The effect of nitrate and monensin on in vitro ruminal fermentation

Two experiments evaluated the effect of calcium ammonium nitrate decahydrate (calcium nitrate [NIT]) and monensin sodium (MON) on in vitro fermentation parameters of 2 contrasting diets (100:0 and 10:90 forage-to-concentrate ratios). Diet addition of NIT (0, 1.25, and 2.5 g/100 g DM) and MON (0, 3, and 6 mg/L) were tested alone and combined (9 treatments total; 5 bottles per treatment). Mixed ruminal microorganisms were incubated in anaerobic media containing 0.5 g of substrate diet, 1 of 9 treatments, and 40 mL buffer solution. Incubations were performed in batch cultures for 48 h at 39°C. Headspace gas volume was measured and sampled at 4, 8, 12, 24, and 48 h, and the VFA profile was assessed at the end of the experiment. Total gas production was reduced by NIT (87.9 vs. 94.6 mL; < 0.01) and MON (78.6 vs. 94.6 mL; < 0.01) and, in Exp. 2, further reduced by NIT+MON when the additives were combined (161.1 vs. 196.9 mL; < 0.01). Methane production from control in Exp. 1 and Exp. 2 averaged 9.1 and 15.3 mL, respectively, and was decreased by NIT (3.4 and 8.3 mL in Exp. 1 and Exp. 2, respectively; P < 0.01), MON (4.1 and 7.7 mL; in Exp. 1 and Exp. 2, respectively; < 0.01) and NIT+MON (1.1 and 1.5 mL; in Exp. 1 and Exp. 2, respectively; < 0.01). Both experiments demonstrated a significant increase in nitrous oxide (NO; < 0.01) when NIT was added. Compared to the control treatment, IVDMD was reduced when NIT+MON was added at the higher doses in EXP1 (31.7 vs. 37.4%; < 0.01) and EXP2 (76.6 vs. 79.9 %; < 0.01). Net VFA production was not affected by treatments ( > 0.10), but molar proportions of acetate and butyrate were reduced by MON ( < 0.01). Propionate molar proportion was increased in both experiments by MON ( < 0.01) and further increased in Exp. 2 when the additives were combined at lower doses ( < 0.01). Compared to the control treatment, the acetate:propionate (A:P) ratio was reduced by MON in Exp. 1(1.2 vs. 2.8; < 0.01) and Exp. 2 (1.0 vs. 2.3; < 0.01). Fermentation efficiency (%) was increased by MON (81.7 vs. 73.7%; < 0.01) and further increased in Exp. 2 when the additives were combined at lower doses (87.2 vs. 76.6%; < 0.01). The combination of NIT and MON in 2 contrasting diets proved beneficial by altering fermentation products toward lower CH and more propionate; however, the addition of NIT consistently increased NO production. Negative effects of the additives on IVDMD were found only when the additives were combined at higher doses.