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.
