The objective of the study was to evaluate the effects of spices on forage utilization and nitrogen (N) emission using in vitro and in vivo approach. A 6 × 5 factorial triplicate arrangement was used to assess the in vitro degradability of rice straw with control (without spices) and individual (40 mg/g rice straw) spices (cumin, coriander, clove, black cumin, turmeric) at five different incubation times. In vitro dry matter (DM) and organic matter (OM) degradability of rice straw were highest in presence of spices except for clove. Clove significantly reduced the total volatile fatty acids concentration, molar proportion of acetate and propionate ratio, but increased propionate production. Acetate and butyrate production were not affected by treatments. The ammonia-nitrogen concentration was lowest for clove and turmeric compared to other spices. Rumen pH was unchanged but gradually decreased over the incubation period. For in vivo study, 12 bucks with average live weight 7.65 ± 0.19 kg were assigned to a completely randomized design with three treatments and four replicates for a 28-day period. Bucks were fed a total mixed ration without (0 g/kg DM) or with (2.5, 7.5 g/kg DM) clove supplementation. DM intake, body weight and apparent total tract digestibility of crude protein (CP) and acid detergent fibre (ADF) were not affected by increasing dietary doses of clove but tended (p = .09) to increase DM and OM digestibility. The urinary N and urine urea N concentration of bucks decreased linearly with incremental doses of clove diet. In contrary, clove supplementation had positive effects on plasma urea N and retained N in bucks. We concluded that though some spices had positive effects on ruminal digestion and fermentation, the dosage level assessed in the current study (up to 7.5 g/kg DM clove) can be promoted as an effective dietary approach to mitigate N losses in bucks.