SUPPLEMENTAL FAT AND IONOPHORES IN FINISHING DIETS - FEEDLOT PERFORMANCE AND RUMINAL DIGESTA KINETICS IN STEERS

One feedlot trial (116 d; 192 steers) with a factorial arrangement of treatments was conducted to determine effects of supplemental fat and ionophores in corn-based finishing diets. Factors were level of fat (0 or 4% tallow) and ionophore type (none [N], lasalocid [L], monensin p lus tylosin [MT], or daily rotation of L and MT [LMT]); L, M, and T we re fed at 31, 25, and 10 ppm (DM basis), respectively. Fat and ionopho re interacted for DMI (P < .10), ADG (P < .01), and gain:feed G/F, P < .05). Compared with 0%-fat diets, 4% fat decreased (P < .005) DMI by steers fed MT by 8.9%. In 0%-fat diets, MT increased (P < .05) ADG and G/F compared with N or L; however, this increase was negated or rever sed in 4%-fat diets. Fat and ionophore interacted (P < .16) on dietary NE concentration. In the 0%-tallow diet, MT increased NE(m) and NE(g) by 5.1 and 7.0%, respectively, but MT had not effect on NE concentrat ion of the 4%-tallow diet. Alternate feeding of L and MT did not incre ase animal performance above that of separate, continuous feeding. Six ruminally fistulated steers were used in a metabolism study to evalua te potential mechanisms underlying the interactions observed in the fe edlot trial. Treatments were the same as the feedlot trial, except tha t LMT was not included. Ruminal digesta kinetics were not altered (P > .30) by the addition of fat or ionophores to the diet. Fat and ionoph ore interacted (P < .05) for molar proportions of ruminal acetate and propionate. Acetate was decreased (P < .05) and propionate was increas ed (P < .05) by ionophore addition to the 0%-fat diets; however, when 4% fat was fed, neither L nor MT affected acetate or propionate. These results suggest that the response of finishing cattle to ionophores m ay be altered by fat supplementation, perhaps in part as a result of n egative associative effects between these additives on the end product s of ruminal fermentation.