Methane (CH4) emissions and nitrogen (N) excretion in ruminant production systems are major contributors to atmospheric greenhouse gas accumulation and groundwater pollution. This project aimed to evaluate the effects of dietary (fresh grass, grass silage, pelleted grass and concentrates) and animal (breed and sex) factors on CH4 emissions and N utilisation and to develop prediction models for CH4 emissions and N excretion in sheep. During each measurement period, sheep were fed ad libitum once daily in the morning and were housed in individual pens for 14 d before being transferred to individual respiration chambers for 4 d with feed intake, faeces and urine outputs and CH4 emissions measured. Data were analysed using ANOVA to evaluate the effects of diet and animal factors on feed intake, nutrient digestibility, CH4 emissions and N utilisation. Linear and multiple regression equations for CH4 emissions and N excretion were also developed. Feeding pelleted grass, rather than fresh grass or grass silage, reduced CH4 emissions per kg dry matter intake and shifted N excretion from urine to faeces. Feeding concentrate supplementation by 0.5 kg/d with fresh grass had no effect on CH4 emissions per kg feed intake and N retention. Dry matter intake and N intake were the best single predictors for CH4 emissions and N excretion, respectively. However, adding grass nutrient concentrations as supporting factors improved prediction accuracy. Models based on farm level data (animal liveweight and grass chemical composition) showed satisfactory accuracy for use in practice. Increasing feeding level and grass metabolisable energy and water soluble carbohydrate contents, and decreasing grass N content, could optimise the mitigation of both CH4 emissions and N excretion in grazing sheep. The equations provide an approach to quantify CH4 emissions and N excretion and consequently to develop mitigation strategies to reduce the environmental footprint in sheep production systems.