The role of condensed tannins in the nutritional value of Lotus pedunculatus for sheep: 2. Quantitative digestion of carbohydrates and proteins

Abstract

1. Primary growth vegetative Lotus pedunculatus containing 46 and 106 g/kg dry matter (DM) of total condensed tannin and 3 and 14 g/kg DM of free condensed tannin, was cut and fed fresh at hourly intervals (750 g DM/d) to sheep fitted with permanent cannulas into the rumen and duodenum. Low- and high-tannin lotus contained respectively 41.3 and 31.6 g total nitrogen/kg DM and 132 and 152 g lignin/kg DM. The two forms of lotus were similar in carbohydrate composition. 2. Nutrient intake was recorded and faecal output measured by direct collection. Digesta flow to the duodenum was estimated by measuring dilution at the duodenum of inert ruthenium phenanthroline (Ru-P) and chromium-EDTA markers continuously infused into the rumen. 3. Effects attributable to condensed tannins were assessed by comparing the digestion of the two diets, and by comparing the digestion of each with predicted values for non-tannin-containing fresh forages fed at similar intakes. 4. Apparent digestibility of all nutrients measured was less for high- than for low-tannin lotus (P< 0.01). The levels of cellulose digested ruminally and post-ruminally in both forms of lotus were similar to predicted values. However, less hemicellulose and readily fermentable carbohydrate (RFC; soluble carbohydrate + pectin) was digested in the rumen in sheep given both forms of lotus than would be predicted for non-tannin-containing fresh forage diets, but this was compensated for by greater post-ruminal absorption of both nutrients. 5. Total N gains across the rumen (duodenal N flow – total N intake) were 1.8 and 10.5 g/d for low- and high-tannin lotus v.predicted losses of 3.7 and 2.1 g/d for non-tannin-containing fresh forages given at the same total N intakes. Post-ruminal digestion of non-amonia-N (NAN; proportion NAN flowing at duodenum) was 0.71 and 0.67 for low- and high-tannin lotus respectively v.0.76 for comparable non-tannin-containing fresh forages. Energy absorbed as amino acids from the small intestine was calculated to be 0.29 of metabolizable energy for both forms of lotus, compared with 0.17 and 0.21 for perennial ryegrass and white clover. 6. It was concluded that the presence of condensed tannins in lotus markedly increased post-ruminal NAN absorption compared with non-tannin-containing fresh forage diets, but depressed ruminal digestion of RFC and hemicellulose.