School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffect of tannins on the nutritive value of tropical shrubs( 1995)Three studies were conducted to assess the effect of condensed tannins (CT) on the nutritive value (NV) of some tropical shrubs. The first study involved two experiments (Experiments la and lb). Experiment la was designed to investigate the nylon bag dry matter digestibility (NBDMD) of 12 tropical shrubs (10 legumes and 2 non-legumes) varying in their content of extractable (ECT) and bound (BCT) condensed tannin. Samples were finely-chopped (- 3 mm) and incubated as either fresh or after oven-drying at 65C for 48 hours in the rumen of 5 steers. The contents of nitrogen (N), neutral (NDF) and acid (ADF) detergent fibres, lignin, ECT by vanillin (ECTvan) and butanol-HC1 (ECTbut) assay methods, protein-bound (PCT) and fibre-bound (FCT) were measured and the relationship between these plant constituents and the NBDMD were determined. Experiment lb was undertaken to determine the effect of drying method of forage samples on the chemical and CT contents, and NBDMD of 6 shrub legumes. A further purpose was to determine the effect of drying method on the prediction of NBDMD from the chemical and CT constituents. Forage samples were either freeze-dried (FD) at -30C or oven-dried (OD) at 65C for 48 hours and then ground through a 1 mm screen before the analyses or nylon bag digestion. The second study was an investigation of the effect of preparation method of fresh forage samples on the NBDMD of two leguminous shrub species, Calliandra calothyrsus and Lysiloma watsonii. Preparation methods included: fresh material coarsely (Ti) or finely (T2) chopped immediately after harvest and inserted in the rumen of steers within 30 minutes of harvest and preparation of sample; fresh material finely chopped and held for 6 hours before incubation either, in the oven at 65C (T3), in a sealed plastic bag and refrigerated at about 4C (T4) or in shade (wilted) under room temperature (approx. 23C, T5); unchopped fresh material was sealed in a plastic bag and refrigerated for 6 hours before being finely-chopped and inserted in the rumen immediately after preparation (T6). All samples were incubated in the rumen of steers for 48 hours. In the third experiment the effect of the addition of polyethylene glycol (PEG) on in vitro dry matter digestibility (IVDMD) of freeze- (FD) and oven-dried (OD) samples of 4 shrub species was determined. Two in vitro techniques were used, the rumen-liquor-pepsin (RLP) and the pepsin-cellulase (PC) methods. Polyethylene glycol was added as a solution at 0, 80, 160, 320 or 640 mg/g DM of sample incubated. The results of the chemical analysis showed significant (P<0.001) variation between species in Experiment 1a but not in Experiment 1b. The contents of ECT and BCT (PCT+FCT) also varied between species and method of analysis, although these differences were not tested. The ranges of the content of ECT in FD samples for vanillin and butanol- HC1 respectively were: 0.35-11.62% and 0-9.34% in Experiment 1a, 0.3-6.5% and 0.1- 7.6% in Experiment 1b. Nylon bag dry matter digestibility was significantly (P<0.001) different between species, ranging from 17-81% in the fresh samples and 75-88% in FD samples. There were significant (P<0.001) linear correlations between NBDMD and N (r = 0.438), NDF (r = -0.663), ADF (r = -0.666) lignin (r = -0.810), ECTvan (r = -0.548), ECTbut (r = -0.620) and total CT (r = -0.601) but not with PCT (r = -0.210) or FCT (r = -0.389) (P>0.05). Oven-drying significantly (P<0.001) increased the contents of NDF, ADF and lignin in Experiment 1a but not in Experiment 1b. The effect of oven-drying on the content of ECT or BCT was not consistent between species or drying methods (FD vs. OD). While some forms of CT were increased by oven-drying in some species, others were decreased but this pattern was not similar for all species. Oven-drying at 650C significantly (P<0.001) depressed NBDMD of C. calothyrsus, L. watsonii and A. currassavica (Experiment 1a), and C. calothyrsus and L pallida (Experiment 1b). The correlation between NBDMD and lignin was significant when FD samples were used (r = -0.55; P<0.001) but not when OD samples were used (r = -0.29; P>0.05) for both the analysis and digestion study. The relationship between NBDMD and other chemical or CT component was not affected by drying. Preparation method of fresh forage significantly (P<0.001) affected NBDMD of both C. calothyrsus and L. watsonii. Coarse-chopping (- 15 mm) gave lower estimates of NBDMD compared to fine-chopping (- 3 mm). Holding freshly-chopped samples for 6 hours significantly (P<0.05) depressed NBDMD and this depression was related to temperature and was greatest in OD samples. In vitro dry matter digestibility differed significantly (P<0.001) between species, method of drying samples, the level of PEG and some of the interactions between these factors. Addition of PEG at 80-160 mg/g DM of sample significantly (P<0.05) improved IVDMD of the tannin-containing shrubs. Increasing the level of PEG beyond 160 mg up to 640 mg/g DM did not show any further improvement in IVDMD. Generally, the PC method gave significantly (P<0.05) higher IVDMD than the RLP method at all levels of PEG (62.77 vs. 54.14%, s.e.d. = 0.28). Similarly, FD samples were significantly (P<0.05) higher in IVDMD than OD samples (59.32 vs. 57.60% respectively, s.e.d. = 0.28). It was concluded from these studies that the composition of CT varied between assay technique and species. Also, the effect of CT on the NV of tanninferous tropical shrubs also varied between species. The variation in the effect of oven-drying on the chemical and CT contents and the digestibility of shrubs was attributed to the differences in the composition and nature of CT present in individual shrubs. It appears that the depressing effect of CT on digestibility was primarily due to the free or extractable form rather than the bound CT in the species studied. However, in addition to CT, other factors such as the fibre and lignin components of shrubs also affect digestibility. Method of preparation of fresh samples should be taken into consideration when conducting a chemical analysis or nylon bag digestibility study. Whenever possible, freshly-chopped forage materials should be used. Otherwise, fresh materials can be stored under low temperature (VC) prior to incubation in the rumen. Polyethylene glycol improved IVDMD irrespective of in vitro technique or drying method but the efficiency of PEG varied from one species to another since the nature and reactivity of CT vary between species. Polyethylene glycol was not able to overcome the depression in IVDMD associated with oven-drying probably due to the formation of some irreversible CT-protein complexes. The Pepsin-cellulase in vitro technique gave higher estimates of IVDMD than the RLP technique after 48 hours of incubation. More studies were suggested to further understand the anti-nutritional effects of CT, most especially in dried forages. Characterisation of tannins in plant materials should precede their quantitative determination. Also, more in vivo studies and feeding trials are recommended since in addition to digestibility and chemical composition, the NV of forage is a function of voluntary feed intake and the efficiency of conversion of digested nutrients to tissue gain and reproductive performance.