In situ and in vitro evaluations of a slow release form of nitrate for ruminants: Nitrate release rate, rumen nitrate metabolism and the production of methane, hydrogen, and nitrous oxide

In situ and in vitro experiments were conducted to determine nitrate (NO3−) release rate from encapsulated NO3− (EN) 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, 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% of soluble N fraction and 54.7% of slowly degradable N fraction. Two in vitro experiments were conducted where UEN and EN were incubated for 24 h in buffer alone without substrate or in buffered-rumen fluid with substrates containing starch and xylan with 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 considerably increased (P < 0.01) for UEN compared with urea and EN without a difference between urea and EN. Nitrous oxide production was greater for UEN followed by EN and urea (20.8, 7.5 and 0.02 μL, respectively; P < 0.05). 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 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− 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.