The Effect of Two Additives on Ruminal Fermentation Using a Semi-Continuous Culture System

Seven 37-d trials tested the effect of nitrate (NIT) and monensin (MON) in diets commonly fed to beef (BEEF; 10:90 forage to concentrate; 3 trials) and dairy cattle (DAIRY; 50:50 forage to concentrate; 4 trials) on rumen fermentation and methane (CH₄) emissions. Additives (0, 1.25 and 2.5% diet DM of NIT; 0 and 4 mg/L of MON) were tested alone and combined (NIT + MON; 6 total treatments; 3 replicates per treatment). Rumen fluid was pooled from 5 non-adapted lactating cows fed 50:50 forage to concentrate diet 3 h after morning feeding, and 1 L of processed inoculum was transferred to 2.2-L vessels. Treatment diets were added to nylon bags which remained in the anaerobic fermentation system for 48 h. Buffer was infused into vessels at 70 ml/h. Gas production and composition and VFA were measured daily. Ammonia nitrogen (NH₃-N) and DMD were measured twice weekly. For BEEF and DAIRY trials, when compared to control, NIT reduced CH₄ production (33.5 vs 43.5 and 24.7 vs. 32.1 mM/d, respectively; P<0.01; SEM= 1.5) and NH₃-N (6.6 vs 8.1 and 17.1 vs. 21.0 mg/dL, respectively; P<0.01; SEM =0.6) while MON reduced CH₄ production (26.6 vs. 43.5 and 26.3 vs. 32.1mM/d, respectively; P<0.01), butyrate (12.6 vs. 14.9 and 16.1 vs. 19.4 mM/d, respectively; P<0.01; SEM =0.5), acetate to propionate ratio (1.6 vs. 1.9 and 1.6 vs. 2.0, respectively; P<0.01; SEM =0.06) and increased propionate (29.6 vs. 26.2 and 26.8 vs. 22.2 mM/d, respectively; P<0.01; SEM =0.6) and total VFA (50.7 vs. 48.1 and 38.3 vs. 35.4 mM/d, respectively; P<0.01; SEM =1.9). The combination of NIT+MON in BEEF and DAIRY trials further reduced CH₄ (19.3 vs 43.5 and 21.3 vs. 32.1 mM/d, respectively; P<0.01) and increased acetate (51.2 vs. 49.5 mM/d; P=0.07 and 53.7 vs 49.9 mM/d; P<0.01, respectively; SEM =0.9). For BEEF only, compared to control, NIT reduced DMD at higher doses (72.3 vs. 74.3 g/100g DM; P<0.05; SEM =1.4) and NIT+MON increased total VFA at lower doses (55.4 vs. 48.1 mM; P<0.01). For DAIRY, butyrate was reduced by NIT (16.1 vs. 19.4 mM/d; P<0.01) and further reduced by NIT+MON (15.2 vs. 19.4 mM/d; P<0.01). The combination of NIT+MON enhanced CH₄ reduction and increased acetate molar ratio. When NIT was present at higher doses, DMD was reduced. The combination of NIT+MON reduced butyrate in DAIRY trials and, at lower doses, increased total VFA production in BEEF trials.