Efficacy of Supplementary Guanidinoacetic Acid at Different Dietary Crude Protein Levels on the Productive Performance, Egg Quality, Intestinal Morphology, Antioxidative Status, and Serum Metabolites of Aged Laying Hens Exposed to High Ambient Temperatures

BACKGROUND: Prior research has established that high ambient temperature and diets with reduced protein content exert detrimental effects on egg production and quality and the overall health of poultry. Supplementation with guanidinoacetic acid (GAA) has demonstrated the capacity to mitigate the negative effects of heat stress on broiler chickens. Notably, the impact of GAA supplementation on late-laying hens subjected to cyclic high ambient temperatures remains largely unexplored. OBJECTIVES: This study was designed to evaluate the influence of dietary GAA supplementation on performance parameters, egg quality attributes, gastrointestinal morphology, antioxidant status, and some blood biochemical parameters in late-laying hens exposed to high ambient temperatures under varying dietary crude protein (CP) concentrations. METHODS: A total of 216 ninety-week-old laying hens were randomly allocated to four experimental groups, with each group further subdivided into six replicates of nine hens. The dietary treatments comprised: (1) a standard protein diet (15.5% CP, control), (2) the standard protein diet supplemented with 0.15% GAA (B-GAA), (3) a diet with a 1% reduction in CP supplemented with 0.15% GAA (RP1-GAA), and (4) a diet with a 2% reduction in CP supplemented with 0.15% GAA (RP2-GAA). The reduction in dietary protein levels was accompanied by a concomitant decrease in the concentration of essential amino acids. RESULTS: The findings revealed a significantly greater body weight gain (p ≤ 0.05) in hens fed the B-GAA diet compared to those receiving the basal or reduced protein diets. In contrast, dietary manipulation did not exert a significant effect on egg production and feed conversion ratio (p > 0.05). Hens in the B-GAA group exhibited enhanced feed intake, egg weight, and egg mass compared to those in the basal or RP2-GAA groups (p ≤ 0.05), while egg quality parameters remained statistically similar across all dietary treatments (p > 0.05). No significant differences were detected among the experimental groups with respect to jejunal villus height, width, surface area, crypt depth, and the villus height-to-crypt depth ratio (p > 0.05). Furthermore, feeding B-GAA and RP1-GAA diets resulted in significantly elevated serum glutathione peroxidase activity compared to the control and RP2-GAA groups (p ≤ 0.05). Moreover, the B-GAA group displayed significantly higher serum superoxide dismutase activity when compared to all other treatment groups (p ≤ 0.05). GAA supplementation led to an increase in serum total antioxidant capacity levels (p ≤ 0.05) and indicated a trend towards reduced serum malondialdehyde levels (p = 0.07). Serum levels of glucose, uric acid, albumin, calcium, and phosphorus were not significantly affected by the dietary treatments (p > 0.05), whereas dietary GAA supplementation significantly increased serum nitric oxide (NO) concentrations (p ≤ 0.05). Serum creatine level was significantly lower in hens fed the RP2-GAA diet compared to all other treatment groups, while the B-GAA group exhibited significantly higher serum total cholesterol and total protein levels (p ≤ 0.05). CONCLUSION: The comparable responses observed in the B-GAA and RP1-GAA groups suggest the potential to reduce dietary protein concentration by 1% in aging laying hens exposed to elevated ambient temperatures through supplementation with 0.15% GAA, without compromising key production performance indicators. This strategy may offer significant economic and environmental advantages for sustainable poultry production.