PS10-8. Tannin Supplementation in Cow-calf Pairs Grazing a Grass Monoculture.

Abstract Cow-calf pairs grazing rangelands rely on grass-based diets that decline in quality over the season, reducing nitrogen (N) use efficiency and increasing enteric methane (CH4) emissions. Grass monocultures also lack plant secondary compounds, such as tannins, which may improve nutrition and reduce environmental impacts. Twenty-four Angus cow-calf pairs grazed six 3.6-ha meadow bromegrass paddocks (4 pairs/paddock) over two consecutive years. All cows received 1 kg/cow/day of DDGs (Control; n = 3 paddocks), while tannin-supplemented cows (TT; n = 3) additionally received a quebracho-chestnut extract included at 0.5% dietary DM. After a basal period, cattle grazed three consecutive 14-d treatment periods (P1, P2, P3). Mixed models included treatment, period, and their interaction as fixed effects; and year, paddock, and animal as random effects; and baseline values as covariate. Biomass removal did not differ between treatments (P > 0.05). TT cows had lower BUN in P3 and lower fecal ADF in P1 than Control cows (P < 0.05), whereas fecal N tended to remain greater in TT cows across periods and urinary N concentration tended to be lower in TT cows during P2 (P = 0.115). Methane production showed no treatment effects for daily emissions, yield, or intensity (P > 0.05). Daily CH4 emissions and CH4 yield peaked in Period 1 and declined in Periods 2–3 for TT cows (P < 0.05), with a smaller decline for Control animals over the same periods (P < 0.05). Across periods, TT cows consistently exhibited numerically lower CH4 emissions, reaching a 20% reduction by Period 3 relative to Control animals. Fecal fiber, N, and CH4 outcomes aligned with body-weight patterns: Cow and calf BW did not differ between treatments (P > 0.05), but TT cows showed less BW loss than Control cows, and calves from TT-supplemented cows gained approximately 7 kg more BW over the study, corresponding to11% greater total gain. Thus, moderate tannin inclusion promoted selection of a higher-quality diet (lower fecal fiber and more fiber remaining in the grazed pasture), improved N-use efficiency, and reduced enteric CH4 emissions by up to 20%, supporting their potential as a strategy to enhance nutrient retention and mitigate environmental impacts in grass-based beef systems.