Effects of nonstructural carbohydrate concentration in alfalfa on fermentation and microbial protein synthesis in continuous culture

• Published in: Journal of dairy science 2010-02, Vol.93 (2), p.693-700
• Main Authors: Berthiaume, R, Benchaar, C, Chaves, A.V, Tremblay, G.F, Castonguay, Y, Bertrand, A, Bélanger, G, Michaud, R, Lafrenière, C, McAllister, T.A, Brito, A.F
• Format: Article
• Language: English
• Subjects: alfalfa; Animal productions; Animals; Bacteria - metabolism; Bacterial Proteins - biosynthesis; Biological and medical sciences; carbohydrate content; Carbohydrates - chemistry; Cattle; continuous fermentation; cultivars; dietary carbohydrate; Digestion - physiology; Female; Fermentation - physiology; Food industries; forage evaluation; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; in vitro digestion; Medicago sativa; Medicago sativa - chemistry; Medicago sativa - metabolism; microbial proteins; Milk and cheese industries. Ice creams; Nitrogen - metabolism; nonstructural carbohydrate concentration; protein synthesis; Rumen - chemistry; Rumen - metabolism; Terrestrial animal productions; Vertebrates
• ISSN: 0022-0302; 1525-3198
• DOI: 10.3168/jds.2009-2399

Insufficient readily fermentable energy combined with extensive degradation of proteins in alfalfa (Medicago sativa L.) may result in poor forage N utilization by ruminants. Using the inherent genetic variability and differences between harvests, our objective was to compare the effect of contrasting concentrations of nonstructural carbohydrates (NSC) in alfalfa on rumen fermentation and microbial protein synthesis. Individual genotypes of the alfalfa cultivar AC Caribou grown near Québec City, Québec, Canada, were harvested at the vegetative and early flowering stages, dried at 55°C, ground, and analyzed for soluble carbohydrates (fructose + sucrose + glucose + pinitol) and starch. Approximately 20 genotypes having, respectively, the highest and lowest NSC concentrations were pooled to constitute 2 contrasted 1-kg forage samples. Samples of high- (17.9% DM) and low- (7.4% DM) NSC alfalfa were respectively allocated to separate dual-flow fermenters in a completely randomized design with 3 replications. Rumen inoculum was obtained from 4 ruminally fistulated cows in early lactation that were fed a TMR with a 50:50 forage to concentrate ratio. A 10-d incubation period was used, with the first 6 d serving as an adaptation period followed by 4 d of sampling with solid and liquid dilution rates in the fermenters set at approximately 2.0 and 4.3%/h, respectively. High versus low NSC concentration in alfalfa significantly enhanced the apparent digestibility of OM (59.1% for high-NSC alfalfa vs. 54.4% for low-NSC alfalfa) and DM (60.0 vs. 54.3%) and the true digestibility of DM (74.1 vs. 64.7%). Increasing NSC concentration in alfalfa (high vs. low) significantly decreased ruminal pH (6.85 vs. 7.08) and NH3-N concentration (26.0 vs. 33.6 mg/dL) and increased total VFA concentration (94.9 vs. 83.0mM). Molar proportions of acetate, isobutyrate, and isovalerate significantly decreased, whereas molar proportions of propionate and butyrate significantly increased with high-NSC alfalfa, resulting in a more glucogenic fermentation. More importantly, microbial N flow (263 vs. 230 mg/d) and bacterial N efficiency (41.1 vs. 29.6% of available N), measured using 15N as a microbial marker, both significantly increased with the high-NSC alfalfa. These results indicate that increasing the concentration of NSC in alfalfa promotes a glucogenic fermentation and enhances microbial N synthesis in the rumen.