l-carnitine (LC) has been widely studied as a natural antioxidant molecule for treating low-fertility gametes. However, the molecular mechanism of the effect of LC supplementation on the testes of aging cocks has not been evaluated. Therefore, the objective of this study was to reveal the mechanism of testicular oxidative defense induced by LC supplementation in relation to the semen quality factor (SQF) in the aging cock through a transcriptome study conducted from a new perspective. A total of 24 Jinghong cocks at 55 wk of age were randomly divided into 2 groups (n = 12). All cocks were fed a standard isocaloric and isonitrogenic breeder diet (control, LC-0), and the other group was supplemented with LC at 150 mg/kg/d (treated, LC-150) for 12 wk. Remarkably, seminal characteristics and enzymes, sex hormones, and cock fertility related to testicular oxidative defense were considerably improved by LC …