This study aimed to evaluate in vitro and in vivo the nutritional value of the spent mushroom (Agaricus bisporus) compost (SMC) silage treated with different levels of molasses. For this purpose, the SMC sam-ples were treated with 0, 7.5, and 15% (dry matter basis) of molasses (SMC0, SMC7.5, and SMC15, respec-tively) and ensiled over 60 days. At the end of the ensiling period, chemical composition and fermentation characteristics (including pH, NH3, water-soluble carbohydrates (WSC) and lactic acid contents, buffering capacity (BC), and flieg point) of the silages were determined. Ruminal fermentation characteristics and total tract digestibility of the silages were determined in vitro (24-h and 144-h incubations) and in vivo (us-ing 18 male lambs), respectively. Molasses treatment improved SMC silage fermentation characteristics by lowering pH and BC (P<0.05). In 24-h incubations, the gas produced over 24 h of incubation (GP24), dry matter digestibility (DMD24), and total volatile fatty acids were higher in SMC7.5 and SMC15 than SMC0 (P<0.05). In 144-h incubations, asymptotic gas production (A), dry matter and organic matter digestibility (DMD144 and OMD144) increased, and lag time (L) decreased in the SMC7.5 and SMC15 (P<0.05). In vivo digestibility of nutrients (except neutral detergent fiber (NDF)) was improved non-linearly with molasses treatment (P<0.05). The NDF digestibility tended to increase linearly with molasses treatment (P=0.056). These results revealed that ensiling spent mushroom compost with 15% molasses significantly improves its nutritional value, making it a cost-effective by-product feedstuff that can be used in ruminant diets.
