves including petioles were considered as growth from the previous season while fresh green leaves including petioles were the current season’s annual growth. Bark sampling of stems/branches was carried out only on plant species which had visually-observable livestock fresh ‘strip marks’ or those in close proximity. Sampling height was restricted to <2.5m to simulate feeding range of cattle, sheep and goats – which are the most important animals kept in Botswana’s extensive rangelands. However in this study dead, fallen and dried leaves as well as fruits/pods were also excluded when sampling, as so were twigs, flowers and tubers even though these may be available to different ruminant animal species. About 500g of each sample (fresh leaves, old leaves, bark) was collected from each species and bagged separately in brown paper bags and thereafter sent for nutritional and chemical analysis at the Soil and Plant Analytical Laboratory in Sebele, Botswana. In instances where samples from one tree or shrub were inadequate, then plant parts from other trees or shrubs of the same species were clipped and pooled to constitute desired sample quantities.

Nutritional Analysis

Samples (fresh leaves, old leaves, bark) of browse species were oven dried at 105^oC for 16 hours, ground to pass through a 2mm screen and analyzed for dry matter (DM), crude protein (CP) and crude fiber (CF) following standard procedure of AOAC (1996). In-vitro true digestibility (IVTD) was determined following Tilley and Terry (1963). The atomic absorption spectrophotometry procedure was used to determine minerals calcium (Ca), magnesium (Mg) and sodium (Na) while phosphorus (P) content was determined using UV-vis-spectrophotometry (AOAC 1996).

Results and Discussion

A total of 16 browse species were sampled, showing a wide diversity of woody species from 10 genera and 7 families (Table 1). In semi arid environments where moisture is a limiting factor, the true worth of browse is apparent during extended dry seasons and during drought years, when the herbaceous component becomes more fibrous and livestock depend more on leaves and succulent twigs of browse species (Shenkute et al 2012). The early green flush in browses especially during the last 2 driest months before the onset of rains is crucial as a potential supplement to pasture during a period of late dormancy in grasses (Bergstrom 1992). Dry matter content of the browse species was found to be high in all the plant parts that were sampled with an average of 98.5%. Generally the forage value of any consumed plant depends on two components namely palatability and nutritive value (Le Houerou 1980). Depending on the objectives of the enterprise (e.g. maintenance, weight gain and milk production), these components need to be sufficiently high for an animal to take in its daily requirement of energy, protein and minerals (Lefroy et al 1992).

Chemical Composition

Crude Protein

Crude protein (CP) content of fresh leaves was highest in Lonchocarpus capassa with 23.9% and lowest in Combretum imberbe with 6.08%. The lowest amount of CP recorded in old browse leaves was 6.09% in Maytenus senegalensis and the highest amount was found to be 13.4% in Colophospermum mopane. Bark of C.mopane contained 4.62% CP while that of Terminalia prunioides contained the highest amount of 11.5%. Olubukola et al (2013) also observed a similar trend that CP content in barks is generally lower than that of leaves alone or leaves plus twigs. The differences in CP content between browse species can be explained by inherent characteristics of each species’ ability to extract and accumulate nutrients from the soil (Njidda 2010). In a study on identification and nutritive value of potential fodder trees  and shrubs in the Mid Rift Valley of Ethiopia, Shenkute et al (2012) reported a high CP content of Acacia pods (about 18.8%), which is sufficient to support microbial rumen fermentation.

According to APRU (1980), mature cattle require 7.8% CP for maintenance while growing beef cattle require 10%, 0.2% P and 0.2-0.25% Ca. Therefore most species in the present study had CP content higher than the required amounts for maintenance. Tsopito (2000) also stated that diets from range contains between 4 to 7% CP with higher concentrations occurring during the wet season and lower concentrations occurring during the dry season. According to Boitumelo (2000), browses may be used alone or as supplements to other feeds since browse leaves are usually higher in protein content of 12-30 %DM as compared to mature grasses with a protein content of 3-10 %DM.

Crude Fiber

Crude fiber (CF) content in fresh leaves ranged from 10% in Combretum apiculatum to 39% in C imberbe. Old leaves had a CF content ranging from 22.4% in C. mopane and 40.4% in Acacia tortilis. Barks contained 30.1-39.3% with C .mopane being the lowest and A. mellifera being the highest. Ladipo (2014) recorded CF values ranging from 15.74% to 15.88%, which are lower than those recorded in the present study. The CF levels in browses can be attributed to the environmental conditions in the area as high temperatures and low precipitation tend to increase cell wall fraction and decrease soluble content of the plants (Boufennara et al 2012).

Digestibility

Digestibility is a major determinant of nutritive value (Ulyatt 1973; Holechek et al 1982). In-vitro digestibility (IVTD) of fresh leaves was found to be highest in C. apiculatum (84.6%) and lowest in C. imberbe (49.5%). A range of 51.2% in A. tortilis to 70.3% in Pappea capensis were recorded in old leaves, while 56.2-58.2% was found in barks with the lowest amount in Grewia bicolor and the highest in T. prunioides. These results show generally high IVTD values in descending order from fresh leaves to old leaves and lastly barks. The relatively higher digestibility could be partially attributed to the higher CP content of the browses. High levels of CP result in increased ruminal ammonia N concentration which in turn enhances microbial activity and growth, resulting in greater DM digestibility (Griswold et al 2003). Low digestibility may be due to anti-nutritional factors (Sanon 2007; Matlebyane et al 2009) which have been shown to decrease the digestibility in browse fodders.

The preceding differences in chemical composition of browse forages are not surprising, and may be attributed to numerous factors including soil type and climate, the plant part (leaf, stem and pod), growth stage and season (Njidda 2010; Abdullah et al 2013).

Mineral Composition

Mineral content also varied among the sampled browse species (Table 1). Minerals found in fresh leaves ranged from 0.61-3.01% Ca, 0.08-0.29% P, 0.2-0.3% Mg, 0.01-0.03% Na and 0.77-1.79% K. On the other hand and interestingly, old leaves still retained reasonable mineral levels of 1.15-4.85% Ca, 0.04-0.16% P, 0.1-0.83% Mg, 0.01-0.05% Na and 0.49-1.62% K. This pattern contradicts expectations that mineral concentrations decrease as the plants lose their green color and dry off (Aganga and Mesho 2008). Ranges of 1.24-21.5% Ca, 0.02-0.05% P, 0.09-0.13% Mg, 0.01% Na and 0.92-1.22% K were recorded for bark. The highest amount of Ca was found in bark of A. mellifera, followed by old leaves sampled from Acacia nigrescens and lastly fresh leaves from Combretum hereroense. Old leaves contained the highest amount of Mg whereas fresh leaves contained the highest amount of K. According to National Research Council (1984) as cited by Abdullah et al (2013), the recommended range on mineral nutrients for all classes of ruminants are 0.19-0.82% Ca, 0.12-0.48% P, 0.5-1.0% K, 0.19-0.82% Na and 0.12-0.20% Mg. Fresh leaves and bark Mg content were lower than the recommended values while K exceeded the recommended range. The amount of Na recorded was lower than the amounts recorded by Abdullah et al (2013) as well as Aganga and Mesho (2008). Na is an important element and McDowell and Valle (2000) cautioned against prolonged Na deficiencies which can ultimately cause weight loss or the loss of appetite, decreased growth and reduced milking. Lastly, P content of the browse species was lower than the recommended values and this could be attributed to P deficiency generally observed in Kalahari soils (APRU 1980; Mogotsi et al 2011).

Anti nutritional factors

Kumar (1992) defined anti-nutritional factors (ANFs) as those substances generated in natural feedstuffs by the normal metabolism of species and by different mechanisms. These substances may inactivate some nutrients; interfere with the digestive process or metabolic utilization of feed which exerts effects contrary to optimum nutrition. Although ANF analysis is not included in this study, it is worth noting that factors such as tannins may have an effect in digestibility of the browse species. According to Shenkute et al (2012), Acacia species contain tannins and other ANFs and the use of their dried pods and wilted leaves is known to reduce the effects of these ANFs. Though C. mopane leaves contains adequate levels of CP, fresh leaves are rarely consumed compared to old fallen dry leaves – thereby suggesting the possible role of anti-nutritional factors (Macala et al 1992). However, ANFs such as tannins are not always harmful to livestock unless they are in high levels. Yami (2011) found out that legume forages rich in tannins are superior as bypass protein sources since tannins link with proteins during mastication and reduce their degradation in the rumen. Since tannins are complex forming hydrosoluble polymers, several techniques can be employed to deactivate such tannins and other secondary compounds such as the use of polyethylene glycol (PEG) (Madibela et al 2006), as well as increased acidity and alkalinity (Russel and Lolley 1989; Salem et al 2005)

Accessibility

One of the problems to consider about browses is their accessibility. According to Le Houerou (1980), the aspect of plant height is related to the height of animals, thus animals have access to what they can reach and without human intervention only part of the feed available can be eaten by the animals. Thus there is this existing height impediment such that most tree leaves will not be consumed until they have fallen naturally to the ground where animals find them – by which time there would likely have experienced a considerable drop in feed value. Browses have also developed defense mechanisms that assist their survival. According to Boitumelo (2000), leguminous trees and shrubs often have thorns, fibrous foliage and growth habits that protect the crown of the tree from grazing ruminants. Thorns and spines as seen in Acacia spp. are well-known physical deterrents and even though such morphological features do not prevent animals from feeding, they certainly reduce bite size and biting rate (Bergstrom 1992).

Furthermore, in some areas heavily bush encroached sites may have impenetrable thickets of usually Acacia species which may render such browse unavailable to livestock (Kgosikoma and Mogotsi 2013).

Conclusion

• From the results of this study, it is evident that browse species could be used as high quality feeds for livestock not only in the dry season where quality feed is scarce but throughout the year. This is indicated by the high nutrient content of the browses especially CP which is generally higher than the recommended amounts for all classes of ruminants. However, due to the relatively lower P content observed, livestock kept in Impala ranch and other regions within the region with a similar problem need to be provided with P supplements.

Acknowledgements

The authors would like to thank Mr Andrew Bakang Moroka for technical support during plant identification and sampling in the field, as well as drivers who tirelessly transported the research team around the ranch during sampling. We also extend our gratitude to the laboratory staff for analysing the browse species samples.

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Received 30 August 2015; Accepted 31 August 2015; Published 1 October 2015

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