Abstract:

【Objective】 An in vitro study was conducted to evaluate the ability of nitrate reduction and its effect on ruminal methano genesis and fermentation characteristics of various microbial fractions. 【Method】 Using the Menke’s gas production method in vitro, pure substrates formulated with soluble starch, Avicel, and sodium nitrate were anaerobicly incubated for 24 hours. Physical centrifugation and chemicals selections were used to differentiate microbial fractions as follows: whole rumen fluid (WRF), protozoa (P), bacteria (B), fungi (F), protozoa plus bacteria (P+B), protozoa plus fungi (P+F), bacteria plus fungi (B+F), and negative control (CTN). 【Result】 The results showed that the degradation of nitrate by fraction P was higher than fraction B (P＜0.001), while fraction F had little contribution to nitrate reduction, even for its co-culture with fractions of P or B. The gas production and nitrate reduction were greatest in WRF, followed by fractions of P+B and P, and the three fractions had greater values in methane (CH4) proportion, total VFA (TVFA) concentration and acetate molar proportion (P＜0.001). Nitrate-N addition increased rumen microbial protein synthesis with the greatest synthesis occurring in P+B fraction. 【Conclusion】 Both ruminal fractions of P and B had the ability to reduce nitrate (NO3-) and nitrite (NO2-) with a greater ability in P fraction. F fraction had minor ability of nitrate reduction in this substrate. Ruminal protozoa and bacteria represented predominant populations responsible for nitrate reduction, methanogenesis, and microbial protein synthesis.

Key words: nitrate reduction, ruminal fermentation, protozoa, bacteria, fungi