Effects of optaflexx (ractopamine hydrochloride) on performance, rate and variation of intake, and acid-base balance in feedlot cattle

Research was conducted to determine the effects of Optaflexx (OP) in feedlot cattle. In Exp. 1, a 3 x 3 factorial was used to study optimal dose (0, 100, or 200 mg•steer-1•d-1) and duration of feeding (28, 35, or 42 d) of OP in a randomized complete block design (n = 360; BW = 545 ± 33 kg). As OP dose increased, there was a linear increase in final BW (FBW; 1.6%; P = 0.011), average daily gain (ADG; 18.2%; P < 0.001), and gain-to-feed ratio (G:F; 15.6%; P < 0.001). As duration of feeding increased, there were quadratic (P = 0.04) and linear (P < 0.004) effects on ADG, with tendencies (P < 0.10) for quadratic effects for FBW, dry matter intake, and G:F. Hot carcass weight increased linearly (1.9%; P = 0.016) as dose of OP increased. Increasing dose of OP from 0 to 200 mg•steer-1•d-1 and duration of feeding from 28 to 42 d, improved feedlot performance, although quadratic responses for duration of feeding indicated little improvement as duration was extended from 35 to 42 d. In Exp. 2, 12 crossbred beef steers (BW = 593 ± 16 kg) were used in a completely randomized design to evaluate effects of OP (0 or 200 mg•steer-1•d-1) on rate of intake, daily deviation in intake, and acid-base balance. There were no differences in feedlot performance, urine pH, blood gas measurements, or daily deviations in intake between treatments; however, cattle fed OP at 200 mg•steer-1•d-1 had increased longissimus muscle area, decreased yield grade, and increased time to consume 50 and 75% of daily intake compared with control cattle. In Exp. 3, feed delivery data from three dose and duration trials were analyzed for day-to-day variation in feed deliveries. Based on analyses of adjusted absolute daily deviations in feed delivered, data indicated that OP did not affect daily variation; however, cattle tended to consume feed sporadically. These studies indicate that feeding OP for 35 d at 200 mg•steer-1•d-1 provided optimal improvement in feedlot performance, and that OP did not affect acid-base balance or variation in feed intake.