This study was conducted to determine the effects of dietary guanidinoacetic acid (GAA) and betaine supplementation on performance, antioxidant status and biochemical parameters of broilers subjected to cold stress. Based on a 2 × 2 factorial arrangement, 384-day-old male broiler chicks (Cobb) were randomly distributed between four experimental diets (with 8 replicates and 12 birds per replicate) included basal diet (as control) and the basal diet supplemented with 1200 mg/kg GAA; 600 mg/kg betaine and 1200 mg/kg GAA + 600 mg/kg betaine. No significant dietary effects were seen on performance, haematological and blood biochemical parameters including plasma glucose, uric acid, total antioxidant status, lactate, lactate dehydrogenase, red blood cell, haemoglobin, haematocrit and heterophil-to-lymphocyte ratios. However, malondialdehyde (MDA), and the activity of creatine kinase (CK), superoxide dismutase (SOD) and glutathione peroxidase (GPx) were affected by the experimental diets. Compared with the other groups, betaine supplementation decreased liver MDA level and SOD activity while increased activity of liver GPx and serum CK and decreased serum level of MDA were observed in birds fed the GAA-included diet. Overall, based on the results, it seems that dietary GAA and betaine could have beneficial effects on antioxidant status of broilers subjected to cold stress. ARTICLE HISTORY

The objectives of the present study were (1) to determine the extent to which the crude protein (CP) content of laying hen diets can be reduced, based on performance criteria, and (2) to determine how egg quality traits and blood biochemical parameters are affected by changes in dietary CP under hot summer conditions. The requirements for egg mass (EM) and feed utilisation (FU) were estimated using quadratic equations and broken-line regression models. Laying hens were raised for a 12-week period (42–54 weeks) in wire cages under a high ambient temperature in an open-sided housing system. A total of 144 Lohmann LSL laying hens were assigned to four treatments with six replicates of six birds each. The four experimental diets (11.51 MJ ME/kg) varied according to four CP levels: normal-CP diet (Control, 16% CP) and low-CP diets containing 13%, 14%, or 15% CP. All diets were fortified with DL-methionine, L-lysine, L-threonine and L-tryptophan at levels sufficient to meet dietary requirements. The results showed that under high ambient temperature conditions, all productive performance traits of birds fed 15% and 14% CP diets were similar to those of birds fed a Control diet (16% CP), whereas feeding a 13% CP diet negatively affected (P < 0.05) EM and FU. Estimations of requirements were of 15.13% and 14.56% CP for EM, and 14.86% and 14.38% CP for FU using quadratic and broken-line models, respectively. Haugh units, blood uric acid levels and albumin levels were significantly lower in birds fed a 13% CP diet compared with the Control group. In contrast, yolk colour index and blood triglyceride level were higher (P < 0.05) for the hens fed the 14% and 13% CP diets than for birds fed the Control diet. Overall, hens consuming 14% and 15% CP diets performed equally as well as the high protein diet group under hot summer conditions.

The results indicated that the dietary supplementation with GAA and betaine could improve FCR under low ambient temperature.