Evaluation of 0 or 300 Mg of Optaflexx® on Growth Performance and Carcass Characteristics of Steers Fed to Different Degrees of Finish

The objective of this study was to test the effect of 0 or 300 mg of Optaflexx^® on growth and carcass performance of yearling steers fed to differing degrees of finish. Crossbred yearling steers (n=342; initial BW = 416, SD = 34 kg) were utilized in a generalized randomized block design (3 BW blocks) with a 2 × 3 + 1 factorial treatment design. Factors included Optaflexx dosage (0 or 300 mg / steer daily) and different days on feed (118, 139, or 160) plus cattle fed 2 weeks longer (174 d) without Optaflexx to evaluate just feeding longer. Steers were fed Optaflexx for the last 35 d prior to harvest. No significant dose × days on feed interactions (P > 0.40) were observed for growth performance. Live final BW was 13 kg heavier (P = 0.01) for steers fed 300 mg of Optaflexx as compared to steers fed 0 mg. Steers fed 300 mg of Optaflexx had greater (P = 0.04) carcass-adjusted ADG compared to steers fed 0 mg. Feeding 300 mg of Optaflexx resulted in an improvement (P = 0.05) in carcass-adjusted G:F. There were no differences (P = 0.24) in DMI between Optaflexx doses. Carcass weight and carcass-adjusted final BW were 6.9 and 10 kg greater (P < 0.06), respectively, for steers fed 300 mg of Optaflexx compared to 0 mg. As days on feed increased, final BW increased linearly (P < 0.01); whereas, intake decreased linearly (P < 0.01). Carcass-adjusted ADG was constant (P > 0.15) which lead to a small linear improvement (P= 0.02) in G:F, due to the reduction in DMI. Cattle performance was negatively influenced by wet, cold, muddy conditions in January and February which lowered ADG and HCW compared to targeted finish weights/HCW for the study. From d 90-97 of the trial, cattle on both Optaflexx treatments had a negative ADG response. During d 111 to 118, cattle performance suffered a reduction in live interim BW for steers fed 118 d for both Optaflexx and control treatments. During these time points, weather was adverse with low comprehensive climate index numbers resulting a reduction in performance for cattle fed 118 or 139 d, as cattle fed longer than 139 d had time to recover in performance. Feeding Optaflexx at 300 mg improved ADG, G:F, and HCW regardless of days on feed (i.e., degree of finish).

Key Words: Finishing, Ractopamine, Yearlings