Estimating the Nutritive Value of Forages Containing Tannin and Phenols by Chemical and Bioassay Methods
1976
Burns, J. C. | Cope, W. A.
Anti-quality substances in crownvetch (Coronilla varia L. and sericea lespedeza [Lespedeza cuneata (Dumont) G. Don] require that analytical methods show their presence and effect on nutritive value of these forages. This study compares a chemical and a bioassay (in vitro dry matter disappearance) method of estimating the nutritive value of six morphologically different crownvetch clones (separated into stems, petioles, leaflets and flowers) and three strains of sericea lespedeza (separated into stems and leaves). The chemical method uses detergents to fractionate the fibrous constituents of forages. Use of the content of fibrous fractions in a summative equation provides an estimate of true digestible dry matter (EDDM). The bioassay relies on rumen microflora to degrade the forage sample, resulting in an in vitro estimation of dry matter disappearance (IVDMD). When total phenol and tannin concentrations were high and fiber low (leaf tissue) agreement was lacking between the chemical and bioassay methods (crownvetch r = —0.22; lespedeza r = —0.38, harvest 1 and r = 0.04, harvest 2). The relationship between methods was significant (P ≤ 0.01) when total phenol and tannin concentrations were low and fiber (stems) of primary concern (crownvetch stems r = 0.93; lespedeza stems r = 0.59, harvest 1 and r = 0.77, harvest 2). The chemical method was insensitive to either total phenol or tannin concentrations, whereas, the bioassay appeared sensitive to both structural constituents and the anti-quality substances. Methods used to estimate nutritive values of forages that reflect only some of the quality constituents are of specific applicability and their indiscriminate use may lead to erroneous conclusions.
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