Effects of dietary supplementation of nitrate on enteric methane production, performance, and rumen microbiome of Hanwoo steers

An in vivo experiment was performed to evaluate the effects of calcium ammonium nitrate (CAN; 5Ca (NO3)2·NH4NO3·10H2O, 75% DM NO₃⁻) supplementation on enteric CH4 emissions, ruminal fermentation and microbial community and productivity in Hanwoo (Korean native cattle) steers. Twenty Hanwoo steers (average body weight 348 ± 18 kg) were randomly allocated to 5 pens (4 head/pen) balanced for body weight, and then randomly assigned to 1 of 2 dietary treatments (2 head/treatment/pen): (1) the control (CON), 500 g/kg long stem timothy hay and 500 g/kg commercial concentrates; (2) NIT, control diet top-dressed with 2.5% CAN (1.578% NO3- in the dietary DM) for 115 d in a completely randomized design. Methane measurement was performed for 3 d starting from days 21, 51, 80, and 112 using whole-body respiratory chambers (RC). Rumen fluid and blood samples were collected on days 25 and 26, respectively. A decrease (P = 0.05) in dry matter intake (DMI) was observed in steers fed NIT both in the pen and RC. However, average daily gain and gain:feed were not adversely affected. Additionally, methemoglobin concentration was not influenced by the treatment, suggesting no potential toxicity of NO₃⁻ in steers fed NIT. A decrease (P < 0.05) in CH4 production (g/d) was noted in NIT throughout the feeding trial, but CH4 yield (g/kg DMI) was not different (P > 0.05) between treatments. Ruminal pH, ammonia concentration, and total volatile fatty acid production were not affected by the treatment, but NIT increased and decreased (P < 0.01) acetate and butyrate proportions, respectively, compared with CON. Steers in NIT group had increased (P < 0.05) ruminal abundance of Ruminococcus sp. and a tendency for increased Succiniclasticum (P = 0.07), and Selenomonas ruminantium (P = 0.07). However, NIT decreased (P < 0.05) the abundance of Olsenella, Atopobium and tended to decrease (P = 0.07) Methanobrevibacter ruminantium in the rumen. Overall, dietary supplementation of NO₃⁻ demonstrated a potential to reduce enteric CH4 production (g/d) by an average of 25.4% in Hanwoo beef cattle over time without causing NO₃⁻ intoxication in the early stage or negatively affecting growth performance throughout the trial. However, the lack of a sustained mitigation effect on CH₄ yield (g/kg DMI) suggests that further evaluation of NO₃⁻ supplementation under different dietary conditions is necessary.