Objective: To evaluate the global warming potential index (GWPI) and in vitro gas production (GP) of fattening diets in lambs fed with silage of agricultural by-products of nopal cladodes and prickly pear (Opuntia ficus-indica L.) - hibiscus grain (Hibiscus sabdariffa L.) - oats straw (Avena sativa L.). Design/methodology/approach: The GP technique was used to obtain the GWPI of isoproteic (crude protein (CP)) and isoenergetic diets (15% CP and 2.8 Mcal ME (metabolizable energy)) without silage (DWS; control), with corn silage (CSD) and with 10 or 20% of nopal-prickly pear-hibiscus grain-oat straw silage (DEN10, DEN20), fed for 60 days to 24 Creole fattening sheep. Results: In vitro dry matter digestibility at 72 h (DIVMS72) was better in CSD, but similar for DEN10, DEN20, and CSD. DEN10 and DEN20 had the lowest CH4 production, GWPI, and environmental impact index (EII). The low fermentable fraction (LF; GP24-72 h) was related to DIVMS72. Findings/conclusions: The cactus pear-hibiscus grain silage inclusion (DEN10, DEN20) in conventional diets had no effect on DIVMS72, but decreased CH4 emissions and the GWPI.