Ractopamine with dietary lysine concentrations above basal requirements of finishing barrows improves growth performance, carcass traits and modifies the mTor signalling pathway

A 28-day study was conducted to evaluate the effects of three step-up levels of ractopamine hydrochloride (RAC) together with two additional levels of standardised ileal digestible lysine (Lys) above the basal requirements on growth performance, carcass characteristics and the mechanism of action on adipose and muscle tissue. In all, 108 finishing pigs (initial bodyweight 75.37 kg ± 2.88) were used for growth data and 54 pigs for carcass data. Samples from 18 pigs were used for the molecular study. Pigs were blocked by initial bodyweight and allotted to one of the following nine treatments: negative control (NC) without addition of RAC or Lys supplementation, constant 7.5 mg/kg RAC, 5 mg/kg RAC for 14 days, followed by 10 mg/kg for 14 days (Step-up 1), 5 mg/kg RAC for 21 days, followed by 10 mg/kg for 7 days (Step-up 2) and 5 mg/kg RAC for 7 days followed by 10 mg/kg for 21 days (Step up 3); on constant and step-up treatments were added 15% or 30% Lys above the basal level, giving a 4 × 2 + 1 factorial with six replicates. Loin muscle and fat tissue were collected for carcass-characteristic analysis and western blotting for p-AKT, p-P70S6K and carnitine palmitoyltransferase I. Feeding RAC increased gain to feed ratio and efficiency of energy utilisation (EF) from Day 0 to Day 13 (P < 0.05) compared with NC. From Day 14 to Day 27, greater average daily gain (ADG) was observed in RAC-treated animals (P < 0.05), except in the Step-up 2 with 30% additional Lys. During the second half of the trial, RAC positively affected ADG, gain to feed ratio and EF (P < 0.01), while a Step-1 versus Step-2 effect was observed for ADG (P < 0.03). For the overall period, RAC-treated pigs had greater ADG than did NC pigs (P < 0.05). An average of 8.1% improvement on feed efficiency and 30% improvement on EF were observed for RAC-fed pigs in comparison to NC pigs (P < 0.05). Chilled carcass weight and loin eye area were increased in pigs fed RAC (P < 0.01). Western blotting showed greater p-P70S6K in muscle samples from pigs fed RAC with 15% additional Lys than in those from NC pigs (P < 0.10). RAC was effective at improving efficiency of production. Lys supplementation of 15% was enough for optimal performance of the pigs in the present study; however, step-up programs did not outperform RAC-constant programs. Results of the present study suggest that RAC stimulates protein synthesis through the mTOR signalling pathway.