Responses of broiler chicken to different oil levels within constant energy levels from 20 to 40 days of age under hot weather conditions

In total, 144 Arbour Acres broiler chickens were distributed among four treatment groups (six replicates per treatment; six chickens per replicate) during days 20-40 of age. The chickens were offered iso-caloric and iso-nitrogenous diets containing four dietary oil levels (DOL): 0 (oil non-supplemented diet, control), 2, 4, and 6% in a relatively low-energy diet (12.4 MJ ME/kg diet). During the experimental period, the chickens were reared under natural hot weather conditions (32.5 +/- 4 degrees C, 54 +/- 7% relative humidity). Growth, feed conversion ratio (FCR), protein (PCR), metabolisable energy (MECR) ratio, and European production efficiency index (EPEI) were similar among groups fed up to 4% DOL but raising DOL to 6% impaired these traits. Besides, DOL at 6% decreased digestibility of dry matter, crude protein, and ash. Furthermore, a 6% DOL showed the lowest digestibility of dry matter, crude protein, and ash. Dressing percentage was the highest in 2%, and abdominal fat percentage showed the same trend in 6% DOL, but the gizzard percentage was the lowest in 6% DOL. The liver percentage increased significantly with fat/oil inclusion compared to the control. Meat dry matter and either extract increased considerably due to offering different DOLs, with maximum values at 6%. The inclusion of dietary oils in diets significantly increased serum malondialdehyde (MDA) but decreased serum total antioxidant capacity (TAC)/MDA ratio compared to the 0% DOL. In conclusion, under natural summer conditions, from 20 to 40 days of age, broilers' best productive characteristics were achieved using 0-2% DOL, and the best immune response was obtained for 4-6% DOL.