IntroductionIt is well known that prices of poultry meat are lower than those of other meats and this created a high demand for poultry meat in many countries around the world within the last few years. Many factors affect costs of poultry production, such as feed price, feed fed amount, crude protein content of feed, and the biological value of nutrients. Because of the limited resources of feedstuffs in developing countries and increasing demand for soybean meal (SBM) as a main source for crude protein, feeding costs have tended to increase (1). The use of untraditional feedstuffs in poultry diets has received great attention in developing countries. Sunflower meal (SFM) is defined as a by-product of the sunflower oil industry (2). The chemical composition of sunflower meal mainly depends on some factors like the seed, the method of processing, and the degree of decortication. SFM contains about 33%-37% crude protein and 18%-23% fiber contents. The meal is a mixture of hulls and kernel in a ratio about 40%/60% as described by Lević et al. (3). The major challenge of the inclusion of SFM in diets of broilers is its high content of fibers (4), which has negative impacts on growth and carcass traits. To solve this problem, certain synthetic enzymes like phytase and β-glucanase could be added to broiler diets having SFM to help in the digestion of fibers and to decrease their negative effect on growth parameters of broiler chickens. Lipiec (5) stated that SFM can be successfully used in the nutrition of monogastric animals at levels of 50-150 g/kg diet. Alagawany et al. (6) reported that there are many factors encouraging the use of SFM in poultry nutrition, like its cheap price compared to SBM and being free from antinutritional factors and toxic molecules. The aforementioned authors reported that SFM as an alternative feedstuff could be utilized profitably at levels of 200 g/kg of broiler diets without any harmful effects on growth and carcass yield. Using enzymes could be a practical way to enhance performance and improve utilization of higher levels of agroindustry Abstract: The present study aimed to investigate the substituting effect of sunflower meal (SFM) for soybean meal (SBM) with or without supplemental Avizyme on growth performance, protein and energy efficiency ratios, carcass traits, and nutrient digestibility of broiler chickens. A 4 × 2 factorial design experiment was conducted with four levels of SFM replacing SBM (0%, 25%, 50%, 75%) and two levels of supplemented Avizyme (0 or 0.1 g/kg diet). Results revealed that increasing SFM substitution for SBM up to 50% was associated with an increase in body weight (P < 0.01). Daily weight gain was statistically (P < 0.01) enhanced with increasing SFM up to 50% through 22 to 42 and 7 to 42 days of age. Feed conversion ratio improved (P < 0.01) in groups given SFM at 25% and 50% compared to the control. No carcass values were affected by SFM inclusion or Avizyme. The inclusion of 25% or 50% SFM increased (P < 0.01) protein efficiency at periods of 7-21 an...