الفهرس 
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Abstract
This study was carried out at a private farm in Shanawan, Minuofiya Governorate, Egypt, throughout the period from March to April 2010. The objective of this investigation was to study the adverse effects of heat stress on performance, dressing, some internal organs percent and some plasma blood constituents of broiler chicks, and to alleviate these adverse effects by supplementation of Vit, C, S.B, and their mixture. The results can be summarized as follows: a- Effect of heat stress: 1- Body weights and body weight gains of broilers exposed to heat stress fed diets with no additive supplementations were significantly decreased as compared to birds reared under normal conditions. 2- Heat stress caused a significant decrease in feed consumption and significant deteriorations in feed conversion. 3- The control group exposed to heat stress recorded the highest mortality rate. 4- Heat stress caused a significantly decrease in dressing, liver, gizzard and giblets %. 5-There was a significant increase in plasma glucose of control group exposed to heat stress. 6- Total protein, albumin, and globulin in blood plasma of heat-stressed broiler chicks were significantly lower than that of the control group. b- Effect of additive supplementations (Vit. C, S.B. or their mixture) : 1- The mixture of Vit. C and S.B treatments alleviated the detrimental effects of heat stress on body weight (2229.2 vs. 1769.8g) and body weight gain (1536 vs. 1083.6 g). 2- Feed consumption was increased by dietary supplementation under heat stress, compared to control + H treatment; being 3218.9, 3340 and 3380 g vs. 2662.9 g at 6 wks of age. 3- Supplementation with Vit. C, S.B and their mixture overcame the deterioration effect of heat stress (2.24, 2.29 and 2.20 in comparison with 2.24 ), respectively at 6 weeks of age. 4- The combination of Vit. C and S.B treatment recorded the lowest mortality rate being ; 2.5% at the end of the experimental period compared to 15.0 % at the control + H- treatment. 5-There are highly significant beneficial effects of Vit.C, S.B and their mixture on dressing, liver, gizzard and giblets percentages. 6- Vitamin C or S.B supplementation had beneficial effects on plasma total protein, albumin and globulin. 7- Supplementing diets with Vit. C, S.B or their mixture caused a significant decrease in plasma T3 and T4 concentrations.