Degradability of alfalfa saponins in the digestive tract of sheep and their rate of accumulation in rumen fluid

Abstract

Two experiments were conducted to examine the fate of alfalfa saponins in the digestive tract of sheep. In the first experiment 14.4 or 28.8 g saponins were administered intraruminally daily, which provided 62 and 78% of the total saponin entering the rumen. Degradabilities of the saponins in the total digestive tract were 96.0 and 97.9% (P = 0.13), respectively, but no attempt was made to determine how closely the products of degradation were related to the original molecules. In the second experiment, the rate of release of saponins from alfalfa into rumen fluid was measured in the presence and absence of viable bacteria. When no viable bacteria were present 74% of the saponins in the alfalfa appeared in rumen fluid after 8 h of incubation and this amount was unchanged at 24 h. Rate of release did not follow first order kinetics, rather an amount equivalent to approximately 8% of the original saponin concentration accumulated in the fluid each hour over the 8-h period. After 4 h, saponin concentrations in rumen fluid were decreased in the presence of viable bacteria indicating microbial metabolism was occurring. Final accumulation of saponins in the rumen fluid after 24 h incubation was 62% of that originally present in alfalfa samples when viable bacteria were present. It was concluded that saponins were rapidly released into rumen fluid and extensively degraded in the digestive tract. The implications of these observations on ruminant bloat are discussed. Key words: Alfalfa, saponins, digestibility, bloat, rumen fluid, bacteria