RESUMEN
Decline in semen quality is considered as a major contributing factor in age-related subfertility of broiler breeder flocks. This study was aimed to investigate the effect of dietary supplementation of Guanidinoacetic acid (GAA), as an alternative energy source along with antioxidant potential, on testicular histology and relative gene expression of some spermatogonial markers (c-Kit and STRA8) in aged roosters. Sixteen 24-week-old male broiler breeders were randomly allocated into four groups and fed a basal diet supplemented with increasing levels of GAA including 0 (GAA-0), 600 (GAA-600), 1200 (GAA-1200) or 1800 (GAA-1800) mg/kg diet/day for 26 successive weeks. At the end of the experiment, all the birds were killed and two ipsilateral testicle samples were taken to either quantify relative gene expression or do histology. Except for seminiferous tubules' diameter, testicular weight, and the number of blood vessels, dietary supplementation of GGA improved the epithelium thickness of seminiferous tubules, the number of spermatogonia and Leydig cells and the relative gene expression of c-Kit and STRA8 (Pâ¯<â¯0.01). Increasing levels of GAA cubically affected (Pâ¯<â¯0.01) the diameter of seminiferous tubules and their epithelium thickness as well as the number of spermatogonia. However, number of Leydig cells and relative expression of c-Kit were linearly, and relative expression of STRA8 was quadratically (Pâ¯<â¯0.01) enhanced in response to graded levels of GAA supplementation. Taking all parameters into account, daily supplementation of 1300-1450â¯mg of GAA/kg diet was estimated as an optimum dosage maximizing the evaluated traits.