Chemical composition, in vitro digestibility and ruminal degradation of pods from major Acacia tree species in semi-arid areas of Southern Ethiopia

Boru Kanu, Kassa Shawel¹ and Ahmed Hassen¹

Department of Animal and Range Sciences, Woilita Sodo University, Sodo, Ethiopia
hassenahmed211@gmail.com
¹ Department of Animal Sciences, Debre Berhan University, P O Box 445, Debre Berhan, Ethiopia

Abstract

Pods of the four dominant Acacia species in Mio district of southern Ethiopia were collected to determine their nutritive value. All of the four species (A. nilotica, tortilis, senegal and mellifera) showed acceptable indices of low to moderate levels of NDF, medium to high content of crude protein and medium to high levels of in sacco DM and crude protein degradability after 24h. A. nilotica was rated highest for all nutritional attributes, followed closely byA. tortilis with lower values for A. senegal and A. mellifora. The study highlighted the potential of the pods of Acacia tree species to be used as supplementary feed for ruminant livestock, particularly during the dry season.

Key words: browse trees, bypass protein, degradability

Introduction

Ruminant production systems in Ethiopia are generally characterized by limitations posed by inadequate feed resources due to prolonged annual dry seasons (Anele et al 2009). Feed shortages during the dry season constitute the greatest challenge to livestock in developing countries including Ethiopia (Masikati 2010; Adugna et al 2012). Due to the large livestock population, the rangelands allocated to grazing are shrinking. As a result, animals are allowed to graze continuously without giving rest which affects the carrying capacity and ultimately livestock production (Ahmed et al 2017). Moreover, livestock production based on rangelands is coming under increasing pressure due to rainfall variability (IPCC 2013). This calls for strategies to bridge the gap between the dry and wet seasons

Indigenous browse species adapted to the existing climatic variability are becoming a major feed resource in semi-arid rangelands by supplying protein and energy to maintain livestock production (Anele et al 2009; Ahmed et al 2017). They reduce seasonal feed resource limitation, produce more biomass, and are more nutritious than natural grazing lands (Belete et al 2012).

Trees and shrubs of the genus Acacia are the most dominant woody species in the dry tropics of sub-Saharan Africa (Mengistu et al 2003). Their particular value in arid zones lies in their resistance to heat, drought, salinity and alkalinity, drifting sand, grazing and repeated cutting (Le Houerou 1980). Pastoralists in Africa have long recognized the importance of Acacia trees and shrubs as a source of fodder (browse and pods) in the dry season. They have over the years developed appropriate techniques of harvesting the fodder, including pollarding, lopping and use of long hooked poles to manually shake down the pods of Acacia species. Consumption of pods considerably improves the quality of small ruminant diets as well as their growth rate (Fadul et al 2014; Uguru et al 2014). Resource-poor pastoralists and agro-pastoralists, particularly in Africa, can use Acacia pods as a strategic dry season supplementary feed to improve the nutritional value of the inherently low quality indigenous forages. Apart from being sources of high quality feed during the dry season, and ability to retain or produce green leaf during this period of low soil moisture. Acacia trees also provide other benefits such as shade, fuelwood, green manure and improved land use systems (Cobbina et al 1990).

The objective of this research was to assess the nutritive value of pods from four Acacia trees commonly found in Ethiopia.

Material and methods

Description of the study area

Mio district is located at an altitude of 750 to 1350 m above sea level, between 4⁰ 01 'N Latitude and 38 ⁰15 'E Longitude at a distance of 152 km from zonal capital Yabello, and 725 km from Addis Ababa. The main rainy season is from March to May which is called ganna and the short rainy season, called hagayya from September to November. The temperature ranges from 16 to 27 ⁰C. The district includes pastoral communities, where livestock rearing is the main livelihood of the people and dependent on communal grazing systems on rangelands as the major feed resource. Only 2% of the population lives in urban areas (Zewdie et al 2015). Population density of the district is 17.5 people per km². Cattle, sheep, goats and camels are the major livestock species owned by the pastoralists in the study district.

Acacia pod collection

Two representative kebelles (the smallest administrative units in the district) were purposely selected on the basis of Acacia species potential, based on group discussion with livestock herders and in consultation with extension agents and livestock development experts in the district. Representative samples of Acacia pods were collected from the four most dominant Acacia species (Acacia tortilis, Acacia mellifera, Acacia nilotica, and Acacia senegal) according to the procedures of previous studies on browse species in Ethiopia (Yayneshet et al 2009; Yisehak et al 2010; Aster et al 2012). Samples of mature pods were collected during early dry season (January to February 2018) by hand picking and use of a long, hooked stick from 15 randomly selected trees of each species in each kebele. The samples were dried at 65 ˚C for 72h then ground in a Wiley mill to pass through a 1mm sieve for chemical analysis andin-vitro digestibility determination, and through a 2mm sieve for in sacco degradability determination.

Chemical analysis

The samples were analyzed for dry matter (DM), crude protein (CP) and ash using the standard procedures of AOAC (1990). Neutral detergent fiber (NDF) was determined by the method of Van Soest et al (1991) whereas acid detergent fiber (ADF), and acid detergent lignin (ADL) were analyzed according to Van Soest and Robertson (1985).

In vitro

Dry matter digestibility (IVDMD) was determined following the method of Tilley and Terry (1963) as modified by Van Soest and Robertson (1985) in which the second stage (pepsin digestion) was substituted with neutral detergent solution.

In sacco degradability

In-sacco DM degradability of the samples was carried out following the procedure of Ørskov and MacDonald (1981) using three rumen-fistulated crossbred (Boran-Holestein) steers. Samples of about 3 g were put in nylon bags (41µm pore size and 6.5 ×14 cm) which were incubated in the rumen of the fistulated steers for 0, 6, 12, 24, 48, 72 and 96h. After incubation the bags were removed and washed by tap water until the rinsing water was clean. Zero-hour disappearance rates were determined by washing not-incubated sample bags in a similar manner. The washed bags were then dried at 60 ⁰C for 72h. The percentage disappearance of DM and CP was determined as follows:

The DMD and CPD data were fitted to the equation described by Ørskov and McDonald (1981) using the Neway Excel program.

Y = a + b (1-e^-ct), where

Y = the potential disappearance of DM at time t

a = rapidly degradable fraction

b = the potentially, but slowly degradable fraction

c = the rate of degradation of b

e = base for natural logarithm

t = degradation time (0, 6, 12, 24, 48, 72, and 96 h)

The potential degradability (PD) was calculated as

PD = a + b

ED = a + bc / k+c where

k = passage rate

c = the rate of degradation of b

Statistical analyses

The data were subjected to analysis of variance using the General Linear Model procedure of statistical analysis system (SAS), version 9.1 (SAS 2008). The Tukey-test was used to determine mean differences at p ≤0.05.

Results

The overall nutritive value data show A. nilotica as the best species followed by A. tortilis with A. senegal and A. mellifear rated poorest (Tables 1-3; Figure 1).

Figure 1. In sacco rumen degradability of DM and crude protein (CP) of four Acacia species after 24h fermentation

Discussion

There were major differences in the apparent nutritive values of the four browse species with A. nilotica showing consistent superiority for the major indices of percentage NDF and in vitro DM degradability at 24h. This is in accordance with the findings of Abebe et al (2012) that pods of A. nilotica had a high rumen degradability.

An important issue concerns the precise role of legume trees in farming systems. According to Preston and Leng (1987) species such as Leucaena leucocephala and Gliricidia sepium are important sources of “bypass” (rumen undegradable) protein. In this case a high rate of protein degradability is not an asset, as protein of lower rumen degradability will be more likely to “escape” the rumen fermentation for more efficient enzymic digestion in the intestines. However, this issue relates to the leaves and not the pods, about which there is much less information.

The pods of A. nilotica were highly degradable after 24h of fermentation (74% for DM and 72% for crude protein). Their role could thus be as a survival feed in the dry season, as a supplement to low quality pasture or as a protein-energy source in more intensive feeding systems based on crop byproducts such as urea-treated straw. In this context, information is needed on the yield of pods from the different tree species, the optimum time for harvesting, and the most appropriate methods for establishing and managing the trees as components of agroforesty systems with low-carbon footprints.

Conclusions

• The four tree species (A. nilotica, totilis, senegal and mellifera), A. niltoica showed acceptable indices of low to moderate levels of NDF, medium to high content of crude protein and medium to high levels of in vitro DM and protein degradability after 24h.
  
  
• A. nilotica rated highest for all nutritional attributes, followed closely byA. tortilis with lower values for A. senegal and A. mellifora.
  
  
• The study highlighted the potential of the pods of Acacia species to be used as supplementary feed to ruminant livestock, particularly during the dry season.

Acknowledgements

We would like to thank the Woilayita University for financial and material support of this study. We also acknowledge the Holetta Agricultural Research Center of Ethiopia for providing analytical services. Our special gratitude also goes to pastoralists in the study areas, for the support given during our study.

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