Anaerobic digestion (AD) is a technology that allows waste management. For this process to be carried out correctly, an appropriate inoculum is required to degrade organic matter to produce biogas and digestate. However, in rural areas of developing countries, an inoculum from an anaerobic treatment plant is not available. In addition, various agricultural and agro-industrial wastes are generated in these areas, but there are also low-cost digesters that operate mainly with cattle manure. Therefore, this study aimed to determine the energy potential of agro-industrial waste using cattle slurry as inoculum in the AD. The substrates used were sugarcane residues, cheese whey, and slaughterhouse wastewater. Biochemical methane potential (BMP) tests were conducted in 500 mL glass flasks at 35 ± 2°C, ISR of 2, and volume displacement measured methane production. Organic matter removal (based on volatile solids) and buffer capacity (VFA/TA ratio) were determined. Sugarcane residues showed a BMP ranging between 0.21–0.28 Nm3CH4/kgVSad, cheese whey between 0.42–0.60 Nm3CH4/kgVSad, and slaughterhouse wastewater 0.15–0.50 Nm3CH4/kgVSad. For the three cases, buffer capacity was <0.35 mg eq acetic acid/mg CaCOO3, evidencing low inhibition risks and removing greater than 70% of organic matter. Cattle slurry was suitable as an inoculum for the AD of agro-industrial and agricultural residues, which could boost the bioeconomy of rural areas in developing countries.