Browse species from Ethiopia: role in methane reduction and nematode control in goats

The aim of the research reported in this thesis was to evaluate browse species collected from Ethiopia for preference by goats, and for their in vitro anthelmintic and methane (CH4) reduction properties. During the conduct of the studies observations were made warranting a further aim, to compare in vitro fermentation patterns of browse species using inocula from goats and cows kept on identical dietary regime. The preference of browse species using dry matter intake (DMI) as a proxy and in combination with polyethylene glycol (PEG), relationships between browse species intake and chemical composition were determined in Chapter 2. Air-dried leaves of Acacia etbaica, Cadaba farinosa, Capparis tomentosa, Dichrostachys cinerea, Dodonaea angustifolia, Euclea racemosa, Maerua angolensis, Maytenus senegalensis, Rhus natalensis and Senna singueana were used. Two cafeteria trials, each lasting 10 days were conducted using goats receiving a daily ration of grass hay and wheat bran, without (trial 1) or with (trial 2) the inclusion of PEG. Preference measured as the first 10 min browse DMI differed significantly among browse species and with PEG (P<0.0001). Browse with higher tannin content, D. cinerea, R. natalensis and A. etbaica were the most preferred species regardless of PEG presence. Preference appeared to be based on digestible fibre fraction, hemicellulose rather than tannin levels in the browse species. Extracts of the 10 browse species were evaluated for their anthelmintic activity against Haemonchus contortus (Chapter 3). The larval exsheathment inhibition assay (LEIA) was applied using H. contortus third stage larvae (L3) in a dose dependent manner with extract concentrations of 0, 150, 300, 600, 1200 µg/ml phosphate buffered saline (PBS). The role of polyphenols in the inhibition against L3 was evaluated using polyvinylpolypyrrolidone (PVPP). All browse extracts significantly (P<0.0001) inhibited larval exsheathment in a dose dependent manner with the dose required to inhibit 50% of the L3 (EC50) being highest in C. farinosa and lowest in E. racemosa and M. senegalensis. Polyvinylpolypyrrolidone treated A. etbaica, C. tomentosa, M. angolensis, R. natalensis and D. cinerea were different (P<0.001) from the control (only PBS), indicating that larval inhibition was largely due to non-phenolic compounds. Absence of significant differences between PVPP treated E. racemosa, M. senegalensis, D. angustifolia and S. singueana, and control suggest that inhibition was mostly attributable to tannins and other polyphenols. Browse species anthelmintic property against H. contortus L3 was due to the presence of phenolic and non-phenolic compounds. In vitro gas production (GP), CH4, volatile fatty acids (VFA) and in vitro organic matter digestibility (IVOMD) of the 10 browse species were determined using PEG 6000 in Chapter 4. Proanthocyanidins (PA) were quantified using a modified HCl-butanol method and PA composition was determined by UPLC-DAD, with detection of other polyphenols by UPLC-ESI-MS/MS. Substrates were inoculated in buffered goat rumen fluid with or without PEG 6000 for 72 h to measure GP with head space gas sample measurements taken at 0, 3, 6, 9, 12, 24, 30, 48, 54, and 72 h for CH4. At the end of incubation, VFA, ammonia (NH3) and IVOMD were determined. Increased (P<0.0001) GP, CH4 and total VFA were observed after PEG addition indicating PA were mainly involved in reducing methanogenesis and to a lower extent also overall fermentability. Prodelphinidins were the major explaining factors for this reduction but other polyphenols like quercetin, myricetin and kaempferol were also involved in CH4 reduction. The effect of PEG addition on IVOMD was variable among browse and could be due to artefacts from the tannin-PEG complexes in the incubation residue. Proanthocyanidins were mainly responsible for the reduced in vitro fermentative activities with possible minor effects of other phenolic and non-phenolic components. Due to unusual fermentation patterns observed in Chapter 4, a comparison was made between goat and cow inocula on in vitro gas and CH4 production and kinetics parameters as well as VFA production in Chapter 5. Leaves of A. etbaica, C. tomentosa, D. cinerea, R. natalensis, freeze-dried maize and grass silage, and a concentrate were inoculated for 72 h to measure GP, in buffered inocula from goats and cows kept on an identical feeding regime. During incubation, headspace gas samples were obtained at 0, 3, 6, 9, 24, 30, 48, 54, and 72 h, and analysed for CH4 with VFA determined at the end of incubation. A triphasic and monophasic modified Michaelis-Menten equation was fitted to the cumulative GP and CH4 curves, respectively. Total GP and CH4 (P<0.0001), half-time for asymptotic (P<0.012) and rate (P<0.0001) of GP were higher for goat inoculum. The total VFA were higher (P<0.0001) in goats and the proportion of individual VFA differed significantly (P<0.002) between animal species. Differences between goat and cow inocula were attributable to variation in the activity and composition of the microbial population, and differences were more pronounced for fermentation of browse species than grass and maize silages. A synthesis of the results from the four research chapters is provided in the general discussion (Chapter 6). The present work highlights the browse species characteristics which can be strategically exploited in goat production systems to improve health and feed utilization efficiency.