Abstract: In this study, two different rice (Oryza sativa) varieties, Liangyoupeijiu (late maturing indica) and Nanjing 44 (early maturing japonica), were used to investigate the mechanisms of gibberellic acid (GA) regulation of the forage value of rice straw during late growth. GA was applied 15 days after anthesis at a rate of 45 g/ha. The endogenous plant hormones [such as GA, 3-indole acetic acid (IAA), and abscisic acid (ABA)], superoxide dismutase (SOD) and peroxidase (POD) activities, and malondialdehyde (MDA) content in the first top leaf were assessed on the 3rd d and 14th d after GA application. Forage quality and fermentation quality of rice straw were also assessed after rice harvest. The results indicated that the forage value Liangyoupeijiu and Nanjing 44 were significantly improved by GA application as a result of reduced senescence. However, there were significant cultivar differences; the green leaf area per tiller of Liangyoupeijiu was significantly higher than that of Nanjing 44 (P<0.01); nonstructural carbohydrates (NSC) and crude protein (CP) content in the straw of Liangyoupeijiu were significantly higher than those of Nanjing 44 (P<0.05). The MDA content of GA treated Liangyoupeijiu was significantly lower than that of the control (P<0.05), while the SOD and POD activities of GA treated plants were significantly higher than those of the control (P<0.05). The GA content and the GA∶ABA ratio of GA treated plants of Liangyoupeiju were significantly higher than those of the control (P<0.05) and Nanjing 44. It was concluded that application of GA during late growth could significantly improve the forage quality of rice straw. The effects of GA were greater in Liangyoupeijiu than in Nanjing 44.