الفهرس 
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Abstract
The present work aimed to investigate the effects of increasing the fat level (soybean oil) in the broiler diet on their physiological parameters during environmental changes (hot and cold environmental temperature).The study was carried out on 120-1-day old Cobb chicks during winter and summer season. In winter season, 60 chicks were used and were divided into 3 groups, normal temperature-normal fat (NTNF)(CW) that was reared under optimum temperature, fed on 3% soybean oil from the 1st to the 42nd day of age and was kept as control, low temperature-low fat group (LTLF) that was reared under the natural season temperature from the 21st to the 42nd day of age and was fed on 3% soybean oil allover the rearing period and low temperature-high fat group (LTHF) that was reared under natural season temperature from the 21st to the 42nd day of age and was fed on 6% soybean oil during the last 3 weeks of age. In summer season, the same design was applied with the 3 groups, normal temperature-normal fat (NTNF)(CS), high temperature-low fat (HTLF) and high temperature-high fat (HTHF) group. Body weights and feed consumption of the chicks were recorded weekly and the body weight gains and feed conversion ratios were also calculated. Blood samples were collected from the wing vein of the broilers at the 28th, 35th and the 42nd days of age to do total red blood cells count, white blood cells count and blood smears for differential leucocytic count and heterophile:lymphocyte ratio determination. Serum samples were separated and stored frozen at 20oC for later analysis of total cholesterol, triacylglycerol, high density lipoproteins, low density lipoproteins, total lipids, total protein, albumin, globulin, alanine-aminotrasnferase, aspirate-aminotransferase, triiodothyronine, thyroxine and corticosterone. The result of this work revealed that: Usage of soybean oil in broiler ration at a level of 3% and increasing this level to 6% from 21st to 42nd days of age improved growth performance parameters in summer and not have a side effect on the other parameters also, its usage in winter improve high density lipoprotein cholesterol (HDL-c).