Study on the in vitro fermentation characteristics and three cellulolytic bacteria of grass fiber from different species and grain size

Understanding grass fiber degradation in ruminants is beneficial to improve their production efficiency. The consistency of digestion and degradation for different grass fibers in ruminants is unclear. Therefore, here, the effects of the grass fiber source and its particle size on in vitro fermentation and the interface characteristics of microorganisms were studied. Two kinds of forage grass (alfalfa and Pennisetum purpureum ‘Guimu No. 1’) were selected. Neutral detergent fiber (NDF) was extracted and crushed into 3 kinds of granules: < 0.15 mm (PS1), 0.15～0.3 mm (PS2), and 0.9～1.2 mm (PS3). The experiment was divided into 6 treatment groups according to fiber source and particle size, with three replicates per treatment. The gas production parameters, fiber degradation rate, fermentation parameters, microbial interface characteristics, and the copy number of fiber-degrading bacteria and total bacteria were determined. The results showed that 1) the maximum gas production (V_f), gas production curve constant (b), time used to reach half of the maximum gas production (T_0.5), fermentation gas production fraction (FRDt_0.5), ammonia nitrogen (NH₃-N) concentration, pH, total volatile fatty acids (TVFA), and molar ratio of propionic acid in the ‘Guimu No. 1’ group were significantly higher than those in the alfalfa group (P < 0.05). In the alfalfa group, the initial gas production fraction (FRD₀), dry matter degradability (DMD), microbial protein (MCP), molar percentage of acetic acid, valeric acid, isobutyric acid, and isovalerate, and the ratio of ethylene to propylene, cell membrane permeability (CMP), liquid surface tension (ST), Fibrobacter succinogenes, Ruminococcus flavefaciens, and total bacteria were significantly higher than those in the ‘Guimu No. 1’ group (P < 0.05). 2) The b value, T_0.5, NH₃-N, and isobutyric acid molar ratio of grass fiber during fermentation tended to increase with the increase of particle size, while the FRD₀, DMD, CMP, and Ruminococcus flavefaciens significantly decreased with the increase of particle size (P < 0.05), and cell surface hydrophobicity (CSH) first decreased and then increased with the increase of particle size. In summary, different grass fibers and their grinding sizes affect fiber fermentation and microbial interface characteristics in vitro. Alfalfa has a better fermentation effect than that of ‘Guimu No. 1’ and better fermentation of fiber with small grain sizes.