Effects of dietary nitrate, fumaric acid, and methanotrophic bacteria supplementation on rumino-intestinal nutrient metabolism and enteric gas exchange in dairy cows

• Methanotrophic bacteria supplementation did not decrease enteric methane emission. • Nitrate decreased emission of methane and increased emission of hydrogen. • Fumaric acid redirected to some extent excess hydrogen into propionate. • Overall, any effects on nutrient digestibility were minor. The objective of the study was to investigate the effects of two approaches for enteric methane (CH 4 ) mitigation and one approach for redirection of excess hydrogen (H 2 ) resulting from CH 4 inhibition, on dairy cows’ gas exchange and nutrient digestibility. Approaches for CH 4 mitigation were methanotrophic bacteria supplementation and dietary nitrate, whereas the approach for H 2 redirection was nitrate combined with fumaric acid. An incomplete 6 × 6 Latin square design experiment was conducted with 4 periods of 21 d using 6 rumen, duodenum, and ileum cannulated Danish Holstein cows with DIM of 123 ± 64.8 d (mean ± SD) and the milk yield was 33.6 ± 9.39 kg/d at beginning of experiment. The treatments were organized in a 2 × 3 factorial arrangement, where the first factor represented treatments without or with methanotrophic bacteria supplementation (MET), and the second factor represented 3 different dietary additive supplementations (DIET). These were a basal diet (BAS; no additives), a diet with nitrate (NIT; 10 g nitrate/kg dry matter (DM)), and a diet with nitrate combined with fumaric acid (NIT-F; NIT + 15 g fumaric acid/kg DM). Cows had ad libitum access to diets with a forage to concentrate ratio of 60 to 40 on dry matter basis. Following adaptation to experimental diets, samples of rumen fluid, digesta from duodenum and ileum, and feces were collected to estimate nutrient digestibility using Cr 2 O 3 and TiO 2 as external flow markers. Then, gas exchanges were measured in respiration chambers. There was no CH 4 mitigating effect of MET. Nitrate reduced CH 4 production (g/d), yield (g/kg DMI), and intensity (g/kg ECM) by 19.5, 11.9, and 17.2 %, respectively, whereas H 2 yield (g/kg DMI) was increased by 261 %. Ruminal redox value was decreased by nitrate, and individual rumen volatile fatty acid proportions reflected a more reduced rumen environment although propionate proportions decreased. Nutrient digestibility was not affected by nitrate although microbial CP efficiency (g of microbial CP/kg true rumen digested organic matter) was decreased. Supplementing fumaric acid in combination with nitrate decreased H 2 production by 26.8 % compared to nitrate fed cows, and this was associated with increased propionate concentrations. However, there was no effect on H 2 emission when corrected for dry matter intake (DMI) or energy-corrected milk (ECM) yield. There were no effects of any of the treatments on DMI or ECM yield. In conclusion, the results demonstrated a CH 4 mitigating effect of nitrate supplementation resulting in increased H 2 emission. The effects on nutrient digestibility and rumen fermentation were minor. Fumaric acid supplementation redirected some H 2 to propionate, although the efficiency was small. Supplementation of methanotrophic bacteria did not suppress CH 4 emission.