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Condensed Tannins in Tropical Forage Legumes: Their Characterisation and Study of Their Nutritional Impact from the Standpoint of Structure-Activity

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Condensed Tannins in Tropical Forage Legumes: Their Characterisation and Study of Their Nutritional Impact from the Standpoint of Structure-Activity The University of Reading Condensed tannins in tropical forage legumes: their characterisation and study of their nutritional impact from the standpoint of structure-activity relationships By Rolando Barahona Rosales B.S. (Kansas State University, 1992) M.Sc. (Kansas State University, 1995) A thesis submitted for the degree of Doctor of Philosophy Department of Agriculture, The University of Reading July 1999 SOLANGE, SAMUEL, SANTIAGO and ISABELA: Contar con ustedes ha sido la bendicion mas grande de mi vida. En esta y en las futuras obras de mi vida se los encontrara a Uds. como mi inagotable fuente de inspiracion. i ACKNOWLEDGEMENTS This thesis is the culmination of a long-held dream, that started in the days I was attending the John F. Kennedy School of Agriculture at San Francisco, Atlantida, Honduras. There, and throughout my days as a student at Kansas State University, my daily interactions with friends and professors kept alive and nurtured those aspirations. Undoubtedly, the number of persons and institutions to whom I am indebted for the completion of my PhD career is enormous. I would have not advanced so far into this journey, however, without the encouragement and inspiration provided by my parents, sisters and brothers, who never made a secret of their satisfaction for my chosen career. Throughout my PhD studies, I was fortunate to receive advice and supervision from four great individuals: At the Centro Internacional de Agricultura Tropical (CIAT) in Cali, Colombia, I received supervision and most invaluably, friendship from Dr. Carlos Lascano. Ever since we met in 1994, Carlos and I have shared the desire to understand the true impact of tannins in ruminant nutrition. At the Institute of Grassland and Environmental Research (IGER) in Aberystwyth, UK, Dr. Philip Morris and Professor Mike Theodorou have earned my gratitude and respect. To Phil I owe my understanding of condensed tannin chemistry and of the analytical methods to determine tannin structure. Mike’s supervision on the microbiological aspects of this thesis was invaluable, but his help was also important to ensure the well being of my family and me during our stay in the United Kingdom. At the University of Reading, UK, I was fortunate to have the supervision of Professor Emyr Owen. I thank him for his genuine interest in my research and for the encouragement he never failed to provide. In the completion of this thesis, and to a greater level than at any other stage of my academic career, I received help, advice and friendship from an outstanding group of professionals. I like to acknowledge that help, without which, I would not be able to complete this task. ii First of all, I must acknowledge the help provided my wife Solange in many areas of this thesis. Her help was invaluable in the conduction of the experiments involving the determination of enzymatic activity in the presence of condensed tannins and in the purification of condensed tannins. At the Forage Quality of CIAT, the help provided by Nelmy Narvaez allowed me to efficiently organise and conduct my experimental work. I acknowledge and thank the help provided by Gustavo Ospinal, Orlando Trujillo and Benilda Garcia in the determination of the neutral and acid detergent fibre, in vitro dry matter digestibility and in the conduction of gas production assays. Axel Schmidt generously provided the samples for the study of the G X E interaction in Desmodium ovalifolium. I also acknowledge the people at the Analytical Lab of CIAT for their help in the crude protein determinations. I also thank Gerardo, Julia, Patricia, Ever, don Eduardo and “el pupilo” for all their help and friendship. Claudia Ximena, Ma. Ximena, Patricia Ximena, Silvia and don Alvarito, your friendship and support made easier my days at CIAT. I would like to thank Dr. A. C. Longland for advice and R. Pilgrim for technical assistance in the determination of non-starch polysaccharides. I thank Dr. John Cockburn and Pauline for their help in the determination of amino acid content in my samples. I thank A.E. Brooks for teaching me how to conduct gas production experiments and for her help in growing the Festuca arundinacea cell walls. Eleanor Bakewell provided technical assistance in the determination of VFAs. I thank M.S. Dhanoa for statistical advice. Dr. W.Y. Zhu and Mr. E. Oskodze generously provided the culture filtrates from Neocallimastix hurleyensis, which I used for the determination of enzymatic activity in the presence of condensed tannins. The clonal ferulic esterase was a gift from Genencor International. Elke, Paul, Andy, Barbara, Marcia, John and Chris, your friendship and support are greatly appreciated. I would like to express my gratitude to Dr. M. Robbins, his help ensured the conclusion of this thesis. My gratitude also goes to the people in the Department of Agriculture at the University of Reading, especially to Professor D. Beever, for all the help provided. iii Research for this project was made possible by a grant from the Livestock Program of DFID (formerly ODA) which was managed by NRI. To these and to all those who helped make my dream a reality, thanks. iv ABSTRACT In the tropics, animal production is compromised by the lack of suitable forage for the animals. As a further constraint, low-fertility, acid soils are prevalent in areas where many small and medium size farmers are dependent upon livestock agriculture as their main source of income. This prevents the incorporation of high quality forage legumes (i.e Leucaena leucocephala, Gliricidia sepium) into those grazing systems, as these legumes do not perform very well under those conditions. In turn, most of the tropical forage legumes with good adaptation to acidic soils are of low quality and poor acceptability by livestock, which might be associated with their high contents of condensed tannins. However, research on the impact of condensed tannins upon the nutritional value of forage has not provided unequivocal results. This could be related to the variation in structure (molecular weight and monomeric composition) observed among condensed tannins from different legumes. This study investigated the impact of condensed tannins from different tropical legumes on the degradation of different substrates by rumen microorganisms and/or their enzymes under the scope of structure-activity relationships. In an initial experiment, the chemical composition and in vitro degradability of immature and mature forage of seven tropical legumes (Calliandra calothyrsus, Clitoria fairchildiana, Desmodium ovalifolium, Flemingia macrophylla, Leucaena leucocephala, Leucaena macrophylla and Leucaena pallida) were determined. Chemical composition varied greatly among the different samples. In turn, gas pool after 144 h of fermentation ranged from 66 to 140 ml and in vitro degradability ranged from 230 to 630 g kg-1 of dry matter. The concentration of condensed tannins was associated with a decrease in the initial rate, but not with the extent, of dry matter degradation. In particular, the concentration of condensed tannins had significant negative relationship with the maximum rate of gas production observed during the fermentation of these samples (p ≤ 0.0011). Results from this experiment suggested that condensed tannins differ in nutritional effects and that factors other than condensed tannins affected the in vitro degradation of the legumes studied. In a second set of experiments, condensed tannins purified from all tanniniferous legumes differed in molecular weight, (number-average molecular weight = 2360 to 4880 dalton). Condensed tannins also differed in monomeric composition, with some v being mostly composed of procyanidin (up to 86 %), while others were mostly composed of prodelphinidin (up to 87 %). Procyanidin content and molecular weight of condensed tannins were negatively associated with the maximum rate of gas production and with the gas accumulation observed in the first experiment, respectively (P < 0.01). Another set of gas production experiments showed that the ability of condensed tannins to inhibit the fermentation of substrates was greater when they were associated to the substrate than when they were present in soluble form. Condensed tannins from L. leucocephala were the least effective at inhibiting substrate fermentation, which was related to their low molecular weight. In a final series of experiments, condensed tannins from L. leucocephala were also among the least effective at inhibiting the activity of fibrolytic enzymes from the gut anaerobic fungi Neocallimastix hurleyensis. Results from this experiment also suggested that the observed degree of inhibition of a given enzyme by condensed tannins is also a function of characteristics of both the enzymes and the substrate they hydrolyse. A final experiment investigated changes in chemical composition in five different accessions of the tropical legume D. ovalifolium grown under different environmental conditions (six sites, two fertilisation levels and two growth seasons) in Colombia, South America. There was considerable variation in chemical composition (condensed tannins included) in response to environmental variables. Likewise, the in vitro dry matter digestibility ranged from 250 to 559 g kg-1 of dry matter. Differences associated with genotype were comparatively smaller. Content of condensed tannins was inversely related to in vitro degradation. However, results from this experiment suggested that in vitro degradation of forages is a function of their entire chemical composition.
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