Effect of supplemental organic acids on performance, carcass characteristics, and serum biochemical metabolites in broilers fed diets containing different crude protein levels

The present study was conducted to investigate the effect of dietary supplementation of organic acids (OA) on performance and carcass characteristics of broiler chickens fed diets with different crude protein (CP) levels. A total of 540 day-old broiler chickens were assigned to 9 dietary treatments in a 3 x 3 factorial arrangement with 4 replicate pens and 15 broiler chickens per pen. Dietary treatments consisted of 3 different CP levels (high, medium, and low) and 3 dietary OA supplementation (control, 2.5 g citric acid/kg, and 2.5 g butyric acid/kg). Dietary CP levels in high-CP diets were 230, 210, and 190 g/kg during starter, grower, and finisher periods, respectively. The respective values were 220, 200, and 180 g/kg for medium-CP, and 210, 190, and 170 g/kg for low-CP diets. The study lasted for 42 d. Performance variables including average daily gain (ADG), feed intake (ADFI), and feed conversion ratio (FCR) were measured biweekly. At the end of study, 2 randomly selected birds per pen were slaughtered to measure carcass traits. Furthermore, serum samples of 42-d-old broilers were analyzed for blood metabolites. Results showed that although ADFI and FCR values were not affected by CP levels, reducing dietary CP level (medium + low) decreased ADG during starter (P = 0.092) and grower (P=0.053) periods. At the finisher phase, however, ADG was similar among different CP groups. Dietary supplementation of citric acid increased ADG (P=0.062) during starter period. In addition, OA supplementation of diets (citric + butyric) increased (P=0.002) ADG at the finisher period, resulting in improved (P=0.027) FCR values. Supplemental OA increased ADG only in broilers fed diets containing higher CP levels (high- and medium-CP diets), resulting in a CP x OA interaction (P=0.024) during the finisher period. The relative liver weight and carcass yield were decreased (P<0.05) by reducing dietary CP level (medium + low), while abdominal fat percentage was increased (P < 0.001), with the greatest (P=0.004) abdominal fat allotted to broilers fed low-CP diet. Dietary OA supplementation (citric + butyric) increased (P=0.016) carcass yield, while reduced (P<0.001) relative gizzard weight. Although serum concentrations of high- and low-density lipoproteins and cholesterol were not affected by dietary CP level, feeding low-CP diet increased (P<0.001) serum triglyceride level compared with higher CP levels. Serum concentration of uric acid was decreased (P<0.001) as the result of reducing dietary CP level (medium + low). The present findings indicate that dietary CP level can be reduced by 2 percentage points if the most limiting amino acids are provided at the sufficient levels. On the other hand, dietary OA supplementation increased weight gain in the finisher period and carcass yield of broiler chickens. (C) 2015 Elsevier B.V. All rights reserved.