In Situ and in Vitro Evaluation of a Slow Release Form of Nitrate for Ruminants: Nitrate Release Rates, Rumen Nitrate Metabolism and Production of Methane, Hydrogen, and Nitrous Oxide

In situ and in vitro experiments were conducted to determine the nitrate (NO₃^-) release rate from encapsulated NO₃^- (EN; 71% NO₃^-) in the rumen and to examine metabolism of EN-NO₃^- in conjunction with methane (CH₄), hydrogen (H₂), and nitrous oxide (N₂O) production. Three ruminally-cannulated beef heifers were used to incubate the following substrates in the rumen for 72 h: soybean meal, EN, and unencapsulated NO₃^- (UEN). Because of immediate solubilisation, the soluble nitrogen (N) fraction of UEN was assumed to be 100%. A non-linear regression model fitted to N disappearance of EN (r² = 0.93, P < 0.01) indicated 35.4% and 54.7% of soluble and slowly degradable N fraction, respectively. Two in vitro experiments were conducted where UEN and EN were incubated for 24 h in buffer alone or with buffered-rumen fluid with substrates containing urea, UEN, or EN as sole N sources. When UEN was incubated with buffer, 99.9% of UEN-NO₃^--N was recovered as NO₃^--N in the buffer over 24 h. However, recovery of EN-NO₃^- as NO₃^--N gradually increased over 24 h up to 58%. When urea, EN or UEN were incubated with buffered-rumen fluid, total gas production decreased (P = 0.022) for UEN compared with urea and EN. Methane production decreased (10.7 and 13.3 vs. 19.6 mL; P < 0.01) for UEN and EN, respectively, compared with urea. Hydrogen production increased (0.2 vs. 0.07 and 0.08 mL; P < 0.01) for UEN compared with urea and EN, respectively. Nitrous oxide production was greater for UEN (20.8, 7.5 and 0.02 μL, respectively; P < 0.05) followed by EN and urea. During the incubation with buffered-rumen fluid, 100% of UEN-NO₃^- was recovered as NO₃^--N in the medium until 6 h and then the recovery gradually decreased to 18% at 24 h while recovery as nitrite (NO₂^-)-N increased up to 22% at 24 h. However, recovery of EN-NO₃^--N as NO₃^--N in the medium increased by 20% at 12 h and then gradually decreased to 5% at 24 h without accumulation of NO₂^--N in the medium. In conclusion, EN released NO₃^- comparatively slowly in the rumen and decreased CH₄ production without negatively affecting microbial fermentation. Increases in H₂ and N₂O production and accumulation of NO₃^‑ and NO₂^- in the medium were not observed for EN compared with UEN, indicating that EN could be a promising NO₃^- source to lower CH₄ production with less risk of NO₃^-/NO₂^- toxicity.