Degradability of frozen and ensiled alfalfa proteins by sheep and assessment of duodenal digesta protein

Six adult ewes equipped with ruminal and duodenal cannulae were used in a crossover experiment to determine the effect of ensiling alfalfa on protein degradation in vivo. Frozen alfalfa and wilted silage prepared from the same crop were fed twice a day (08:00 and 16:00 h) with a mineral supplement. Water was available free choice. Rumen digesta out-flow rate and duodenal microbial nitrogen (N) were estimated using chromium mordanted alfalfa and RNA respectively. Animals fed frozen alfalfa had significantly higher (P < 0.05) duodenal non-ammonia N (g day−1). Ensiling alfalfa did not alter ruminal degradation of forage N, the passage of duodenal microbial N (g day−1) and the apparent efficiency of microbial N synthesis in the rumen (g kg−1 organic matter apparently digested in the rumen). Plasma levels of branched chain amino acids in ewes fed frozen alfalfa were significantly higher (P < 0.05) than in ewes fed silage. There were no treatment differences (P > 0.05) in the composition of amino acids flowing to the duodenum. Attempts to identify proteins entering the duodenum indicated the presence of only low molecular weight proteins below 40 kDa for sheep fed frozen alfalfa and largely below 18 kDa for sheep fed silage. The results obtained indicate that alfalfa protein is a source of highly degradable protein with 13% and 10% undegradable protein in frozen and ensiled alfalfa respectively, and sheep fed alfalfa silage diets had less intestinal amino acid absorption compared with those fed frozen alfalfa.