The role of condensed tannins in the nutritional value of Lotus pedunculatus for sheep

Abstract

1. Voluntary intake was determined with vegetativeLotus pedunculatuscut and fed fresh to growing sheep of 42–46 kg live weight. Effects attributable to condensed tannins were assessed by growing the plant under high and low levels of soil fertility, inducing low and high concentrations of tannin (Expt 1), or by binding the tannins through spraying the herbage with polyethylene glycol (molecular weight 3350, PEG; Expt 2). Primary-growth lotus was used in Expt 1 and secondary-growth lotus in Expt 2. Concentrations of total and free condensed tannin were determined in fresh lotus, free tanning being defined as that not bound by mascerates of the plant.2. In Expt 1 the herbages fed contained respectively 46 and 106 g total condensed tannin/kg dry matter (DM) and 3 and 14 g free condensed tannin/kg DM. Mean metabolizable energy (ME) intakes were 0.89 and 0.77 MJ/kg live weight0.75per d (P< 0.05) respectively.3. The lotus used in Expt 2 contained 63 and 5 g total reactive condensed tannin and free condensed tannin/kg DM respectively. After spraying with PEG at 2.4 g/g total condensed tannin, these values were reduced to 7 and 0.5 g/kg DM respectively. PEG addition increased apparent digestibility (proportion of each nutrient ingested) of cellulose, hemicellulose and nitrogen by 0.05, 0.08 and 0.26, and increased ME intake from 0.48 to 0.69 MJ/kg live weight0.75per d.4. It was concluded that high concentrations of condensed tannins depressed ME intake, due to depressions in both the voluntary intake and digestion of organic matter. The stimulation of hemicellulose and cellulose digestion by PEG addition suggests that the condensed tannin was depressing rumen digestion.5. It is considered that free tannins are most likely to have been responsible for the depressive effects in the present study. This could be due to their reaction with microbial enzymes in the rumen, the tannin content of the plant exceeding the capacity of the plant protein to bind it, or to reaction with enzymes secreted into the small intestine following the absorption of amino acids. In either instance free tannins could also react with proteins of the gut wall.