348. The Effect of Increasing Inclusion Level of Different Rumen Buffers or Alkalizing Agents on in vitro Fermentation Outcomes When Added to a High-roughage Diet for Growing Beef Cattle.

Abstract Reducing enteric methane (CH4) emissions from beef cattle remains an opportunity to promote production efficiency and minimize the environmental footprint of beef production. Rumen buffers and alkalinizing agents decrease the concentration of free hydrogen ions in the rumen that may be reduced to form dihydrogen, a primary substrate for methanogenesis. Four individual completely randomized design experiments were conducted to evaluate the effects of increasing levels of limestone, sodium bicarbonate (NaHCO3), potassium carbonate (K2CO3), or urea on in vitro CH4 production, digestibility, and other fermentation outcomes when used to replace steam-flaked corn in a high-roughage (35% of DM) diet for growing beef cattle. Treatments included (DM basis) either 0, 1, or 2% of NaHCO3, K2CO3, or urea while limestone inclusions were 0, 1.5, or 3%. Ground diet samples (0.5 g) were added to fiber filter bags in duplicate, randomly assigned, and added to 1 of 36 (n = 12/treatment level) ANKOM radio frequency gas production jars containing buffer and rumen fluid. Gas collection bags were attached to each jar and incubated for 48 h at 39 °C. Gas samples were analyzed for %CH4 and rumen fluid solution samples were analyzed for volatile fatty acid (VFA) and ammonia concentrations. Statistical analyses were conducted using a general linear model (jar = experimental unit; treatment = fixed effect). Covariates included gas production (GP) at h 0 and total sample weight. Orthogonal contrasts were conducted to characterize relationships between treatment levels and response variables. Significance was defined as P ≤ 0.05 with a tendency defined as 0.05 < P ≤ 0.15. Limestone, K2CO3, or urea inclusion did not affect GP, %CH4, or CH4 yield (P ≥ 0.17). Increasing NaHCO3 tended to linearly decrease %CH4 (P = 0.09) but did not affect CH4 yield or GP (P ≥ 0.17). Increasing limestone linearly decreased in-vitro dry matter digestibility (IVDMD; P = 0.01) whereas increasing urea linearly increased IVDMD (P = 0.03). Increasing K2CO3 or NaHCO3 did not affect IVDMD (P ≥ 0.27). True dry matter digestibility (TDMD) was unaffected by limestone, K2CO3, or urea (P ≥ 0.22), but was linearly increased by increasing NaHCO3 (P < 0.01). Final pH was linearly increased by increasing limestone, urea, or K2CO3 (P ≤ 0.04), but was not affected by NaHCO3 (P = 0.29). Total VFA production was unaffected by limestone, K2CO3, or urea (P ≥ 0.20). However, increasing NaHCO3 linearly increased total VFA production (P = 0.03). Ammonia production was unaffected by limestone, NaHCO3, or K2CO3 (P ≥ 0.35), but was linearly increased by increasing urea (P < 0.01). These results indicate that in vitro CH4 production was relatively unaffected by buffers or alkalinizing agents added to a high-roughage diet, however, other effects on rumen fermentation warrant further investigation.