In this study, two experiments were conducted to investigate the growth performance and meat quality of broilers as affected by dietary soybean lecithin and xylanase enzyme supplementation with energy levels. In the 1st experiment, two feed additives were used (soybean lecithin and xylanase enzyme), each was supplemented at three levels being 0, 0.5 and 1g / kg diet for crude soybean lecithin and 0, 0.1 and 0.2g / kg diet for xylanase enzyme. All these additives were added to two levels of energy (recommended and -100 kcal lower than recommendation). A total number of 384 unsexed day old Cobb500 broiler chicks were assigned into 12 treatments; each one contained four replicates of 8 chicks per replicate. Chicks were wing banded, individually weighed to the nearest 1 gram and then randomly distributed into 12 groups. Supplementation was offered to experimental diets during starter (1-10 days), grower (11-22 days) and finisher (23-35 days) periods. All these sources and levels were used either with the recommended level of metabolizable energy (3001, 3001 and 3100 Kcal / kg diet during starter, grower and finisher periods, respectively) or -100 Kcal / kg diet lower than energy recommendation (2901, 2901 and 3001 Kcal / kg diet during starter, grower and finisher periods, respectively).
The 2nd experiment was designed according to the result of experiment 1, by applying the best level of each additive as a single dose or in combination, with two dietary energy levels (recommended and -150 kcal lower than recommendation). A total number of 256 unsexed one day old Cobb-500 broiler chicks, obtained from the same hatchery used in the previous experiment, were divided into 8 treatments; each contained 4 replicates of eight chicks per replicate. The chicks were wing banded, individually weighed to the nearest 1 gram then randomly distributed into 8 treatments. The best level of crude soy lecithin (0.5g / kg) and xylanase (0.2g / kg) were used as a single dose or in combination between them, with the recommended level of metabolizable energy (3001, 3001 and 3100 Kcal / kg diet during starter, grower and finisher periods, respectively). Or -150 Kcal / kg diet lower than energy recommendation being 2851, 2851 and 2951 Kcal / kg diet at starter, grower and finisher periods, respectively.
Experimental diets and water were offered ad-libitum all over the experimental periods. Chicks in all experimental treatments were kept under similar management, hygienic and environmental conditions. Birds of each experimental group were individually weighed to the nearest gram at the start of the experiment (one day) then weighted at a certain periods up to the end of the experimental period (35 days). Since, weight gain between two successive periods was individually calculated. Feed intake by all chicks was weekly recorded for each treatment. Feed conversion was then calculated as a ratio between feed intake (g) and weight gain (g). Mortality rate was estimated. At the end of the experimental period (35 days of bird's age), digestibility trails were carried out and a slaughter test was performed to determine carcass traits. Also, physical and chemical meat quality was performed. Some blood plasma parameters, asparatate aminotransferase (AST), alanine aminotransferase (ALT), total cholesterol, triglycerides, HDL and LDL were determined and economical efficiency was calculated.
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