ARDACIN FOR STEERS GRAZING ENDOPHYTE-FREE FESCUE PASTURE - EFFECTS ONLIVE WEIGHT-GAIN, FORAGE INTAKE, NITROGEN AND FIBER DIGESTION, RUMINAL FLUID KINETICS, RUMINAL FERMENTATION, AND SERUM HORMONES AND METABOLITES

Growth and digestion studies were conducted to evaluate the use of ard acin as a feed-grade antibiotic for enhancing digestive function and g rowth in grazing steers. In Exp. 1, 90 yearling steers (average initia l BW of 248 kg) used in a randomized complete block design (block = we ight group) grazed fescue pasture without supplementation (CON) or wit h daily supplements (DM basis) of .4% of BW supplemental ground corn ( CRN) or .4% of BW supplemental corn supplying 120 mg of ardacin (ARD). In Exp. 2, 12 ruminally and duodenally cannulated steers and three ru minally cannulated steers (Hereford x Angus; average BW of 347 kg) wer e used to evaluate the effects of the same supplements used in Exp. 1 on ruminal fermentation and digestion. In Exp. 1, ARD-supplemented ste ers weighed more (P < .01) at the conclusion of the study than CRN ste ers, which together weighed more (P < .01) than CON steers. Average da ily gain was greater (P < .10) in supplemented than in CON steers; ARD steers had greater (P < .01) ADG than CRN steers. In Exp. 2, forage i ntake and harvesting efficiency did not vary (P > .10) with supplement ation or type of supplement, but total intake reflected (P = .03) the addition of corn to the forage diet. Addition of ardacin increased (P = .02) ruminal pH compared with CRN steers. Ardacin decreased ruminal molar proportions of acetate and increased (P = .01) propionate propor tions when compared with CRN steers. Total tract N digestibility was a ffected (P < .10) by supplementation and by addition of ardacin to the diet. Addition of ardacin to the ground corn supplement increased ADG , in part by enhancing acetate:propionate ratios and increasing N dige stion.