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When feeding excessive amounts of protein, the nitrogen eliminated by the pigs in swine facilities has an important impact in the environment. Therefore, it is important to define nutritional strategies that promote a more efficient use of protein. This study was conducted to evaluate the effect of a low-protein non-amino acid supplemented diet and ractopamine (Paylean®) on performance of late-finishing pigs. Thirty-six finishing barrows and gilts with an initial body weight of 153.4 lb were used in a 42- day experiment. Pigs were penned individually and had ad libitum access to feed and water. The pigs were randomly allotted to one of four dietary treatments with different dietary protein (10 or 16 % CP) and ractopamine (0 or 20 ppm) concentrations. 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 serum was collected. Data were analyzed as repeated measures and by orthogonal contrast (to examine differences among means). There were treatment differences for ADG (P < 0.05) for the overall experimental period with the highest ADG (2.26 lb/ day) corresponding to the pigs receiving 16% CP and 20 ppm ractopamine. There was no ractopamine effect on serum urea nitrogen( SUN) for any weekly period or overall. Average daily feed intake was lower for diets with 16% CP compared to diets with 10% CP, (P< 0.05). For diets with 10% CP (vs. 16% CP), ADG/ADFI was lower (P < 0.05). There was an effect of protein on ADG and ADG/ADFI (P < 0.05), but not on SUN or ADFI. The lack of an effect of ractopamine on ADG, ADFI, ADG/ADFI or SUN was possible due to an inadequate protein or amino acid intake. Ractopamine tended to increase growth performance. In summary, the highest CP concentration used in this experiment failed to provide an adequate amino acid supply to allow ractopamine to increase growth performance of late-finishing pigs. It appears that ractopamine requires dietary CP concentrations greater than 16% to improve growth performance in late finishing pigs from the University of Nebraska–Lincoln herd.