This study was conducted to determine the effects of a by-product feed (BF) based silage diet on the performance, blood metabolite parameters, and carcass characteristics of Hanwoo heifers. The BF-based silage was composed of 50% spent mushroom substrate, 21% recycled poultry bedding, 15% cut ryegrass straw, 10.8% rice bran, 2% molasses, 0.6% bentonite, and 0.6% microbial additive (on a wet basis), and ensiled for over 5 d. Twelve heifers were allocated to three diets during the growing and fattening periods (6.2 and 8.8 months, respectively): a control diet (concentrate mix and free access to rice straw), a 50% BF-based silage diet (control diet + 50% of maximum BF-based silage intake), and a 100% BF-based silage diet (concentrate mix and ad libitum BF-based silage). The BF-based silage was fed during the growing and fattening periods and was replaced with rice straw during the finishing period. After 22.4 months, the resulting steers were slaughtered. Compared with the control diet, the 100% BF-based silage diet significantly increased the average daily gain and feed efficiency of the heifers by 27 and 18% respectively (P<0.05), due to an increase in voluntary feed intake. Compared with the control diet, the BF-based silage diet had little effect on levels of serum constituents, electrolytes, or enzymes or on blood cell profiles in growing heifers. The BF-based silage diet did not affect cold carcass weight, yield traits such as back fat thickness, longissimus muscle area, yield index, and yield grade, or quality traits such as meat colour, fat colour, texture, maturity, marbling score, and quality grade. In addition, good quality meat was more frequently obtained when the heifers were fed the BF-based silage diet (75% for the control group vs. 100% for the BF-based silage-fed groups). In conclusion, BF-based silage can be used as a good quality roughage source for Hanwoo heifers.