Effects of a low dose of 3-nitrooxypropanol and a combination of direct-fed microbials on methane emission, oral swab bacterial community, and performance in dairy cows

Abstract Background 3-Nitrooxypropanol (3-NOP) can inhibit ruminal methane production by inactivating the key enzyme during methanogenesis. Combining 3-NOP and direct-fed microbials (DFM) can potentially provide a dual-action approach to reduce ruminal methane production. The objective of this study was to investigate the effects of supplementation of a low dose of 3-NOP and a mixture of DFM, on methane emissions, performance, and rumen microbial community of dairy cows. Results Forty-eight Danish Holstein dairy cows were used in a 14-week randomized complete block design. Due to a mixing error, 3-NOP was included at an unexpected low dose (6.0 mg/kg DM). Cows were fed with one of four experimental diets: (1) control diet, and other treatment diets were control diet supplemented with (2) 3-NOP, or with (3) DFM (12 mg/kg P. acidipropionici strain P169 and 120 mg/kg Bacillus subtilis and B. licheniformis mixture), or with (4) a combination of 3-NOP and DFM. 3-NOP inclusion did not affect DMI, feed efficiency, or milk yield and composition. DFM supplementation reduced partial mixed ration intake and total DMI. 3-NOP supplementation significantly reduced CH 4 production (g/d) (8%) and CH 4 intensity (g/kg ECM) (9%) but did not affect CH 4 yield (g/kg DMI). Furthermore, 3-NOP supplementation increased H 2 production, yield, and intensity. DFM supplementation reduced milk yield from weeks 3 to 8 and ECM yield from weeks 3 to 7, but these effects diminished over time. 3-NOP alone or in combination with DFM resulted in higher abundance of Megasphaera and Prevotella genera in oral swab samples. DFM supplementation had no effect on gas emissions, and it only tended to improve feed efficiency (ECM/DMI, kg/kg) during the later stage of the experiment. DFM supplementation led to higher abundance of certain species within the Lactobacillaceae family particularly at later stages of the experiment. Conclusions An unforeseen reduced dose of 3-NOP supplementation still can effectively mitigate methane emissions (both production and intensity) and increase H 2 emissions without affecting DMI or performance. DFM supplementation reduced DMI and milk yield at the onset of the experiment, but the effects subsided over time, resulting in an increased feed efficiency by the end of the experiment.