Effect of condensed tannins on ruminal degradability of purple prairie clover (Dalea purpurea Vent.) harvested at two growth stages

The native legume purple prairie clover (PPC; Dalea purpurea Vent.) is adapted to the prairie region of North America and contains high levels of condensed tannins (CT) with activity against Escherichia coli O157:H7. However, information on the variation of CT concentration in PPC at different growth stages is lacking. Whole plant PPC was harvested from 5 paddocks (4 samples/paddock) from native pastures at vegetative/early flowering (VEG) and full flower/early seed (FL) stages. Plants were dissected into leaf, stem and inflorescence, freeze dried and ground for determination of total phenolics (TP), CT and other nutrients. Plants harvested at VEG had similar (101 versus 111 g/kg DM) TP but lower CT (59 versus 94 g/kg DM; P<0.01) than those harvested at the FL stage. Inflorescences contained the highest CT (198-213 g/kg DM), followed by leaves (50-64 g/kg DM) and stems (17 g/kg DM). Contents of organic matter, neutral detergent fibre (aNDF) and acid detergent fibre were higher (P<0.01), but N content was lower (P<0.01) for FL than VEG. Ground PPC samples from 3 paddocks at both growth stages were incubated with a mixture of ruminal fluid and mineral buffer in three DaisyII fermentors to assess ruminal degradability of DM, crude protein (CP) and aNDF. Polyethylene glycol (PEG) was added to half of the jars to define effects of CT on digestion. Bags were retrieved at intervals over 72 h incubation and analyzed for N and microbial 15N. There was no growth stage by PEG interaction for the kinetic parameters a, b and c of DM degradation. Plants harvested at the VEG stage had higher (P<0.01) in vitro degradability of DM and aNDF, slowly degradable fraction (b) of DM, aNDF and CP than those harvested at the FL stage. More (P<0.01) rapidly degradable fraction (a) of DM, but less (P<0.05) rapidly degradable fraction (a) of CP occurred for VEG than FL. Inclusion of PEG increased (P<0.01) the degradation rate of the b DM fraction for both growth stages and that of aNDF at FL, while reducing (P<0.05) lag time of CP degradation for FL. Results demonstrate that PPC contains high levels of a unique CT that has a minimal impact on DM degradability, suggesting that feeding PPC may have potential to reduce E coli O157:H7 in the ruminant digestive tract without adversely impacting digestion.