INTERACTION OF FORAGE LEVEL AND MONENSIN IN DIETS FOR FEEDLOT CATTLE ON GROWTH-PERFORMANCE AND DIGESTIVE FUNCTION

A feedlot growth-performance trial and a metabolism trial were conduct ed to evaluate the interaction of forage level (10 vs 20%) and monensi n (0 vs 28 mg/kg, air-dry basis) on utilization of a steam-flaked corn -based finishing diet. There were no treatment interactions (P > .10) on feedlot cattle growth performance or site and extent of digestion o f OM, ADF, N, and starch. Monensin supplementation did not influence ( P > .10) ADG, DM intake, feed efficiency, estimated NE value of the di et, or luminal and total tract digestibility of OM, ADF, and starch. M onensin decreased (14.5%, P < .10) passage of microbial N to the small intestine and ruminal digestion of feed N (10.4%, P < .05). Ruminal p H tended to be slightly lower (1.9%, P < .10) with monensin supplement ation. Monensin did not affect (P > .10) ruminal molar proportions-of acetate and butyrate. There was an interaction between forage level an d monensin on ruminal molar proportions of propionate. With the low-fo rage diet, molar proportions of propionate were 9.4% higher with monen sin supplementation. In contrast, with the high-forage diet ruminal mo lar proportions of propionate were 5.5% lower with supplemental monens in. Monensin did not affect (P > .10) estimates of methane production. Decreasing the forage in the diet from 20 to 10% increased ADG (10.8% , P < .10), feed efficiency (11.6%, P < .01), and diet NE(g) (11.3%, P < .05). Ruminal digestibility of ADF, OM, and starch were not affecte d (P > .10) by forage level. However, ruminal digestibility of feed N was 20% greater (P < .05) with the high-forage diet. Increasing forage level in the diet decreased total tract digestion of OM (2.4%, P < .0 5), DE (2.7%, P < .05), and ME (4.8%, P < .01) and increased ruminal p H (4.4%, P < .01) and ruminal molar proportions of acetate (13.0%, P < .01) and decreased ruminal molar proportions of propionate (10.2%, P < .05) and increased estimated methane production (19.4%, P < .05). It is concluded. that failure of monensin to elicit a growth-performance response in feedlot steers fed a steam-flaked corn-based finishing di et is not due to differences in the diets' forage level.