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

Abstract

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 degrees 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 NH(3)-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 (15)N 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.

Effect of glycerol in combination with alfalfa on in vitro gas production and microbial protein synthesis

“…The influence of genetic selection was reflected in the chemical composition of HNSC and LNSC alfalfa (Table 1) with detailed results in Berthiaume et al (2010). Briefly, HNSC and LNSC alfalfa had contrasting concentrations of NSC with 17.9 and 7.4% of DM, respectively (Table 1).…”

“…A total of seven treatments were evaluated and denominated as following: LNSC 01.25 g of DM LNSC, LNSC'PG 0LNSC'0.18 g of PG, LNSC'CG 0 LNSC'0.18 g of CG, LNSC'ES 0LNSC'0. Berthiaume et al (2010). Briefly, 50 genotypes with superior NSC and 50 genotypes with inferior NSC were identified and harvested (between 1330 and 1530, with a temperature of 188C) on 2005 Jul.…”

“…Chemical determination of carbohydrates contained in the two pooled samples confirmed their distinct concentrations: 17.9 vs. 7.4% DM for high-and low-NSC alfalfa, respectively. Complete chemical profiles of these forages were published in Berthiaume et al (2010) and are summarized in Table 1.…”

“…Agronomic strategies such as genetic selection and cutting management (sunset vs. sunrise cut) have reportedly increased nonstructural carbohydrates (NSC) in alfalfa (Chouinard-Michaud et al 2009). In vitro results from the same group (Berthiaume et al 2007(Berthiaume et al , 2010 indicate that increasing the concentration of NSC in alfalfa enhances microbial N synthesis in the rumen. However, because the higher concentration of NSC is reached at the expense of neutral detergent fibre (NDF), we hypothesized that the addition of crude glycerol (15% of DM) would be more beneficial with low NSC than with high NSC alfalfa because the latter contains less NDF that is also more digestible.…”

“…For example, ammonia utilization by rumen microbes depends primarily on the rate of release and the balance between carbohydrates and N (Hristov et al 2005). With alfalfa (Medicago sativa L.), high soluble proteins and limited readily fermentable carbohydrates can lead to an inefficient use of forage N and an excessive absorption of ammonia (Broderick and Radloff 2004;Berthiaume et al 2010). In order to obtain an adequate and inexpensive source of energy, it has been proposed that the availability of ruminal fermentable energy can be enhanced through forages.…”