Effect of different levels of L-carnitine on growth performance, carcass characteristics, abdominal fat, Blood plasma components, immune system, microbial flora of the cecum, small intestinal histology, sensory properties of breast meat and profiles of fatty acids of breast meat in broilers

L-Carnitine (β-hydroxy-γ-trimethylaminobutyric) is produced in living organisms through food and the biosynthesis of the amino acids lysine and methionine in the liver, kidney, skeletal muscle and brain and then enters the bloodstream. One of the most important functions of L-carnitine in the body is to produce energy, facilitate the entry of long chain fatty acids into the mitochondria, facilitate the exit of short and medium chain fatty acids into the mitochondria, eliminate the toxic effects of acyl groups in cells, regulate the ratio of coenzyme-A versus acyl coenzyme-A in the cytosol and mitochondria. L-carnitine is a plasma lipid-lowering drug that lowers cholesterol, triglycerides, free fatty acids, phospholipids and low-density lipoproteins, while increases high-density lipoproteins. Supplementation of poultry diets with L-carnitine in has been reported to be effective in controlling blood lipids, abdominal fat pad and poultry health. This study aims to investigate the effects of L-carnitine on growth performance, carcass characteristics, abdominal fat, blood biochemical parameters, immune system, cecal microflora, histology of jejenum, taste properties of breast meat and fatty acid composition of breast meat of broilers. Material and methods: The experiment was performed in a completely randomized design with 3 treatments, 4 replicates, with 10 broiler chicks with the approximate weights of 45±2g in each pen having the dimension of 1.5×1.5m2 and a total number of 120 chicks during 42 days. The experimental treatments included 3 levels of L-carnitine (0, 200 and 400 mg/kg) in corn-soybean meal basal diet. The effects of experimental treatments were analyzed by SAS software and the comparison of the means with Duncan’s multiple-range test at 5% significance level. Body weight gain, feed intake, and feed conversion ratio (FCR) were calculated at the end of feeding phases to evaluate the traits. European Production Efficiency Factor (Aviagen 2018), carcass characteristics (Farrokhyan et al. 2014), blood parameters (Hosseinitabar et al. 2015), immune system (Seidavi et al. 2014), breast meat fatty acid (Folch et al. 1957) cecal microbial population, histology of the small intestine were calculated at the end of experiment (42 days). Results and discussion: In the finisher period of the experiment, the chickens fed by a diet containing 400 mg/kg L-carnitine significantly had lower feed intake and FCR as well as the highest weight gain compared to the control (p < 0.05) ). The reason for the increase in weight of the chickens fed by L-carnitine may be due to the effect of this substance on increasing the insulin-like growth factor-I and also the elevated access of chickens to the energy of feed (Kita et al. 2002). Rabie and Szilagyi (1998) reported that the effect of L-carnitine on improving FCR is related to improving nitrogen metabolism. Elevated levels of L-carnitine significantly increased the European Production Efficiency Factor (P