M. Blümmel

Formation of complexes between polyvinyl pyrrolidones or polyethylene glycols and tannins, and their implication in gas production and true digestibility in in vitro techniques

Various tannin-complexing agents have been used to study the potential adverse effects of tannins on rumen metabolism. Using a method based on turbidity formation, the binding of various tannin-complexing agents (polyvinyl polypyrrolidone (PVPP), polyethylene glycol (PEG) of molecular weights 2000 to 35,000, and polyvinyl pyrrolidone (PVP) of molecular weight 10,000, 40,000 and 360,000) to tannins (tannic acid, purified tannins from quebracho (Aspidosperma quebracho) and leaves of trees and shrubs (Acioa barteri, Dichostachys cinerea, Guiera senegalensis, Piliostigma reticulatum)) was investigated at different pH values. The binding of all the tannins with PVPP was highest at pH 3-4 and lowest at pH 7. For all the pH range (3-7) studied, the binding of PEG was higher than that of PVP. For all the tannins except tannic acid, the binding to PVP was the same from pH 4.7 to 7. Similar results were observed for the PEG of molecular weight 6000 or higher for all the tannins except quebracho tannins for which the binding increased as the pH increased from 3 to 7. The binding with PEG 2000 decreased to a greater extent as the pH reached near neutral and for PEG 4000 this decrease was slightly lower. Addition of these tannin-complexing agents to the in vitro gas system resulted in higher gas production from tannin-rich feeds (increase varied from 0 to 135%). The PEG were the most effective followed by PVP and PVPP. The PEG 35,000 was least effective. The efficiency of other PEG was similar. The PEG 6000 was preferred to PEG 2000 or 4000 as its binding to tannins was higher at near neutral pH values. The gas production increased with an increase in the amount of PEG 6000 up to 0.6 g/40 ml rumen-fluid-containing medium containing 0.5 g tannin-rich feed, beyond which no increase was observed. The percentage increase in gas value at 24 h fermentation correlated significantly with tannin values, the highest correlation (r 0.95) being with protein precipitation capacity of tannins. The increase in gas production was associated with higher production of short-chain fatty acids with little change in their molar proportions, suggesting an increase in organic matter digestibility by inclusion of the PEG in tannin-rich feeds. However, apparent and true digestibilities were lower on addition of the PEG, due to the presence of PEG-tannin complexes in the residues. The use of this bioassay (percentage increase in gas production in the presence of PEG 6000) along with other tannin assays would provide a better insight into the nutritional significance of tannins.

In vitro gas measuring techniques for assessment of nutritional quality of feeds: a review

The close association between rumen fermentation and gas production has been recognised for over a century, but it is only since the 1940s that quantification techniques for measuring gas production have been evolved. The gas measuring technique has been widely used for evaluation of nutritive value of feeds. More recently, the upsurge of interest in the efficient utilisation of roughage diets has led to an increase in the use of this technique due to the advantage in studying fermentation kinetics. Gas measurement provides a useful data on digestion kinetics of both soluble and insoluble fractions of feedstuffs. This review describes the available in vitro gas measuring techniques used for feed evaluation with emphasis on assessing their relative advantages and disadvantages. Origin of gas, stoichiometry of gas production, and various areas for application of gas measurement in feed evaluation are discussed. Some important results obtained using gas measuring techniques have been highlighted, and the potential of gas techniques for tackling some interesting areas of research are presented. The need to consider substrate incorporation into microbial cells in gas measuring technique is pointed out.

The relationship between in vitro gas production, in vitro microbial biomass yield and 15N incorporation and its implications for the prediction of voluntary feed intake of roughages.

The relationship between in vitro gas production, concomitant in vitro apparent and true DM degradability has been examined in forty-two roughages. The partitioning of truly-degraded substrate between gas volume and microbial biomass yield and 15N incorporation into cells was also investigated. The relevance of this partitioning for the regulation of DM intake (DMI) was examined for fifty-four roughages. The results can be summarized as follows. In vitro gas production and in vitro apparent and true degradability are highly correlated (P < 0.0001), r being 0.96 and 0.95 respectively. There is an inverse relationship between in vitro gas production and microbial biomass yield (r--0.67, (P < 0.0001) and also 15N enrichment (P < 0.001) when the variables were related to a given unit of substrate truly degraded. Selecting roughages by in vitro gas production may well be a selection against maximum microbial yield and a combination of in vitro gas volume measurements with a complementary determination of the substrate truly degraded is proposed, to calculate a partitioning factor (PF) reflecting the variation of short-chain fatty acid production per unit substrate degraded. PF is calculated as the ratio, substrate truly degraded: gas produced by it. PF was highly significant (P < 0.0001) in DMI prediction when included in stepwise multiple correlations together with in vitro gas volume variables reflecting the extent and rate of gas production; 11% of the variation in DMI was accounted for by the PF. The total model, including extent and rate of gas production and the PF, accounted for 84% of the variation in DMI. Roughages producing proportionally less gas per unit substrate truly degraded had higher feed intakes.

In vitro rumen apparent and true digestibilities of tannin-rich forages

Abstract Presence of condensed tannins could interfere in the determination of in vitro rumen true digestibility. The neutral detergent fibre (NDF) and acid detergent fibre (ADF) values obtained after removal, by centrifugation, of the liquid phase of the incubation medium containing tannins (centrifuged set) were compared with those obtained directly (direct set) for four tannin-rich leaves ( Dichostachys cinerea, Cassia sieberiana, Robinia pseudoacacia and Acioa barteri ). The NDF values in the centrifuged set were significantly higher than in the direct set. On the other hand, ADF values were significantly higher in the direct set. The contents of both nitrogen and condensed tannins were significantly higher in the NDF of the centrifuged set compared with in the direct set, while nitrogen and condensed tannins were significantly higher in the ADF of the direct set. Nitrogen and condensed tannins were present in the ADF of both centrifuged and direct sets at levels higher than in the ADF of the sample incubated. This was similar for the case of NDF of the centrifuged set. These observations suggest that ADF values obtained in the centrifuged or direct set and NDF values in the centrifuged set are incorrect. Following fermentation, the centrifugation process for recovering residue, comprising of unfermented residue and the microbes used for determination of apparent digestibility, and then removal of microbes from this residue by neutral detergent solution treatment for determination of true digestibility lead to serious artefacts in these digestibility values. Chromatographic assays based on measurement of free monomeric constituents are recommended for determination of fibre in tannin-rich feeds.

The prediction of dry matter intake of temperate and tropical roughages from in vitro digestibility/gas-production data, and the dry matter intake and in vitro digestibility of African roughages in relation to ruminant liveweight gain

Abstract The dry matter intake (DMI) of 54 roughages has been predicted by various parameters obtained from the incubation in an in vitro gas-production system. Rate and extent of gas production from whole roughage and isolated NDF were described by two exponential models y = B (1 − e − c ( t − lag ) ) and y = B (1 − e − ct ) for t > lag , where y = gas volume at a given time t , B = potential cumulative gas production, c = rate of gas production and lag = initial lag for the onset of fermentation. Approximately 69% ( P B (59.6%), lag (5.6%) and c (3.4%). In the model where t > lag , B accounted for 53.6% of the variation in DMI and c for 16.1%. Approx. 81% ( P B NDF 77.5%, C NDF 3.7%, lag NDF not significant). In the model where t > lag , B NDF accounted for 77.5% and c NDF for 5% of the variation in DMI. Combinations of in vitro true degradabilities at 36 or 48 h with gas volumes measured in the same incubation either at 4, 6 or 8 h accounted for approximately 82.0% of the variation in DMI and the inclusion of the partitioning factor (PF) improved the correlation ( r 2 ) to 0.87. The PF reflects the partitioning of truly degraded substrate between short chain fatty acids and microbial cells, and roughages having proportionally higher microbial yield had higher DMI. These parameters together also predicted liveweight gain (LWG) of sheep fed six roughages from Ethiopia ( r 2 = 0.98–0.99). The importance of DMI information for LWG is also discussed.