Ruminants turn human inedible into human edible products, but at a cost of excretion of various pollutants. Implications of dietary measures for cattle to reduce faecal and urinary nitrogen losses on methane emissions are reviewed. Reducing the dietary protein content decreases nitrogen excretion in faeces and urine, but the effect on methane production is less clear. Methane production may decline if starch or digestible nutrients escaping rumen fermentation replace protein, but will rise if dietary fibre levels increase. Hence, mitigation options aimed at reducing urinary nitrogen excretion may result in elevated methane emission levels, depending on fibre level. The trade-off between nitrogen excretion and enteric methane production needs to be understood at the animal scale to allow accurate data to be used at the whole farm level.