Abstract: Background and Objective: The mathematical models would be of high benefit in rural areas of Eastern Cape province of South Africa that keep plenty of donkeys, cattle, horses and goats. The aim of this study was to investigate and establish the optimum mixing ratio for the co-digestion of cow dung, donkey dung, horse dung and goat dung and develop a mathematical model for the mixture producing optimum methane yield. Materials and Methods: The research was carried out using a 1 m3 field batch biogas digester. The substrates were analysed for total solids, volatile solids, chemical oxygen demand, ammonium nitrogen, total alkalinity, pH and calorific value before they were co-digested in different mixing ratios. A multiple regression model was developed for the methane yield as a function of pH, chemical oxygen demand, ammonium nitrogen, digester temperature and total alkalinity. Results: The pH for all the mixtures was within the expected range for optimum methane production and the alkalinity values were all high showing good buffering capacity. The highest methane content of 75% was obtained from the co-digestion of 25% cow dung and 25% donkey dung with 25% horse dung and 25% donkey dung as compared with other co-digestion trials because of balanced nutrients as well as buffering capacity that enhanced the anaerobic digestion process. Conclusion: All the batch digestion tests were performed within a retention period of 30 days. The beneficial effect of the co-digestion is mainly due to the optimisation of the nutrient balance in the co-mixtures. The developed mathematical model can be used to predict methane yields from different mixtures of cow dung, donkey dung, horse dung and goat dung.