Rice whole stillage (RWS) is disposed to aerobic spoilage when stored for a prolonged period. The objectives of the present study were to evaluate the fermentation characteristics of ensiled RWS with wheat straw (WS) and wheat bran (WB) inoculated with Amylomyces rouxii, as well as to evaluate their impact on the metabolic profiles and the growth performance in Japanese black steers. Ensiled blends containing RWS, WS and WB were prepared either with A. rouxii inoculation (AS) or without (US). Fifteen silo bags were made for each blend; each bag had a width × height of 110 × 110 cm. The silos were analysed on Days 0, 7, 14, 28, 42 and 60 after ensiling. Notably, crude protein concentrations were found to be higher in the US treatment than in the AS treatment. In contrast, ash and acetic acid concentrations were elevated in the AS treatment compared with the US treatment. Lactic acid (LA) concentrations did not differ between the two treatments. Furthermore, the feeding efficacies of the prepared silages were evaluated as a feeding material for Japanese black steers. Control diets were composed of total mixed ration (TMR) and timothy hay, and the test diets were composed of TMR supplemented with either 1 kg/head.day US silage blend (USTMR) or 1 kg/head.day AS silage blend (ASTMR). Steers fed on the ASTMR diet exhibited a decrease in blood LA concentrations compared with the other groups. Moreover, growth performance revealed that daily gain and final bodyweight values were similar among treatments. Collectively, RWS can be effectively ensiled with or without A. rouxii and used as a feedstuff for maintaining an adequate growth rate of steers.