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 fi stulated 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 sulfi de gas (H 2 S) and liquid sulfi de (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 0.05) for H 2 S or S 2- concentrations in Exp. 2. Feeding 60% DDGS diets increased mean H 2 S by 71% when compared with feeding 25% DDGS diets. Similar to the response observed for H 2 S, feeding 60% DDGS diets increased mean S 2- by 64% when compared with feeding 25% DDGS diets. Although these studies did not show benefi cial effects of monensin supplementation on ruminal pH, VFA, or H 2 S concentrations, adverse rumen conditions, notably low ruminal pH, when high DDGS diets were fed may have precluded the effects of monensin from being realized.