| Two experiments were conducted in the present study to investigate the effects of high temperature and the level of vitamin A on the development of the intestine, biochemical characters of plasma, immune response and growth performance of the broilers.In Exp. 1, 60 AA broilers with individual live weigh no more than 5 grams were randomly assigned to each of the four treatments. Two groups of the birds were raised under the proper temperature given by the guide-book of "The AA Broiler Feeding" , the other two groups were raised under the temperature which was 4癈 higher than the proper. For each temperature groups, no vitamin A or loOOIU/kg recommended by XRC in 1994 was supplemented. At the age of 21, blood samples from six birds of each group were taken from the wing vein to measure the activity of AST and ALT. Then liver, thymus, bursa, spleen, and proximal duodenal tissue samples were taken from these birds to examine the ultra-structure of liver, SOD activity ad MOD content in immune organs, and histological aspects of small intestine.Results of Exp. 1 indicated that there were significant difference of AST and ALT activity (p<0. 05), pathological changes of liver (p<0.05), SOD activity (p<0.05), MDA content in the immune organs (p<0.'05), histological aspects of small intestine (p<0.05) between the broilers raised under different temperature. The supplementation of vitamin A benefits the broilers by raising their resistance to the damage of the oxidation, depressing the pathologic changes of the liver and offsetting the hightemperature' s damage to the intestine.In Exp. 2, 30 AA broilers with a difference of individual live wight of no more than than 5 grams were randomly assigned to each of the eight treatments. There were two duplicates in each treatment. All the birds were raised under the proper temperature in the first 7 days. 4 treatments were exposed to 6 hours high temperature stress everyday from the 8lh day. The stress temperature was 4癈 higher than the proper temperature. The levels of the vitamin A were 3000,6000, 12000,24000IU/kg respectvely in the 8 treatments.The results of the Exp. 2 indicated that the level of the vitamin A has a significant effect on body weight and feed intake at the day of 14, on body weight at the day of 21, and on the villu height of the duodenum (p<0. 05), and it also has a significant effect on AST and ALT activity (p<0.05), BUN and UA concentration (p<0. 05) at 7, Hand 21 day after hatching. The body weight, some intestine ratio, intestine villu height, and ALT activity in the serum are all significantly different between two temperature groups (p<0. 05). Vitamin A has the significant influence on the titer of the .VDV antibody at the day of 21 (p<0. 05).Our research demonstrates that under the high temperature stress, the extent of the lipid per-oxidation in the poultry' s immunity organs is aggravated and the function of the intestine is reduced and the liver shows some pathological changes, which can be intensified by vitamin A deficiency. The supplementation of vitamin A has beneficial effects on the broilers as it can reduce the poultry' s high temperature stress, enhance the immunity organs' antioxidant ability and maintain the integrity of the intestine' s tissue and function. The proper high level of vitamin A will promote the broiler' s growth performance and intestine development. Make allowance for the intestine development, growth performance, the immunity function, and no adverse influence to the function of the liver and the kidney, we conclude that the optimal level of the vitamin A should be 12000IU kg.
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