Effects of monensin supplementation on ruminal metabolism of feedlot cattle fed diets containing dried distillers grains

Two experiments were conducted to evaluate the effects of monensin and dried distillers grains with solubles (DDGS) on ruminal metabolism in 8 fistulated steers. In Exp. 1, treatments were (DM basis): 1) 0 mg monensin/kg diet DM, 2) 22 mg monensin/ kg diet DM, 3) 33 mg monensin/kg diet DM, and 4) 44 mg monensin/kg diet DM. The remainder of the diet was 10% corn silage, 60% DDGS, 10% corn, and 20% mineral supplement that used ground corn as the carrier. There was no effect (P > 0.80) of dietary monensin inclusion on DMI. Increasing dietary monensin did not affect (P > 0.05) ruminal VFA concentrations or lactic acid concentrations. There was no effect (P > 0.15) of increasing dietary monensin concentration on ruminal hydrogen sulfide gas (H₂S) and liquid sulfide (S^2-) concentrations, or ruminal pH. In Exp. 2, treatments were arranged in a 2 × 2 factorial and contained (DM basis): 1) 0 mg monensin/kg diet DM + 25% DDGS inclusion, 2) 0 mg monensin/kg diet DM + 60% DDGS inclusion, 3) 44 mg monensin/kg diet DM + 25% DDGS inclusion, and 4) 44 mg monensin/kg diet DM + 60% DDGS inclusion. The remainder of the diet was 15% corn silage, corn, and 20% mineral supplement that used ground corn as a carrier. With 60% dietary DDGS inclusion, DMI decreased (P < 0.01) when compared with 25% DDGS inclusion. With 25% DDGS in the diet, 0 h postfeeding acetate concentration was decreased compared with when 60% DDGS was fed (P < 0.01). A similar response (P < 0.01) occurred for total VFA concentrations at 0 h postfeeding. However, at 3 and 6 h postfeeding, propionate concentrations increased (P ≤ 0.05) in cattle fed the 60% DDGS diets, regardless of monensin inclusion. This increase in propionate concentrations contributed to the increase (P = 0.03) in total VFA concentrations at 3 h postfeeding when 60% DDGS diets were fed. There was no interaction detected (P > 0.05) for H²S or S^2-concentrations in Exp. 2. Feeding 60% DDGS diets increased mean H2S by 71% when compared with feeding 25% DDGS diets. Similar to the response observed for H₂S, feeding 60% DDGS diets increased mean S^2-by 64% when compared with feeding 25% DDGS diets. Although these studies did not show beneficial effects of monensin supplementation on ruminal pH, VFA, or H₂S concentrations, adverse rumen conditions, notably low ruminal pH, when high DDGS diets were fed may have precluded the effects of monensin from being realized.