Limited Effects of Tannin Supplementation on the Dairy Cattle Fecal Microbiome with Modulation of Metabolites

Matthew L Klein¹Christian B Erikson²Conor J McCabe¹Laibin Huang³Jorge L.M. Rodrigues^2,4Frank M Mitloehner^1*

• ¹Department of Animal Science, College of Agricultural and Environmental Sciences, University of California, Davis, Davis, California, United States
• ²Department of Land, Air, and Water Resources, College of Agricultural and Environmental Sciences, University of California, Davis, Davis, California, United States
• ³Department of Biology, Saint Louis University, St. Louis, Missouri, United States
• ⁴Division of Environmental Genomics and Systems Biology, Berkeley Lab (DOE), Berkeley, California, United States

Tannins, a class of plant secondary metabolite, are known to bind organic carbon (C) and nitrogen (N). In ruminant feeding systems, this can modify bioavailability of substrates for enteric fermentation, in turn reducing greenhouse gas production (GHG) and directing more dietary protein to fecal excretion. Since tannins are known to be resistant to microbial degradation in the rumen, and may persist throughout the small and large intestines, the present study was implemented to determine the impact of a tannin-based feed additive on fecal microbial diversity, GHGs, and fecal chemical properties. Twenty-four early to mid-lactation dairy cows were randomized to receive either the top-dressed feed additive tannin treatment (TRT), a mixture of condensed and hydrolyzable tannins derived from quebracho [Schinopsis quebracho-colorado (Schltdl.)], or an un-supplemented control diet (CON) for a 64-day trial. Cows were blocked on parity, dry matter intake, milk yield, body weight, and days in milk and group-housed and individually fed twice per day. Feces were collected from each cow at baseline (day 0), and at days 16, 32, and 64, following treatment administration. Using 16S rRNA gene amplicon sequencing, a total of 1,538 amplicon sequence variants were identified across all samples, with Firmicutes as the dominant group. Phylogenetic diversity of the prokaryotic community in fecal samples exhibited a treatment × day effect (p < 0.01), with samples from cows in the TRT group exhibiting decreased diversity versus those in the CON group between day 16 and 64. Fecal C (p < 0.01) and N (p < 0.01) and fecal indole-3-lactate (p = 0.02) also exhibited a treatment × day effect, with samples from the TRT group having lower total C and organic N on day 16 and greater indole-3-lactate on day 64. Greenhouse gas emissions measured for feces in a 14-day lab incubation, were similar among cows in the TRT versus CON groups for N2O, CH4, or CO2 indicating that the feed additive reduced diversity of the prokaryotic community while minimally impacting fecal chemical parameters.