Effect of Ractopamine (Paylean) and Lysine:net Energy Ratio on Growth Performance and Plasma Urea Nitrogen Concentration of Late-finishing Barrows Fed Low-protein Amino Acid-supplemented Diets

University of Nebraska 2007 Swine Report. The feeding of reduced dietary crude protein (CP) to late-finishing pig diets has been a strategy to reduce the amount of nitrogen (N) excreted. Nitrogen from the excess of amino acids is later eliminated by the pigs in the feces and urine, and depending on the waste disposal system may contaminate water supplies. This study was conducted to evaluate the effect of Ractopamine (RAC; Paylean) and lysine (lys):Net energy (NE) ratio on growth performance and plasma urea nitrogen concentration (PUN) of late-finishing barrows fed low-protein amino acid-supplemented diets. Thirty-six late-finishing barrows with an initial body weight of 170 lb were used in a 28-day experiment. Pigs were individually penned and had ad libitum access to feed and water. The pigs were randomly allotted to one of six dietary treatments consisting of one standard diet and two low-protein amino acid-supplemented diets with different lys:NE (5.23 or 6.31g lys/Mcal NE) and RAC counterparts (0 and 20 ppm). Body weight and feed disappearance were measured weekly. Average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (ADG/ADFI) were calculated. Blood samples were collected weekly by venipuncture and plasma separated. During the overall experimental period, RAC increased ADG (P = 0.019) and ADG/ADFI (P = 0.021). The inclusion of RAC resulted in a reduction on PUN on day 7 (P = 0.0286), and 28 (P = 0.0069) as well as in the overall experimental period (P = 0.0179). Growth performance was not affected by reduction in dietary CP concentration; however PUN decreased in response to CP reduction for all sampling days and the overall experimental period (P = 0.0003). The increased Lys:NE in low-protein amino acid-supplemented diets did not change growth performance or PUN for any of the experimental weeks or the overall experimental period. Ractopamine inclusion increased ADG, and ADG/ADFI for weeks 1 and 2 (P = 0.025) and the overall experimental period ( P = 0.017) when added to low-protein amino acid-supplemented diets. These results indicate that RAC (20 ppm) inclusion and low-protein amino acid- supplemented diets increased the efficiency of amino acid utilization for late-finishing pigs. Increasing lys:NE of low-protein amino acid-supplemented diets fed to finishing pigs did not improve growth performance or reduce PUN. The results of this study indicate that RAC (20 ppm) inclusion increased growth performance and the efficiency of amino acid utilization for growth in late-finishing pigs fed different dietary CP concentrations (17.4 or 14%). Increasing lys:NE did not improve growth performance in pigs fed low-protein (14 %) amino acid-supplemented diets. Low-protein amino-acid supplemented diets can provide amino acids and energy in adequate amounts to allow RAC to increase growth performance of pigs from the UNL herd fed for 28 days to a target weight of 240 lb.