And Allow for More Specific Nutritional Recommendations. Feeding Rac- Potential Impact of Ractopamine on Environ- Mental Stewardship

Several research trials have evaluated the impact of ractopamine (RAC, Paylean r , Elanco Animal Health) on barrows and gilts of various genetic populations (GP). Overall, the desirable response of RAC to increase daily carcass lean gain, improve feed efficiency, and increase carcass lean percentage has been observed in GP of substantially different lean growth rates and carcass lean percentages. Three trials have evaluated the magnitude of GP × RAC interactions. Carcass muscle accretion (g/d) increased with RAC to a greater extent in high lean gain (HL) barrows than low lean gain (LL) barrows (P < .02, Bark et al., 1992, J. Anim. Sci. 70:3391). Dissected fat accretion (g/d) was reduced by a greater magnitude in the HL than the LL barrows (P < .04). Gu et al. (1991, J. Anim. Sci. 69:2694) evaluated the RAC response in five GP of barrows and found significant RAC by GP interactions (P < .05) for daily carcass lean gain. Regression of GP means for carcass lean gain with RAC on the carcass lean gain of the control for the five GP indicated that the RAC response was best described as a constant percentage (25%) increase in daily lean gain above the controls. The third trial (Herr et al., 2001) evaluated the response of Paylean in 300 gilts in a 3 × 4 factorial with three GP (commercial terminal crosses) and four RAC levels (0, 5, 10, and 20 ppm). The GP had similar carcass lean percentage. No GP by RAC interactions were found (P > .10). Overall, the research indicates that RAC has a positive impact on barrows and gilts with substantially different genetic potentials for lean growth and carcass lean percentage. The RAC response to increase lean growth has been found to be proportional to the genetic potential of the GP. Recent research has found significant GP by environmental interactions for pigs reared in different health status environments. Environment by RAC and environment by RAC by GP interactions for compositional growth in pigs need to be evaluated.