Ractopamine hydrochloride is a phenethanolamine β-adrenergic agonist that redirects nutrients away from fat deposition and towards lean deposition. The mode of action of ractopamine primarily involves the modulation of metabolic pathways and signals in muscle and lipid cells to enhance protein accretion. Other mechanisms also include regulation of hormone release and modification of blood flow (Mersmann, 1998).
Ractopamine inclusion in finishing diets has been consistently related to improvements in growth rate, feed efficiency, and carcass leanness with minimal effects on pork quality (Apple et al., 2007; Bohrer et al., 2013). Ractopamine seems to enhance lean gain by partitioning energy towards protein deposition, which also improves efficiency because protein requires less energy to be deposited than fat (Mersmann, 1998). The effects of ractopamine progressively decrease over time as prolonged exposure to β-adrenergic agonist causes desensitization of receptors (Mersmann, 1998). Also, the effects of ractopamine are lost once the inclusion of ractopamine in the diet is stopped.
Ractopamine is labeled for addition to finishing diets at 5 to 10 mg/kg for the last 45 to 90 lb of gain before marketing, which is about 21 to 35 days. Ractopamine should be continuously supplied up to marketing as the beneficial effects of ractopamine are quickly lost after withdrawal. Also, appropriate nutritional adjustments must be made when adding ractopamine in finishing diets to support greater rate of lean gain. It is legally required that diets with ractopamine contain at least 16% crude protein. It is recommended to increase lysine levels by 5 to 6 g of SID Lys per day (0.2 to 0.25 percentage units of SID Lys) while maintaining other amino acids at a proper ratio to lysine (Jacela et al., 2009b).
Use of ractopamine has raised concerns regarding swine welfare, particularly on behavior, ease of handling, susceptibility to stress, and incidence of fatigued, non-ambulatory market-weight pigs (Marchant-Forde et al., 2003). Finishing pigs fed ractopamine have been found to be more difficult to handle and more susceptible to fatigue and stress with aggressive handling, especially at high inclusion of ractopamine (James et al., 2013c; Ritter et al., 2017). Implementation of low stress handling and transportation practices is particularly important with pigs fed ractopamine in order to safeguard welfare and minimize losses.