Effects of supplementary nitrate and urea on gas production and nitrate metabolism in rumen contents in vitro

Methane (CH₄) production by ruminants contributes to global warming and represents a loss of metabolisable energy. When ruminants ingest diets containing nitrate (NO3⁻), they release less CH₄ per unit feed digested, apparently because microbial use of H₂ to reduce NO3⁻ to nitrite (NO3⁻) and ammonia (NH3) occurs more readily under rumen conditions than the reduction of CO₂ to CH₄ (Allison and Reddy, 1984). Nitrate is also able to replace ammonia or urea as a non-protein nitrogen (N) supplement (Leng, 2008). The objective of this study was to determine gas production, pH and NO3⁻, NO₂⁻ and NH₃ concentrations in 150 mL syringes after addition of KNO3 or urea to rumen fluid from two donor sheep. Replicate syringes were prepared for each of the five treatments: NO3⁻ (16.8 or 8 mg N per 120 mL), urea (16.8 or 8 mg N per 120 mL) and the control (no additives). Rumen fluid that had been collected anaerobically and held at 39°C was mixed continuously while 60 mL aliquots thereof were distributed to each of the syringes followed by 60 mL of gas-free phosphate buffer that did not contain bicarbonate salts. Production of gas was determined from the movement of the syringe plungers while the syringes were maintained at 39°C.