A Conceptual Model of Protein-Precipitable Polyphenols (Condensed Tannins) on Protein Binding and Protein Digestion in Ruminants

Abstract Text:    There is a need to better understand the mechanisms by which biologically active protein-precipitable polyphenols (PPP) from forages bind to dietary protein in the rumen and how this impacts the ruminant animal’s requirement for protein. The objective of this study was to model the effect of biologically active PPP on ruminal protein binding and the potential protection of rumen-bypass protein. Green live-leaf material from six species of warm-season perennial legumes was sampled during August of 2011 and 2012. The effect of biologically active PPP on protein-binding ability was evaluated by combining rhizoma peanut with experimental forages to create separate basal diets (by year) containing PPP ranging from 1-9%. Protein-binding ability was determined by an in vitro protein-precipitable phenolics assay and nitrogen analysis of the protein-phenolic precipitates. Data were analyzed in linear-segmented and quadratic-segmented regressions (PROC NLIN of SAS) to model the effects of biologically active PPP on protein-binding ability. The Model Evaluation System (MES; http://nutritionmodels.tamu.edu/mes.html) was used to compare the correctness of the linear- versus the quadratic-segmented regression. For the linear-segmented regression, β₀ is equal to α, β₁ is equal to β, and α, β, X_m and Y_m were equal to -3.2935, 6.3329, 6.4 and 37.24, respectively. For the quadratic-segmented regression, β₀ is equal to α, β₁ is equal to β, β₂ is equal to γ and α, β, γ, X_m and Y_m were equal to -10.7117, 11.4534,  -0.6854, 8.32 and 37.14, respectively. The data pattern suggests that the relationship between PPP and protein bound is not linear in nature. Beyond a certain point, in this case a concentration of PPP occurring between 6.4 and 8.3%, the amount of protein bound does not increase as the concentration of PPP in the diet increases. While both segmented regressions were similar in terms of adequacy, the linear-segmented regression was slightly more precise than the quadratic-segmented regression (R² 0.811 and 0.809, respectively) at accounting for variation that occurred in observed values. Overall, the regression comparison indicated that the linear-segmented regression is 1.2 times more likely to correctly predict observed values of protein binding by PPP than the quadratic-segmented regression. However, either model would be an acceptable tool for use in modeling the effect of forage PPP on rumen-protein binding and potential protection of rumen-bypass protein and in a decision support system.

Keywords: Legume, protein, tannin