SỬ DỤNG CÂY KHOAI MÌ (SẮN) ĐỂ PHÁT TRIỂN CHĂN NUÔI DÊ Ở AN GIANG, VIỆT NAM

HUE UNIVERSITY HUE UNIVERSITY OF AGRICULTURE AND FORESTRY LE THI THUY HANG UTILIZATION OF CASSAVA FORAGES FOR GOAT PRODUCTION IN AN GIANG PROVINCE, VIETNAM DOCTOR OF PHILOSOPHY IN ANIMAL SCIENCES HUE, 2020 HUE UNIVERSITY HUE UNIVERSITY OF AGRICULTURE AND FORESTRY LE THI THUY HANG UTILIZATION OF CASSAVA FORAGES FOR GOAT PRODUCTION IN AN GIANG PROVINCE, VIETNAM SPECIALIZATION: ANIMAL SCIENCES CODE: 9620105 DOCTOR OF PHILOSOPHY IN ANIMAL SCIENCES SUPERVISOR 1: Assoc Prof Nguyen Xuan Ba SUPERVISOR 2: Dr Dinh Van Dung HUE, 2020 DECLARATION I hereby guarantee that scientific work in this thesis is mine All results described in this thesis are righteous and objective They have been published in Journal of Livestock Research for Rural Development (LRRD) http://www.lrrd.org Hue University, 2020 Le Thi Thuy Hang, PhD Student i DEDICATION To my parents who taught me the good things in my life, my loving husband and my sons ii ACKNOWLEDGEMENTS These studies were carried out at An Giang University, Hue University of Agricultural and Forestry, Hue University with financial support from the Mekong Basin Animal Research Network (MEKARN II) Project I am grateful for their support for the thesis research and the scholarship for the PhD study I would like to express my sincere thanks to: Associate Professor Nguyen Xuan Ba, my main supervisor, for all his ideas, knowledge and experience Thanks for unceasing support in research and social activities He has given me invaluable support, encouragement and guidance throughout my study His reading, editing and follow -up of this thesis are gratefully acknowledged Dr Dinh Van Dung, my second supervisor, who has given me invaluable support, encouragement, criticism, excellent skilled technical assistance and guidance throughout my study Professor Thomas R Preston, who has given me invaluable support, for valuable advice, encouragement, enthusiasm and discussions throughout the study His reading and correcting and follow-up of this thesis from the beginning to the end have enabled me to accomplish this work successfully, especially in correction of my English Associate Professor Le Van An, Rector of Hue University of Agriculture and Forestry for giving us the best conditions, encouragement and support during our studies in Hue Dr Khieu - Borin, Regional Coordinator of Mekarn II Project “Vietnam – SAREC Sustainable Livestock Production Systems” project, for valuable advice and discussions Associate Professor Duong Nguyen Khang, Consultant of the “Vietnam – SAREC Sustainable Livestock Production Systems”, for valuable advice Professors, Lectures and assistant lecturers in courses which I have attended during my studies for sharing their valuable knowledge Associate Prof Dr Vo Van Thang, Rector of An Giang University for giving me permission to study, facilitation and encouragement iii My Dean, Dr Ho Thanh Binh, Dean of Agriculture and Natural Resources Faculty of An Giang university for giving me permission to study, facilitation and encouragement My colleagues at the Department of Animal Husbandry and Veterinary Medicine of Agriculture and Natural Resources Faculty of An Giang University for performing the chemical analyses and sharing experiences in scientific research and social activities My students, for help me in taking care the experiments To my friends in the PhD Course, from Lao, Cambodia and Vietnam for giving me a warm and friendly atmosphere To my big family, for all their support and encouragement throughout my study And special thanks to my husband Tran Xuan Hien, two children’s who understood my work, and shared the happiness and sadness with me, for his loving, unceasing support and patience for my whole- life study iv ABSTRACT The aims of the study were to improve utilization cassava forage for increasing performance and reducing enteric methane emission in goat fed cassava forage restricted level of brewery grain and biochar in An Giang province, Vietnam There were one survey and four experiments in this study The survey of cassava and goat systems in Tinh Bien and Tri Ton districts, An Giang province showed that there is an increasing tendency to plant cassava At the same time there are major trends in the population of goats increasing However, goat production systems were still extensive, exploiting natural feed resources with small herds of indigenous goats, which have small sizes and low growth rates Feed and feeding for goats were mainly natural grass and by-products, from crop growing, low nutrition It is not enough feed in rainy and flooding season Whole, cassava forage averaginge tons/ha was available, but the farmers did not use them as feed for goats The impact of different levels of urea added to cassava stems (CS) and its chemical properties was investigated (Experiment 1) The urea treated cassava stems (UCS) (3% in DM) made good quality ensilage, with no loss in nutritive value that could be stored up to weeks An additional benefit was that the urea treatment reduced the content of HCN in the ensiled stems Base on these results of the experiment 1, experiment determined the effect on feed intake, digestibility and N- retention in goats of supplementing the urea treated cassava stems (UCS) with fresh water spinach and biochar (Experiment 2) DM intake was increased 18% by supplementing the UCS with biochar; and by 24% by addition of water spinach The combined effect of biochar plus water spinach was to increase DM intake by 41% Biochar increased daily N retention by 46% and the biological value of the absorbed N by 12% It is thought that this major benefit from biochar arises from the role it plays as physical support for biofilms acting as habitat for diverse microbial communities working for the benefit of the host animal and thus acting as a form of prebiotic Experiment describes the addition of increasing levels of brewers’ grains (0 to 6%) in a diet of ad libitum sweet cassava forage for growing goats The 4% level of brewers’ grains increased the DM intake, the apparent DM digestibility, the N retention v and the biological value of the absorbed nitrogenous compounds The methane levels in eructed gas increased with a curvilinear trend as the proportion of brewers’ grains in the diet was increased The benefits of biochar were tested further in experiment Twelve growing male goats of the Bach Thao breed, were given a basal diet of ad libitum fresh cassava forage supplemented with 4% (DM basis) of brewers’ grain The biochar was supplied over the range of to 1.5% in diet DM Responses in feed intake, live weight gain and feed conversion to biochar followed curvilinear trends with optimum benefits when biochar was added at 0.86% of the diet DM By contrast, the eructed methane production was decreased linearly with level of biochar Key word: Cassava stems, cassava forage, brewers’ grain, liveweight gain, biochar, methane emission vi TABLE OF CONTENTS DECLARATION i DEDICATION ii ACKNOWLEDGEMENTS iii ABSTRACT .v TABLE OF CONTENTS vii LIST OF FIGURES xii LIST OF TABLES xiv LIST OF ABBREVIATIONS, SYMBOLS AND EQUIVALENTS xvi INTRODUCTION 1 PROBLEM STATEMENT AIMS AND OBJECTIVES OF THE STUDY .2 2.1 THE AIMS OF THE STUDY 2.2 OBJECTIVES OF THE STUDY RESEARCH HYPOTHESES .2 SIGNIFICANCE/INNOVATION OF THE DISSERTATION 4.1 SCIENTIFIC SIGNIFICANCE 4.2 PRACTICAL SIGNIFICANCE CHAPTER OVERVIEW OF RESEARCH ISSUES GOAT PRODUCTION SYSTEMS IN AN GIANG 1.1 GEOGRAPHICAL LOCATION AND CLIMATE IN AN GIANG 1.2 GOAT RAISING SYSTEMS IN AN GIANG 1.2.1 Goat population and management 1.2.2 Feed and feeding management for goat 1.3 OPPORTUNITIES AND CHALLENGE FOR GOAT PRODUCTION THE DIGESTIVE SYSTEMS AND ENTERIC METHANE EMISSION IN RUMINANTS .11 2.1 RUMEN FERMENTATION AND METHANE PRODUCTION 11 2.1.1 Rumen fermentation .11 2.1.2 Volatile fatty acids pattern 12 2.1.3 Protein metabolism .13 2.2 METHANE PRODUCTION 14 2.2.1 Pathway of methane production 14 vii 2.2.2 Manipulation in mitigation of methane production 16 POTENTIAL OF CASSAVA FORAGE FOR GOAT PRODUCTION 19 3.1 PLANT AREA AND DISTRIBUTION OF CASSAVA AND YIELD OF CASSAVA IN VIETNAM, AN GIANG .19 3.2 POTENTIAL OF CASSAVA FORAGE FOR GOAT PRODUCTION 21 3.2.1 Proportion yield of parts of cassava forage 21 3.2.2 Composition of cassava forage, parts of cassava forage 21 3.2.3 Using cassava foliage for goat production 21 3.2.4 Antinutritional factors (Tannin and HCN) of cassava forage .22 3.2.5 Reducing methods antinutrients factor in cassava foliage 24 IMPROVING GOAT PRODUCTION AND REduction of METHANE EMISSION PRODUCTION .26 4.1 IMPROVING STRATEGY GOAT PRODUCTION 26 4.2 CLIMATE CHANGE AND REduction of METHANE EMISSION PRODUCTION .27 CONCLUSIONS 29 CHAPTER EVALUATION OF THE POTENTIAL OF CASSAVA forage AS FEED FOR GOATS IN AN GIANG PROVINCE, VIETNAM .42 INTRODUCTION 42 MATERIALS AND METHODS .43 2.1 THE FOLLOWING INDICATORS WERE USED IN THE INVESTIGATION OF THE SURVEY 43 2.2 DATA COLLECTION AND CALCULATION .44 2.3 CHEMICAL ANALYSIS 45 2.4 STATISTICAL ANALYSIS 45 RESULTS AND DISCUSSION 46 3.1 CASSAVA PRODUCTION IN AN GIANG PROVINCE .46 3.1.1 The production and yiled of cassava 46 3.1.2 Plant area of cassava by district from 2014-2017 46 3.1.3 Yield of cassava with different of variety in An Giang 48 3.1.4 Planted area and the purposes of cassava cultivation 48 3.1.5 Evaluation of chemical composition of cassava parts 50 3.1.6 The fresh and dry yield of cassava proportion with different variety 50 3.2 GOAT PRODUCTION IN AN GIANG PROVINCE 51 viii Lanh N V., Bich N H., Hung B.N., Quyen N N and Preston T R., 2019: Water retention capacity of biochar and its effect on growth of maize Livestock Research for Rural Development Volume 31, Article #95 Olivier P., 2010 The Small-Scale Production of Food, Fuel, Feed and Fertilizer; a Strategy for the Sustainable Management of Biodegradable Waste Binh P L T., Preston T R, Van N H and Dung D V., 2019 Effect of additives (brewer’s grains and biochar) and cassava variety (sweet versus bitter) on nitrogen retention, thiocyanate excretion and methane production by Bach Thao goats Livestock Research for Rural Development Volume 31, Article #1 Preston T R., 2001 Potential of cassava in integrated faming systems In: Proceedings of International Workshop on Current Research and Development on Use of Cassava as Animal feed (Editors: T R Preston, B Ogle and M Wanapat), organized by Khon Kaen University and SIDA- SAREC, Sweden July23-24, 2000 Preston T R., 2015 The role of biochar in farming systems producing food and energy from biomass In: Geotherapy: Innovative Methods of Soil Fertility Restoration, Carbon Sequestration and Reversing CO2 Increase (Editor: Thomas J Goreau) CRC Press, Tayler and Francis Group, Boca Raton, Florida USA Saroeun K, Preston T R and Leng R A., 2018 Rice distillers’ byproduct and molassesurea blocks containing biochar improved the growth performance of local Yellow cattle fed ensiled cassava roots, cassava forage and rice straw Livestock Research for Rural Development Volume 30, Article #162 Saleem, A M., Ribeiro Jr G O., Yang W Z., Ran T., Beauchemin K A., McGeough E J., Ominski K.H., Okine E.K and McAllister T A., 2018 Effect of engineered biocarbon on rumen fermentation, microbial protein synthesis, and methane production in an artificial rumen (RUSITEC) fed a high forage diet1 Journal of Animal Science 96:3121–3130 Sengsouly P and Preston T R., 2016 Effect of rice-wine distillers’ byproduct and biochar on growth performance and methane emissions in local “Yellow” cattle fed ensiled cassava root, urea, cassava forage and rice straw Livestock Research for Rural Development Volume 28, Article #178 Silivong P and Preston T R., 2015 Growth performance of goats was improved when a basal diet of foliage of Bauhinia acuminata was supplemented with water spinach and biochar Livestock Research for Rural Development Volume 27, Article #58 Silivong P and Preston T R., 2016 Supplements of water spinach (Ipomoea aquatica) and biochar improved feed intake, digestibility, N retention and growth performance of goats fed forage of Bauhinia acuminata as the basal diet Livestock Research for Rural Development Volume 28, Article #98 Silivong P., Preston T R, Van N H and Hai D T., 2018 Brewers’ grains (5% of diet DM) increases the digestibility, nitrogen retention and growth performance of goats fed a basal diet of Bauhinia accuminata and forage from cassava (Manihot esculenta Crantz) or water spinach (Ipomoea aquatica) Livestock Research for Rural Development Volume 30, Article #55 115 Sina V., Preston T R and Tham T H , 2017 Brewers’ grains have a synergistic effect on growth rate of goats fed fresh cassava forage (Manihot esculenta Crantz) as basal diet Livestock Research for Rural Development Volume 29, Article #137 Thuy Hang L T., Preston T R., Leng R A and Ba N X., 2018 Effect of biochar and water spinach on feed intake, digestibility and N-retention in goats fed ureatreated cassava stems Livestock Research for Rural Development Volume 30, Article #93 Thomas M Winders, Melissa L Jolly-Breithaupt, Hannah C Wilson, James C MacDonald, Galen E Erickson, and Andrea K Watson, 2019 Evaluation of the effects of biochar on diet digestibility and methane production from growing and finishing steers American Society of Animal Science 2019.3:775–783 Van Soest P J., Robertson J B and Lewis B A., 1991 Methods for Dietary Fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in Relation to Animal Nutrition In: Symposium: Carbohydrate methodology, metabolism, and nutritional implications in dairy cattle Journal of Dairy Science, Volume 74: 3583 – 3597 116 CHAPTER GENERAL DISCUSSION AND CONCLUSIONS GENERAL DISCUSSION 1.1 POTENTIAL OF CASSAVA IN VIETNAM Cassava (Manihot esculenta Crantz) is a major crop in Vietnam, grown on 532,501 producing annually some million tonnes of roots (FAOSTAT, 2017) The roots are used mainly for manufacture of starch and as an ingredient in livestock feed At the time of root harvest, the cassava forage (leaves and petioles) and cassava stems remain as a byproduct The stems are used partly as plant material for the next crop, but the greater part is discarded after root harvest The ready availability of this waste product has led to experiments in our laboratory to utilize them as the basal diet for goats Figure 6.1 Forage and stems that remain when the cassava roots are harvested Growing the crop as a semi-perennial forage with repeated harvesting at to 3month intervals is a recent development (Wanapat, 1997; Preston et al., 2000; San Thy and Preston, 2001) Feeding goats with fresh cassava forage was pioneered in Cambodia where it was shown to be a means of controlling strongyle infection as well as supporting good growth rates (Kerya et al., 19 ) More recently it was fed as the primary diet of growing goats with growth rates reaching 160 g/day when it was supplemented with small quantities of brewers’ grains (4% of diet DM) (Vor Sina et al., 2017) Further developments (Phuong et al., 2018) with goats highlighted the potential benefits from supplementing the cassava forage with biochar, the byproduct of the carbonization of biomass at high temperatures (Lehmann, 2005) and this was particularly effective when the cassava forage was from a “bitter” variety with a high concentration of cyanogenic 117 glucosides Phalla (2007) had earlier shown that cassava stems were as effective as other sources of fibrous biomass for the production of biochar The aims of this thesis were to provide more information on the use of the different parts of the cassava plant as the basal diet of growing goats 1.2 EFFECT ON NUTRITIVE VALUE OF CASSAVA (MANIHOT ESCULENTA CRANTZ) STEMS OF ENSILING THEM WITH UREA Cassava stems contain about 33% DM but only about 6% crude protein (CP) in the DM It was therefore hypothesized that there could be a double benefit from ensiling the cassava stems with urea: (i) to provide the ammonia needed by rumen organisms; and (ii) to improve the digestibility of the stems DM as has been widely proven in the urea-ensiling of low-protein, fibrous feeds such as rice straw The positive effects of storing (ensiling) the cassava stems with addition of urea were the reduction in HCN levels and the possible synthesis of protein from the ammonia derived from the urea On the negative side was the considerable loss of biomass (about 24%) resulting from the fermentation of part of the cassava stems carbohydrate 1.3 DIGESTIBILITY, NITROGEN BALANCE AND METHANE EMISSIONS IN GOATS FED CASSAVA FORAGE AND RESTRICTED LEVELS OF BREWERS’ GRAINS This experiment was designed to provide evidence for the prebiotic effect of brewers’ grains in a basal diet of cassava forage fed to growing goats Proportions of ensiled brewers’ grains above (6%) and below (2%) the 4% level were compared to identify the optimum level Four “Bach Thao” goats (14 ± kg) were fed fresh cassava forage (sweet variety) ad libitum and levels (0, 2, and 6%, DM basis) of brewers’ grains in a 4*4 Latin square changeover design with periods of 14 days Adding 4% of brewers’ grains to the diet of cassava forage increased the DM intake, the apparent DM digestibility, the N retention and the biological value of the absorbed nitrogenous compounds The methane levels in eructed gas decreased with a curvilinear trend as the proportion of brewers’ grains in the diet was increased The benefits of small quantities of brewers’ grains in the diet are believed to be related to 118 their “prebiotic” qualities in enhancing the action of beneficial microbial communities along the digestive tract of the animal 1.4 EFFECT OF BIOCHAR AND WATER SPINACH ON FEED INTAKE, DIGESTIBILITY AND N-RETENTION IN GOATS FED UREA-TREATED CASSAVA STEMS In the first reported study on addition of 1% of biochar in the diet of ruminants (Leng et al., 2012), growth rates were increased 20% but were probably constrained by errors in management of the feed resource (fresh cassava root) that probably propitiated growth of mycotoxins More recent studies have shown synergistic effects from combining biochar with rice distillers’ byproduct in a cassava-based diet for fattening cattle (Sengsouly et al., 2016) and by combining biochar with water spinach in diets of goats (Silivong et al., 2015, 2016) On the basis of this background, the present experiment was designed with the aim of determining if the synergistic effects of biochar and water spinach on growth of goats fed forage of Bauhinia accuminata would be equally manifested when the basal diet was composed of urea-treated cassava stems, shown to be a potential feed resource for goats by Thanh et al., (2013) Four “Bach Thao” goats (LW 14 ± kg) were fed urea-treated cassava stems alone (UCS) or with a supplement of water spinach at 1% of LW (DM basis) (UCSW), with biochar (derived by carbonization of rice husks in an updraft gasifier stove) at 1% of DM intake (UCSB) or with water spinach + biochar (CSWB) The design was a Latin square with four treatments and four periods, each lasting 15 days (ten days for adaptation and days for collection of feces and urine) Urea treatment of the cassava stems increased the crude protein from 5.5 to 11.7% in DM DM intake was increased 18% by supplementing the urea-treated cassava stems with biochar Addition of water spinach increased total DM intake by 25% while the combined effect of biochar plus water spinach was to increase intake by 41% Biochar increased daily N retention by 46% and the biological value of the absorbed N by 12% Biochar provides no protein to the diet, thus it was postulated that the increase in N retained and in its biological value came about as a result of the biochar stimulating rumen microbial growth resulting in an increase in synthesis and hence of 119 absorption of amino acids We suggest this is further evidence that biochar effectively functions as a “prebiotic” – stimulating the activity of beneficial microbial communities through its support for biofilms in the digestive tract of the animal 1.5 EFFECT OF BIOCHAR ON GROWTH AND METHANE EMISSIONS OF GOATS FED FRESH CASSAVA FORAGE The hypothesis underlying the research was that there would be a dose response in growth rate to biochar over the range of to 1.5% in diet DM Twelve growing male goats of the Bach Thao breed, with an initial body weight from 14 to 16 kg, were housed in individual cages and given a basal diet of ad libitum fresh cassava forage (sweet variety) supplemented with 4% (DM basis) of ensiled brewers’ grain The length of the trial was 12 weeks after a period of 10 days to accustom the goats to the diets For all the growth criteria (feed intake, live weight gain and feed conversion), the responses were curvilinear with positive effects from increasing biochar supplementation from to 0.8% of the diet DM followed by a decline as the biochar level was raised to 1.3% in diet DM By contrast, in terms of effects on the rumen fermentation the improvement (decrease in methane production) was linear It is concluded that the beneficial effects of biochar on growth of goats and cattle fed cassava products is because the biochar provides habitat for microbial communities that reduce the toxic effects of the HCN (or its precursors) while still retaining its beneficial effects in modifying the sites of digestion with positive effects on growth and feed conversion GENERAL CONCLUSIONS - The positive effects of storing (ensiling) the cassava stems with addition of urea are the reduction in HCN levels and the possible synthesis of protein from the ammonia derived from the urea and the fermentation of part of the carbohydrate in the cassava stems Urea treatment of the cassava stems (with 3% in DM) increased the crude protein from 5.5 to 11.7% in DM and can be preserved up to weeks - Cassava stems treated with 3% urea in DM improves nutrietive value and DM intake up to 18% by supplementing with biochar Addition of water spinach increased total DM intake by 25% while the combined effect of biochar plus water spinach was to increase intake by 41% Biochar increased daily N retention by 46% and the biological 120 value of the absorbed N by 12% Biochar provides no protein to the diet, thus it is postulated that the increase in N retained and in its biological value came about as a result of the biochar stimulating rumen microbialgrowth resulting in an increase in synthesis and hence of absorption of amino acids - Adding 4% of brewers’ grains to a diet of cassava forage increased the DM intake, the apparent DM digestibility, the N retention and the biological value of the absorbed nitrogenous compounds The benefits of such small quantities of brewers’ grains are believed to be related to their “prebiotic” qualities in enhancing the action of beneficial microbial communities along the digestive tract of the animal - Feed intake, live weight gain and feed conversion were improved by increasing biochar supplementation from to 0.8% of the diet DM followed by a decline as the biochar level was raised to 1.3% in diet DM Rumen methane emissions were reduced with a linear trend as the level of biochar in the diet was increased IMPLICATION AND FUTHER RESEARCH 3.1 IMPLICATIONS The research described in this thesis adds to the increasing degree of appreciation that the cassava crop can play the same role in the tropics that is played by maize in temperate latitudes, as the basis for more intensive systems of ruminant production It has been shown in the research reported in thesis, cassava offers more products useful to animal production than does maize: (i) the root is a proven replacement carbohydrate for maize grain; (ii) the forage from cassava is an excellent source of bypass protein for intensification of ruminant productivity; (iii) the stems can be the basal diet of growing goats as well as being a potential source of renewable energy by gasification to a combustible gas (Phalla, 2007) By contrast, the forage from maize is of negligible value at the time of harvesting the grain An additional factor in favor of cassava is that it is more tolerant to high ambient temperatures and is not likely to suffer the decline in yield, as is the case predicted for maize, as a result of climate change (Jarvis et al., 2012) A major issue that has developed from the research with cassava forage is the role of the cyanogenic glucosides present in both roots and forage of cassava that can give rise to toxic HCN when exposed to enzymes in the digestive tract of animals and humans On the other hand, the presence of these cyanogenic glucosides has been 121 shown to be associated with benefits such as reduction in total gas and methane by rumen organisms (Binh et al., 2017) In Paper 3, we hypothesized that the beneficial effect of biochar on growth of goats was because the biochar provides habitat for microbial communities that reduce the toxic effects of the HCN while still retaining its beneficial effects in modifying the sites of digestion as discussed by Inthapanya et al., (2019) The concept is that low levels of cyanogenic glucosides act to reduce the rate of rumen fermentation thus facilitating rumen escape of protein for more efficient enzymic digestion in the small intestine, while potentially fermentable carbohydrate continues to the cecum where the fermentation is acetogenic such that losses as methane are avoided A recent paper from Houda et al., (2017) describes a similar shift in digestion sites as a result of including small amounts of thymol in the diet pf lactating cows In vitro rumen gas production was reduced by the additive, milk production was increased and methane in eructed gas was reduced 3.2 FUTURE RESEARCH The new information that has resulted from this research is the role of additives such as brewers’ grains and biochar which appear to act in the ruminant animal as “prebiotics”, that “facilitate” the activity of the resident microflora or act by sequestering toxic substances such as mycotoxins and other anti-nutritional compounds such as the precursors of hydrocyanic acid (HCN) The research in this thesis has been with “sweet” varieties of cassava which have lower concentrations of HCN precursors than the “bitter” varieties (Phuong et al., 2019) However, bitter varieties have higher yields of roots than the sweet varieties and are exclusively planted when the aim is industrial starch production (reference) Relevant observations are that goats having free access to forage of both sweet and bitter varieties opted to consume equal proportions of each (Phuong et al., 2019), despite observations elsewhere (Chiv Phiny., 2019), that when goats were fed exclusively on bitter cassava forage there was high mortality caused by HCN toxicity The fact that growth rates, as measured by N retention, were better and that methane in eructed gases was reduced, for the 50:50 balance of sweet and bitter varieties compared with a sweet variety fed alone, is further justification for promoting research equally with bitter and sweet varieties In such research, the relative responses to prebiotic additives should have high priority 122 The final issue that requires research – and which should have highest priority – concerns the role of biochar in sequestering atmospheric carbon dioxide Biochar contains of the order of 60% carbon derived from the atmosphere and which, applied directly to soils, is resistant to further oxidation – hence its role in mitigation of climate change by sequestering atmospheric carbon dioxide It is assumed that, because of its resistance to further chemical change, biochar fed to animals will be excreted largely unchanged and that when retuned to soil as fertilizer, the excreta from animals fed biochar will continue to have beneficial effects on soil fertility as well as a continued role in sequestering carbon The area of research should have high priority 123 REFERENCES Binh P L.T., Preston T R, Khang D.N and Leng R A., 2017 A low concentration (4% in diet dry matter) of brewers’ grains improves the growth rate and reduces thiocyanate excretion of cattle fed cassava pulp-urea and “bitter” cassava forage Livestock Research for Rural Development Volume 29, Article #104 Binh P L T., Preston T R, Van N.H and Dung D.V., 2018 Methane production in an in vitro rumen incubation of cassava pulp-urea with additives of brewers’ grain, rice wine yeast culture, yeast-fermented cassava pulp and leaves of sweet or bitter cassava variety Livestock Research for Rural Development Volume 30, Article #77 FAOSTAT, 2017 FAO Statistical Database (Food and Agriculture Oranization of the United Rome) Inthapanya S, Preston T R, Leng R A, Phung L D and Ngoan L D, 2019 Simulating rice distillers’ by-product with fermented sticky rice; effects on methane production in an in vitro rumen fermentation of ensiled cassava root, cassava forage and urea Livestock Research for Rural Development Submitted Leng R A, Preston T R and Inthapanya S., 2012 Biochar reduces enteric methane and improves growth and feed conversion in local “Yellow” cattle fed cassava root chips and fresh cassava forage Livestock Research for Rural Development Volume 24, Article #199 Binh P.L T., Preston T R, Van N H and Dung D V., 2019 Effect of additives (brewer’s grains and biochar) and cassava variety (sweet versus bitter) on nitrogen retention, thiocyanate excretion and methane production by Bach Thao goats Livestock Research for Rural Development Volume 31, Article #1 Sengsouly P and Preston T R., 2016 Effect of rice-wine distillers’ byproduct and biochar on growth performance and methane emissions in local “Yellow” cattle fed ensiled cassava root, urea, cassava forage and rice straw Livestock Research for Rural Development Volume 28, Article #178 Silivong P., Preston T R., Van N H and Hai D T., 2018 Brewers’ grains 5% of diet DM) increases the digestibility, nitrogen retention and growth performance of goats fed a basal diet of Bauhinia accuminata and forage from cassava (Manihot esculentaCrantz) or water spinach (Ipomoea aquatica) Livestock Research for Rural Development Volume 30, Article #55 Silivong P and Preston T R., 2015 Growth performance of goats was improved when a basal diet of forage of Bauhinia acuminata was supplemented with water spinach and biochar Livestock Research for Rural Development Volume 27, Article #58 Silivong P and Preston T R., 2016 Supplements of water spinach (Ipomoea aquatica) and biochar improved feed intake, digestibility, N retention and growth performance of goats fed forage of Bauhinia acuminata as the basal diet Livestock Research for Rural Development Volume 28, Article #98 124 Sina V., Preston T R and Tham T H., 2017 Brewers’ grains have a synergistic effect on growth rate of goats fed fresh cassava forage (Manihot esculenta Crantz) as basal diet Livestock Research for Rural Development Volume 29, Article #137 Thanh T X., Hue K T, Anh N N and Preston T R., 2013 Comparison of different forages as supplements to a basal diet of chopped cassava stems for growing goats Livestock Research for Rural Development Volume 25, Article #7 Wanapat M., Pimpa O., Petlum A and Boontao U., 1997 Cassava hay: A new strategic feed for ruminants during the dry season Livestock Research for Rural Development 9(2), 1-5 125 PUBLICATION LIST This thesis is based on the work contained in the following papers: Paper 1: Digestibility, nitrogen balance and methane emissions in goats fed cassava forage and restricted levels of brewers’ grains Livestock Research for Rural Development Volume 30, Article #68 from http://www.lrrd.org/lrrd30/4/thuy30068.html Paper 2: Effect of biochar and water spinach on feed intake, digestibility and Nretention in goats fed urea-treated cassava stems Livestock Research for Rural Development Volume 30, Article #93 from http://www.lrrd.org/lrrd30/5/thuyh30093.html Paper 3: Effect of biochar on growth and methane emissions of goats fed fresh cassava forage Livestock Research for Rural Development Volume 31, Article #67 from http://www.lrrd.org/lrrd31/5/thuyhang31067.html Paper 4: Effect on nutritive value of cassava (Manihot esculenta Crantz) stems of ensiling them with urea Livestock Research for Rural Development Volume 31, Article #92 from http://www.lrrd.org/lrrd31/6/thuyh31092.html 126 APPENDICES Experiment 2: Cassava stems untreated and treated with difference level of urea (0, 1,2,3 and 4% in DM) Cassava stems before and after mixing urea 0% urea 1% urea 0% urea 1% urea 2% urea 3% urea After two weeks storing 2% urea After weeks storing a 3% urea 4% urea 4% urea 0% urea 0% urea 1% urea 1% urea 2% urea After weeks storing 2% urea 3% urea 3% urea 4% urea 4% urea After weeks storing Experiment & 5: A wood frame covered with clear glass and Gasmet infra-red meter (GASMET 4030 b Mau p4s3,p5s3,p8s3,p19s3,45,64-66,79-81,92-95,97,98,104106,108,110,112,117,p1s4,p2s4 Đen p1s2-p3s3,p6s3,p7s3,p9s3-p18s3,20-44,46-63,67-78,82-91,96,99103,107,109,111,113-116,118-126 c ... LOCATION AND CLIMATE IN AN GIANG 1.2 GOAT RAISING SYSTEMS IN AN GIANG 1.2.1 Goat population and management 1.2.2 Feed and feeding management for goat 1.3 OPPORTUNITIES AND... grain and biochar in An Giang province, Vietnam There were one survey and four experiments in this study The survey of cassava and goat systems in Tinh Bien and Tri Ton districts, An Giang province... for all their support and encouragement throughout my study And special thanks to my husband Tran Xuan Hien, two children’s who understood my work, and shared the happiness and sadness with me,