The study investigated the effect of low-crude protein (CP), essential aminoacids (EAAs) and conjugated linoleic acid (CLA) on broiler growth performance. From 1 to 6 days of age, 196 male broiler chicks fed the same basal diet containing 22.5% of crude protein. From 7-28 days the chicks were assigned to 7 treatments (4 replicates, 7 birds/replicate). The control group fed a 21.5% CP diet, supplemented with DL-methionine (Met) and L-lysine (Lys). The low-CP diet (19% CP) was supplemented with Met þ Lys (2EAAs group) or with Met, Lys, L-arginine, L-tryptophan and L-threonine (5EAAs group); 2EAAs and 5EAAs groups were also supplemented with 0, 2 or 4% CLA. The body weight gain (BWG) was the biggest (p < .01) in the control group; 2EAAs and 2EAAs 4% CLA groups had lower BWG than the three groups fed 5EAAs diets. Diet 5EAAs þ2% CLA induced lower (p < .01) feed conversion ratio (FCR) than 5EAAs þ4% CLA and also showed the lowest (p < .01) protein intake. Reducing CP in broiler diet increased (p < .01) the digestibility of CP. The total protein content of plasma was greater (p < .01) in 2EAAs þ4%CLA and 5EAAs þ4%CLA than in the control. Decreasing the protein content of the diet from 21.5 to 19.0% penalised the growth performance of broilers from 7 to 28 days of age and the administration of CLA at 2 or 4% was not able to support growth. However, addition of 5EAAs without or with 2 or 4% of CLA resulted in recovery of FCR and PCR.ARTICLE HISTORY

Broiler males (n= 140) were used in a straight-run experimental design and distributed randomly among four treatment groups with seven replicates per treatment and five broilers per replicate. During 21-42 d old, the chickens were fed iso-caloric and iso-nitrogenous diets containing four levels of dietary vegetable oils (DVO), of 2.7, 4, 6 and 8%. During d 25-27, 31-33, and 38-40 of age, broilers were exposed to heat stress for 4 h a day (1000-1400 h) at 34 ºC, 70-75 % relative humidity. Feeding an 8% DVO diet significantly increased body weight gain compared to the other DVO levels. The feed conversion ratio, protein conversion ratio, metabolizable energy conversion ratio and European production index were Revista Mexicana de Ciencias Pecuarias Volumen 9 Número 2 2018 221 significantly enhanced due to feeding an 8% DVO diet compared to a diet containing 6% DVO. Feeding 8% DVO significantly increased the meat protein and lipid percentages, compared to the control group (2.7 % DVO), but decreased the plasma low-density lipoprotein, very-low-density lipoprotein and lymphocytes. Feeding 8% DVO significantly increased the mean cell volume and mean cell hemoglobin, and bursa weight and percentage compared to the control. In addition, 6 and 8% DVO significantly increased the plasma total antioxidant capacity compared to the control group, but decreased the malondialdehyde. Thus, broilers fed a diet containing 8% DVO have an increased tolerance to heat stress, as evidenced by increasing the productive performance, meat quality, blood hematological and biochemical traits, antioxidants and immunity.