Two experiments were conducted to determine the CP concentration below which N retention and growth performance are reduced when low-protein, amino acid-supplemented, corn-soybean meal diets are fed. In a N balance trial (Exp. 1), 12 gilts (initial weight 41 kg) were fitted with urinary catheters and fed six different diets during three 7-d periods in an incomplete block design. The diets were: 1) 18% CP; 2) 14% CP + AA, 3) 16% CP; 4) 12% CP + AA; 5) 14% CP; and 6) 10% CP + AA. Amino acids (lysine, threonine, tryptophan, and methionine) were supplemented such that the concentrations in the low-protein diets were equal to those in their standard (4% CP higher) counterparts. Nitrogen retention (g/d) decreased (P < 0.01) as CP decreased, in both standard (27.10, 24.53, and 20.99) and low-protein (21.51, 19.18, and 15.83) diets, but was lower (P < 0.01) in low-protein diets. There were no differences among treatments (P > 0.05) in biological value (68.2% standard vs 71.0% low-protein). In a growth performance trial (Exp. 2), 36 gilts (initial weight 19.5 kg) were penned individually and fed one of six diets for 35 d in a randomized complete block design. Dietary treatments were a 16% CP standard diet and low-protein diets formulated to contain 15, 14, 13, 12, and 11% CP supplemented with crystalline lysine, tryptophan, threonine, and methionine to equal the total concentrations in the standard diet. Protein concentration affected (P < or = 0.05) ADG, ADFI, feed efficiency, fat-free lean gain, longissimus muscle area, plasma urea, and plasma concentrations of most essential AA. For most of these traits, the major difference was poor performance of pigs fed the 11% CP diet. Thus, in Exp. 1, at AA concentrations from deficient to excess, low-protein, amino acid-supplemented diets failed to produce the same N retention as the equivalent corn-soybean meal diets. However in Exp. 2, the same performance was obtained with 16, 15, 14, 13, and 12% CP. Based on these data, we suggest that N balance is more sensitive than growth to amino acid adequacy andthat other AA (e.g., isoleucine and valine) may limit growth performance when the protein concentration is reduced by more than four percentage units.

Three experiments were conducted to determine the fifth-limiting amino acid for growing pigs in an 11% CP, corn-soybean meal diet. In each experiment, 36 gilts (initial weight 19.5, 21.9, and 21.0 kg, respectively) were penned individually and fed one of six diets in a randomized block design for 35 d. Diets containing 16, 12, and 11% CP were fed in each experiment. All 12 and 11% CP diets were supplemented with lysine, tryptophan, threonine, and methionine to provide the same total concentrations as those in the 16% CP diet. In Exp. 1, the 11% CP diet was supplemented with isoleucine, valine, or isoleucine + valine to concentrations equal to those in the 16% CP diet. In Exp. 2, the 11% CP diet was supplemented with histidine, histidine + valine, or histidine + isoleucine + valine. In Exp. 3, the 11% CP diet was supplemented with valine, histidine + valine, or isoleucine + valine. Gilts were allowed free access to feed and water. In all experiments, ADG and feed efficiency (G/F) were reduced (P < or = 0.07) as dietary protein was reduced. Supplementation of isoleucine alone further reduced (P < 0.05) ADG, ADFI, G/F, and fat-free lean gain. In contrast, supplementation of valine alone resulted in numerical increases in ADG and ADFI in two experiments, although the differences were not significant (P > 0.05). Supplementation with histidine and valine together resulted in growth performance equal to or greater than that of pigs fed the 12% CP diet, but less than that of pigs fed the 16% CP diet. Supplementation of isoleucine and valine together resulted in better growth performance (P < 0.05) than supplementation of either amino acid alone. In two experiments (Exp. 1 and 3), supplementation of the 11% CP diet with isoleucine and valine together resulted in ADG that were not significantly different (P > 0.05) from those of pigs fed the 16% CP diet. Supplementation of all three amino acids (Exp. 2) did not improve performance over supplementations with histidine and valine. Plasma urea concentrations were reduced (P < 0.05) as dietary protein was lowered from 16 to 12%. Additions of crystalline amino acids did not affect plasma urea levels. Plasma amino acid concentrations reflected the dietary additions of crystalline amino acids, but did not assist in the identification of the sequence of limiting amino acids. These data suggest that valine is the fifth-limiting amino acid and that either histidine or isoleucine is the sixth-limiting amino acid in an 11% CP diet.

Figueroa, J.L., Martinez, M., Trujillo, J.E., Zamora, V., Cordero, J.L. and Sanchez-Torres, M.T. 2008. Plasma urea nitrogen concentration and growth performance of finishing pigs fed sorghum-soybean meal, low-protein diets. J. Appl. h i m . Res., 33: 7-12. To determine the lowest CP level i n sorghum-soybean meal diets of finishing pigs using plasma urea nitrogen (PUN) concentration as rapid response criterion

Metaanálisis del efecto de dietas bajas en proteína y adicionada con aminoácidos sintéticos para cerdos machos castrados en finalizaciónMeta-analysis of the effect of low protein diets for finishing barrows SUMMARY A meta-analysis was carried out using individually pig data from seven experiments involving 222 hybrid finishing pigs with initial body weight of 52.69 ± 3.74 kg, individually housed, fed low-protein diets. Growth performance, carcass characteristics and plasma urea nitrogen concentration were evaluated. An analysis of variance and regression using a response surface model with RSREG procedure of SAS (2009) were performed considering level of crude protein (CP), effect of the experiment, year, time of year, type and level food additive supplemented. Optimum levels of CP were determined (12.8, 11.3 and 9.5%) and corroborated through an additional experiment using 36 hybrid finishing pigs with 54.01 kg of initial weight, during 42 days. Average daily gain, daily feed intake, feed conversion, final weight, gain lean meat, lean meat percentage, longissimus muscle area and plasma urea nitrogen concentration were not affected (P > 0.05) at a concentration of 11.5% CP, however, with 9.5% PC these variables were affected (P ≤ 0.05); backfat increased by lowering CP (P ≤ 0.05). In the experiment to corroborate optimal levels of PC, there were no differences (P > 0.05) for weight gain, feed intake, final weight and gain lean to lower the PC diet to 11.5%. For backfat, longissimus muscle area and lean meat percentage there were no differences (P > 0.05) when decreasing prot to 12.8 9.5% PC. Plasma urea concentration was lowered (P ≤ 0.05) to decrease the PC diet. Reducing dietary protein to 11.5% does not adversely affect the variables analysed.Key words: Low-protein diets, meta-analysis, finishing barrows. RESUMENSe realizó un metaanálisis con datos originales de siete experimentos con 222 cerdos híbridos en finalización alojados individualmente con peso vivo inicial promedio de 52,69±3,74 kg, alimentados con dietas bajas en proteína. Se evaluó el comportamiento productivo, las características de la canal y la concentración de urea en plasma. Se realizó análisis de varianza y de regresión utilizando un modelo de superficie de respuesta mediante RSREG de SAS (2009), considerando nivel de proteína cruda (PC) y efecto del experimento, año, época del año, tipo y nivel del aditivo alimenticio agregado. Se determinaron los niveles óptimos de PC (12,8, 11,3 y 9,5%), corroborándolos con un experimento adicional utilizando 36 cerdos híbridos en finalización con 54,01 kg de peso inicial durante 42 días. La ganancia diaria de peso, consumo diario de alimento, conversión alimenticia, peso final, ganancia de carne magra, porcentaje de carne magra, área del músculo longissimus y concentración de urea en plasma, no se afectan (P > 0,05) con 11,5% de PC pero al disminuir la PC hasta 9,5% sí son afectadas (P ≤ 0,05); el grosor de la grasa dorsal aumenta cuando disminuye la PC (P ≤ 0,05). Al corroborar los niveles óptimos de PC ...

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