3-Nitrooxypropanol supplementation had little effect on fiber degradation and microbial colonization of forage particles when evaluated using the in situ ruminal incubation technique

3-Nitrooxypropanol (3-NOP) is an investigational compound that acts as an enzyme inhibitor to decrease ruminal methanogenesis. We hypothesized that when feeding 3-NOP to cattle fed a high-forage diet, H-2 would accumulate in the rumen, which could suppress microbial colonization of feed particles and fiber degradation. Therefore, the study investigated the effects of supplementing a high-forage diet with 3-NOP on ruminal fiber degradability and microbial colonization of feed particles using the in situ technique. Eight ruminally cannulated beef cattle were allocated to 2 groups (4 cattle/group) in a crossover design with 2 periods and 2 dietary treatments. The treatments were control (basal diet) and 3-NOP (basal diet supplemented with 3-NOP, 150 mg/kg of dry matter). The basal diet consisted of 45% barley silage, 45% chopped grass hay, and 10% concentrate (dry matter basis). Samples of dried, ground barley silage and grass hay were incubated in the rumen of each animal for 0, 4, 12, 24, 36, 48, 96, 120, 216, and 288 h to determine neutral detergent fiber (NDF) degradation kinetics. An additional 2 bags were incubated for 4 and 48 h to evaluate the bacterial community attached to the incubated forages. Dietary supplementation of 3-NOP decreased (-53%) the dissolved methane concentration and increased (+780%) the dissolved H-2 concentration in ruminal fluid, but did not substantially alter in situ NDF degradation. The addition of 3-NOP resulted in a decrease in the alpha-diversity of the microbial community with colonizing communities showing reduced numbers of amplicon sequence variants and phylogenetic diversity compared with control diets. Principal coordinate analysis plots indicated that forages incubated in animals fed 3-NOP resulted in highly specific changes to targeted microbes compared with control diets based on unweighted analysis (considering only absence and presence of taxa), but did not alter the overall composition of the colonizing community based on weighted UniFrac distances; unchanged relative abundances of major taxa included phyla Bacteroidetes, Firmicutes, and Fibrobacteres. The effect of 3-NOP on colonizing methanogenic microbes differed depending upon the forage incubated, as abundance of genus Methanobrevibacter was decreased for barley silage but not for grass hay. In conclusion, 3-NOP supplementation of a high-forage diet decreased ruminal methanogenesis and increased dissolved H-2 concentration, but had no negative effects on ruminal fiber degradation and only minor effects on relative abundances of the major taxa of bacteria adhered to forage substrates incubated in the rumen.