The Effects of Arginine, Guanidinoacetic Acid and Citrulline Supplementation to Reduced Protein Diets for Aged Laying Hens

This study evaluated the effects of arginine (Arg), guanidinoacetic acid (GAA), and citrulline (Cit) supplementation in reduced protein (RP) diets on the production performance, serum uric acid level and bone quality of aged laying hens. A total of 208 Hy-Line Brown laying hens from 60 to 75 weeks of age were assigned to eight dietary treatments, with 13 replicate cages per treatment and two hens per cage. Experimental diets comprised two protein levels (standard protein at 15.8% crude protein versus RP at 13.8% crude protein), and six RP diets supplemented with two inclusion levels (0.06% and 0.12%) of Arg, GAA, or Cit. Overall, increasing Cit supplementation improved productive performance compared with Arg and GAA, particularly by increasing egg weight (p = 0.048), egg mass (p = 0.019), and feed intake (p = 0.014) over the entire experimental period. In addition, Cit increased egg weight during 60–67 weeks (p = 0.049) and improved egg mass during 68–75 weeks (p = 0.014). Furthermore, supplementation of 0.12% Cit in RP diets significantly increased feed intake compared with standard protein (SP) and RP diets from 60 to 67 weeks (p < 0.001; one-way ANOVA). Both Cit and Arg supplementation increased yolk weight compared to GAA (p = 0.018). Further, Arg supplementation to the RP diet at 0.06% significantly increased energy digestibility compared to the SP and RP diets (p = 0.037). Increasing Arg and GAA levels reduced bone breaking strength compared with their respective low levels, whereas increasing Cit level enhanced tibia (p = 0.014) and femur (p = 0.006) bone breaking strength. Low-level GAA supplementation also increased tibia breaking strength. Thus, a moderate reduction in dietary protein level by two percentage points, combined with Cit supplementation at the high level (0.12%), was effective in enhancing egg mass, egg weight, yolk weight, and bone breaking strength in aged laying hens. These findings suggest that RP feeding strategies supplemented with functional Arg precursors may support productive efficiency, skeletal health, and nutrient utilization in late-lay hens.