عنوان مقاله [English]
Introduction Wheat is one of the important ingredients for preparing poultry diets. However, wheat contains some anti-nutritional factors such as non-starch polysaccharides (NSP) which enhance digesta viscosity and thus decrease nutrients digestion and absorption. These negative effects could damage performance and feed efficiency. Increase in digesta viscosity also imposes negative effects regarding intestinal epithelium. Vitamin A (vitA) is an important nutrient in maintaining the health of intestinal epithelium. Since wheat is deficient in vitA content and also wheat-based diets (WBD) can cause less vitA absorption from the intestinal lumen (by disturbing lipid digestion and absorption), it seems that supplementation of the WBD with vitA would be effective in mitigating some of the anti-nutritional effects of the WBD.
Materials and Methods In this study, the effect of a WBD supplemented with different levels of vitA on performance and immune system of broiler chickens were investigated. Five dietary treatments were prepared as follow: 1) WBD supplemented with 1500 IU vitA/kg (control); 2) WBD supplemented with 4500 IU vitA/kg; 3) WBD supplemented with 7500 IU vitA/kg; 4) WBD supplemented with 10500 IU vitA/kg; and 5) WBD supplemented with 13500 IU vitA/kg. Two hundred Ross 308 male day-old broiler chicks in a completely randomized design with four replicates of ten chicks per each were used. Feed intake (FI), body weight gain (BWG), and feed to gain ratio (FCR) were recorded periodically during the experiment. Sheep red blood cells (SRBC) antigen (2%) was injected to thigh muscle of two birds from each replicate on day 28. Seven and 14 days after injection, blood samples were collected and antibody titers against SRBC were measured. In order to evaluate cutaneous basophil hypersensitivity (CBH) response, on day 41, eight chicks per treatment were randomly selected and phytohemagglutinin (PHA-P) was injected (100 µg) subcutaneously into toes web of each bird. 12 and 24 hours after injection, the thickness of the web was measured and CBH response was calculated. At the end of the experimental period (day 42), two chickens from each replicate were selected randomly and euthanized. Abdominal cavity was opened and digestive organs as well as the spleen, bursa of Fabricius, liver, and abdominal fat pad were excised and weighted. The small intestinal parts including duodenum, jejunum, and ileum were excised, weighted, and measured in length. Afterward, the whole carcass and carcass parts were weighted. The weight of each organ was then expressed compared to the live body weight. For the viscosity measurement, jejunal and ileal contents were collected and centrifuged (500 ×g, 15 min). The supernatants were then used for the viscosity measurement. Serum malondialdehyde (MDA) was measured as an indicator of lipid peroxidation. For measurement of total tract apparent digestibility (TTAD) of nutrients, chromium oxide (Cr2O3) was used in diets as an external marker. On day 21, samples of excreta were collected every 6 hour (four samples during 24 hours) and pooled. Feed and excreta samples were then analyzed for crude protein (CP) and ether extract (EE). Chromic oxide concentration in feed and excreta samples was measured and TTAD of CP and EE were then calculated. The collected data were analyzed using the general linear model (GLM) procedure of the SAS software.
Results and Discussion In starter period, supplementing the WBD with 10500 or 13500 IU vitA/kg, significantly reduced FI compared to groups consuming 1500 or 4500 IU vitA/kg of diet. This could be attributed to vitA reservoir in yolk sac at the life beginning which increases the probability of vitA toxicity with higher levels of vitA in early growth period. No significant difference was found for FI among treatment groups during grower period (22-42 days of age) as well as entire period (1-42 days of age) of the experiment. VitA at different levels used in this study, had no significant effect on BWG and FCR during starter, grower, and the entire period of the experiment. The highest production index was seen in the group receiving the diet supplemented with 7500 IU vitA/kg. Furthermore, the lowest relative feed cost per kg of live weight or carcass weight were seen in groups receiving diet supplemented with 7500 and 4500 IU vitA/kg, respectively. Feeding WBD containing 13500 IU vitA/kg caused a significant increase in anti-SRBC titer on 7 days after injection, compared to control group. According to this result, it seems that high level of dietary vitA (13500 IU/kg) would be effective in stimulating antibody-mediated immunity in broiler chickens. Improvement in humoral immune responses of broiler chickens by supplemental vitA has been reported in other studies. No significant difference was seen regarding CBH response on 12 and 24 hours after PHA injection, among treatment groups. There was no significant difference among treatment groups as for spleen and bursa of Fabricius weights. VitA at levels of 4500 and 7500 IU/kg of diet, caused a significant decrease in serum MDA concentration. Decrease in serum MDA concentration by increasing dietary vitA level indicates anti-oxidative effects of this vitamin. Supplementation of the WBD with vitA at levels used in this study had no significant effect on the length and weight of duodenum and jejunum. However, vitA at 4500 and 7500 IU/kg of diet, decreased ileum relative weight. Digesta viscosity in the jejunum was decreased by dietary vitA at levels higher than 7500 IU/kg. In addition, abdominal fat pad weight was decreased by supplemental vitA at 4500 IU/kg of diet, comparing to the control. Supplementing the WBD with different levels of vitA had no significant effect on the TTAD of CP and EE.
Conclusion The results of this study showed that supplementation of the WBD with vitA at 7500 IU/kg, would not affect performance but could stimulate humoral immune response (antibody production) and anti-oxidative defense of the broiler chickens. Also, vitA supplementation at the level of 7500 IU/kg of the diet improved production index and decreased feed cost per kg of live weight. Furthermore, it has been evidenced that higher levels of supplemental vitA (more than 1500 IU/kg as the requirements) could be effective in decreasing abdominal fat pad in broiler chickens.