Manure management, particularly anaerobic digestion, is an alternative for reducing the environmental impacts of cattle raising and energy generation. The aim of this study was to produce biogas and biofertilizer from waste from Nellore cows fed sodium monensin, essential oils, exogenous enzymes, and their combinations. The experimental batch-type biodigesters were placed inside a climatic chamber (30–35 °C). They were organized in a completely randomized design in a 2 × 2 × 2 factorial arrangement, with waste tested from Nellore cows fed the presence or absence of essential oil, exogenous enzyme, and monensin, with four repetitions totaling 32 experimental units (represented by the manure of the animals that received the different additives and their associations). The use of monensin reduced the amount of N inserted into the biodigesters, reduced the nutrient removal efficiency, and reduced biogas production by 39.26%, in addition to altering the composition of the biofertilizer produced. The other tested additives together with the combination of additives did not influence the biodigestion process. Therefore, the addition of monensin reduces nutrient removal efficiency, compromises biogas production, and reduces the concentration of nutrients in the biofertilizer, while essential oils and enzymes do not affect the biodigestion process. In addition, no associative effect was observed among the tested additives
