It was hypothesized that dietary guanidinoacetic acid (GAA), the precursor of creatine (Cr), would be beneficial to heat-stressed finisher broilers owing to improved cellular energy status and arginine sparing effects. A total of 720 one-day-old male Ross 308 broilers were allocated to 3 treatments, 0 (control), 0.6, or 1.2 g/kg of GAA added to complete corn–soybean meal diets, and were fed for 39 D, with 12 replicates (20 birds each) per treatment. A chronic cyclic heat stress model (at a temperature of 34C and 50 to 60% relative humidity for 7 h daily) was applied in the finisher phase (day 25–39). Samples were taken on day 26 and 39 to determine thrombocyte, white blood cell, corticosterone, protein and amino acid levels in blood and Cr, phosphocreatine (PCr), and adenosine triphosphate levels in the breast muscle. Meat quality was assessed on day 40 after overnight fasting. Guanidinoacetic acid at a dose of 1.2 g/kg decreased feed-to-gain ratio compared with the control in the grower phase (1.32 vs. 1.35, respectively; P ,0.05). In the finisher period, the supplementation of 1.2 g/kg of GAA reduced feed intake compared with the control (–3.3%, P ,0.05), whereas both GAA supplementation levels improved feed efficiency markedly (1.76, 1.66, and 1.67 for 0 [control], 0.6, and 1.2 g/kg of GAA, respectively, P ,0.05). Mortality outcomes highlight that GAA feeding improved survival during heat stress, supported by lower panting frequency (linear effect, P ,0.05). Plasma arginine was higher with increase in dietary GAA concentration on day 26 (118.3 and 1 30.8% for 0.6 and 1.2 g/kg of GAA, respectively; P ,0.05). This suggests enhanced availability of arginine for other metabolic purposes than de novo GAA formation. In the breast muscle, PCr (day 39, P ,0.05), free Cr (day 39, P ,0.05), total Cr (both days, P ,0.05), and PCr-to-adenosine triphosphate ratio (day 39, P ,0.05) levels were increased with higher GAA content in diet. Guanidinoacetic acid supplementation improved feed conversion and survival during chronic cyclic heat stress, which may be associated with enhanced breast muscle energy status and arginine sparing effect.

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