Two experiments were completed to determine the effects of diet mix uniformity and ractopamine HCl on finishing pig performance. In Exp. 1, a total of 200 pigs (BW = 90 kg) were used in a 33-d growth assay arranged in a randomized complete block design with 5 pigs/pen and 8 pens/treatment. Treatments were a corn-soybean meal-based control diet mixed for 360 s or the mixed control diet with 10 mg/kg ractopamine added before additional mixing for 0, 30, 120, or 360 s. Chromium was added to the diet and analyzed to determine mix uniformity of the ractopamine. Therefore, this experiment was designed to determine the effects of nutrient utilization from a thoroughly mixed diet with a potentially nonuniform distribution of ractopamine. Pigs fed diets with ractopamine had greater (P < 0.05) ADG, G: F, final BW, HCW, dressing percentage, loin depth, and percentage carcass lean with lower (P = 0.005) 10th rib backfat thickness. Increasing mix time from 0 to 360 s decreased (quadratic, P = 0.001) CV for Cr from 67 to 12% but had no effect on the response to ractopamine for any growth or carcass measurements. In Exp. 2, a total of 160 pigs (BW = 93 kg) were used in a 27-d growth assay arranged in a completely randomized design with 2 pigs/pen and 16 pens/treatment. Treatments were a corn-soybean meal-based control diet mixed for 360 s or the control diet with 10 mg/kg ractopamine mixed for 0, 30, 120, or 360 s. Diet mix uniformity was determined by measuring Cr and using Quantab Cl titrators (low range 0.005 to 0.1% as NaCl; Environmental Test Systems Inc., Elkhart, IN) to measure the concentration of salt. This experiment was designed to determine the combined effects of potentially nonuniform distribution of both nutrients and ractopamine. The use of ractopamine increased (P < 0.05) ADG, G: F, final BW, HCW, dressing percentage, percentage lean, and loin depth. Increasing mix times from 0 to 360 s decreased (quadratic, P = 0.050) CV for salt and Cr from 51 to 12% and 51 to 15%, respectively, with no effect on ADG, ADFI, final BW CV, HCW, dressing percentage, backfat thickness, loin depth, or percentage carcass lean; however, G: F tended to increase (linear, P = 0.07) as mix time increased from 0 to 360 s. In conclusion, increasing mix time of diets from 0 to 360 s did not affect the response of finishing pigs to ractopamine.
