Condensed tannin relationships in vitro forage quality analyses for birdsfoot trefoil

Condensed tannins (CT) in birdsfoot trefoil (Lotus corniculatus L.) are associated with reduced degradation of protein and dry matter digestibility by ruminant animals. The objectives of this study were to isolate the effect of CT concentration on in vitro degradable crude protein (IVDCP) and in vitro digestible dry matter (IVDDM) in birdsfoot trefoil herbage containing diverse CT concentrations, to determine the relative influence of IVDCP compared with IVDDM, and to determine minimum CT concentrations effecting reductions in IVDCP and IVDDM. Twelve birdsfoot trefoil clones, genetically diverse for CT concentration, were harvested at St. Paul and Rosemount, MN. Herbage samples with uniform fiber concentrations were arrayed in five tannin groups (n = 3) at regular intervals from minimum to maximum CT concentrations at both locations. Herbage samples were compared for differences between polyethylene glycol (PEG) and control treatments for IVDCP and IVDDM analyses. The protease ficin was used to determine IVDCP. As CT concentration increased, the difference between PEG and control treatments increased for IVDCP and IVDDM ([R.sub.2] = 0.91 and 0.69, respectively). For tannin groups with mean CT concentrations [is greater than or equal to] 27.0 g catechin equivalent (CE) [kg.sup.-1] DM, the PEG and control treatments differed for IVDCP compared with tannin groups containing minimum CT concentrations at both locations. For the tannin group with the maximum CT concentration at Rosemount (85.3 g CE [kg.sup.-1] DM), the PEG and control treatments differed for IVDDM compared with the tannin group containing the minimum CT concentration. It was concluded that paired-subsample comparisons between PEG and control treatments was a convenient and consistent method for separating the effect of CT concentration from other sources of variation. These data imply that CT concentrations in the range of 25 to 85 g CE [kg.sup.-1] DM may reduce ruminal protein degradation, with little or no corresponding reduction in dry matter digestibility.