Three experiments were undertaken to determine the effect of condensed tannin (CT) in Lotuspedunculutus (45–55 g extractable CT/kg DM) on the digestion of the principal leaf protein, ribulose-1,5-bis phosphate carboxylase EC 4.1.1.39; Rubisco; fraction 1 leaf protein). In two of the experiments Lotus pedunculutus was fed to sheep, with one group receiving a continuous intraruminal infusion (per fistulum) of PEG (molecular weight 3500) to bind and inactivate the CT (PEG group). The other group, which did not receive PEG, was termed the control sheep (CT acting). Expt 3 involved in vitvo incubations of Lotus pedunculutus in buffered rumen fluid, with and without PEG added. In all experiments the results have been interpreted in terms of the effects of CT on Rubisco solubilization and degradation. Disappearance of N and Rubisco from Lotus pedunculutus suspended in polyester bags in the rumen was used as a measure of solubilization. Degradation was defined as the disappearance of Rubisco from in vitro incubations of Lotus pedunculatusinrumen fluid. In Expt 1, CT reduced the digestion of Rubisco in the rumen from 0.96 to 0.72 of intake (P < 0.01). Rubisco digestion in the small intestine was 0.27 of intake in control sheep and 0.04 of intake in PEG sheep. In Expt 2, PEG had no effect on the loss of Rubisco from Lotus pehneulatus contained in polyester bags which were incubated in the rumen, hence CT did not affect the solubilization of Rubisco. Observations in Expt 1 were confirmed by in vitro incubations in Expt 3, where PEG addition substantially increased the rate of degradation of plant protein to NH2. Addition of PEG decreased the period of time taken to degrade 50% of the Rubisco from about 13.8 h to about 3.0 h. It was concluded that the action of CT reduced the digestion of Rubisco in the rumen of sheep fed on fresh Lotus pedunculutus, and that this was primarily due to the ability of CT to slow its degradation by rumen micro-organisms, without affecting its solubilization. Both fresh-minced, and freeze-dried and ground lotus were used for in saccoand in vitro incubations; however, fresh-minced lotus was more suitable for the evaluation of protein solubilization and degradation in fresh forages.
