Effect of tannin-containing legume forages on crude protein degradation in vitro

Abstract Text: Research in our laboratory has demonstrated greater than 10% improvements in feed efficiency by balancing beef cattle diets for post ruminal amino acids. To achieve this, protein sources with low rumen degradability must be utilized. Expensive animal proteins or treated soybean meal (SBM) are the most commonly available sources. Therefore, it is prudent to examine forages with low rumen degradability as alternatives. Tannin-containing forages have been shown to create high proportions of rumen undegradable protein (RUP) due to protein binding properties of condensed tannins (CT). The objective of this study was to evaluate the ability of selected warm-season perennial legumes containing CT to decrease ruminal crude protein (CP) degradation in vitro. We hypothesized the extent of ruminal CP degradation would be inversely related to forage CT protein precipitation ability. Six legume species were evaluated. Three field replications of each CT-containing forage species and one replication of alfalfa (minimal CT) were mixed in a 1:1 ratio with SBM.  Forage: SBM samples were weighed into tubes, inoculated with a 1:3 mixture of strained rumen fluid and McDougall’s buffer, and closed with stoppers fitted with one-way valves. Samples were fermented for 0, 12, 24 and 48 h in a 39°C shaking water bath. Tubes were centrifuged to remove supernatant, dried at 55°C and analyzed for dry matter (DM) and CP disappearance. Supernatant subsamples were taken for ammonia analysis 0, 4, 8, 12, 16, 24, 36, and 48 hours of fermentation, at which times pH was measured. All tannin-containing forages were more effective at protecting protein from degradation, in vitro than alfalfa. Increased CT concentration decreased the potential extent of CP degradation and tannin-containing forages had greater %DM and CP remaining at all hours compared to alfalfa and a SBM control. Plant protein precipitable phenolics concentration, a measure of biologically active CT, was negatively correlated (P<0.0001) with CP digestibility (R²=-0.72), protein solubility (R²=-0.75), and the extent of CP degradation (R²=-0.78). Lespedeza stuevei Nutt. (tall lespedeza) and two ecotypes of Acacia angustissima var hirta(Nutt.) B.L. Rob (Prairie acacia) were identified as plants contributing exceptionally high protection from in vitro ruminal SBM protein degradation when mixed in a 1:1 ratio. Current research is expanding upon these results to evaluate these plants as RUP sources in ruminant diets.

Keywords: condensed tannins, forage legumes, rumen undegradable protein