Effects of Ractopamine Hydrochloride (Actogain) on Nutrient Digestibility and Nitrogen Excretion of Finishing Steers

The objective was to evaluate the effects of ractopamine hydrochloride (Actogain; Zoetis, Kalamazoo, MI) on nutrient digestibility and N excretion. In experiment 1, twelve Simmental × Angus steers were used in a randomized complete block design. Dietary treatments were top-dressed and included: 1) a control without Actogain (CON) or 2) 400 mg·steer^-1·d^-1 ractopamine hydrochloride (RAC) for 35 d before slaughter. Steers were blocked by weight (BW = 571 ± 21 kg). Diets contained 55% dry rolled corn, 20% corn silage, 15% modified wet distillers grains with solubles, and 10% supplement on a DM basis. Two 5 d sampling periods were conducted for each block for total collection of feed, orts, feces and urine. No interaction (P > 0.10) between treatment and collection period was observed for any parameter evaluated. Dietary treatment had no effect (P = 0.38) on DMI, but RAC-fed steers had decreased fecal DM output (2.1 vs. 2.5 kg DM/d; P = 0.04) compared with CON-fed steers. Steers fed RAC had greater apparent total tract DM digestibility (72.8 vs. 68.9%; P = 0.02), NDF digestibility (59.2 vs. 53.3%; P < 0.01), and ADF digestibility (53.8 vs. 47.9%; P = 0.05) than CON-fed steers. Although dietary treatment did not affect N intake (P = 0.51) or N digestibility (P = 0.14), RAC-fed steers excreted less total N (120.8 vs. 138.2 g/d; P = 0.02) than CON-fed steers due to a tendency for decreased fecal N output (61.5 vs. 71.8 g/d; P = 0.09) in RAC-fed steers compared with CON-fed steers. Dietary treatment had no effect (P = 0.31) on urinary N output. Experiment 2 was an in vitro experiment conducted in split-plot design to validate the effects of RAC on nutrient digestibility using a separate, yet contemporary group of heifers (N = 20). Rumen fluid was collected by stomach tube from CON- and RAC-fed heifers (whole-plot) to inoculate tubes containing a CON or RAC substrate (split-plot). No interaction between rumen fluid source and in vitro substrate was observed (P = 0.44). Greater IVDMD (64.0 vs. 58.6%; P = 0.01) was observed in rumen fluid from RAC-fed heifers compared with rumen fluid from CON-fed heifers. Inclusion of RAC in the in vitro substrate increased IVDMD (63.1 vs. 59.5%; P < 0.01). Overall, feeding RAC impacted microbial digestion of the finishing diet to increase total tract digestion and reduce N excretion in the 35 d period prior to slaughter.