Methane production by ruminant animals is an important anthropogenic source of methane to the atmosphere and is considered an important source to more accurately estimate and to control. Large variations in methane production by ruminants have been reported. However very few of these data were obtained using contemporary diets used in the UK, ie grass silage based diets and were also limited to sheep fed at maintenance (M). This has limited the understanding of the dietary factors which may be used to predict methane production. Attempts to predict methane production from either diet chemical composition or digestibility have not been successful (Moss and Givens, 1990; Johnson et al., 1991). Accordingly, different concentrates were used to supplement grass silage at different rates (forage.concentrate, (F:C) ratios) at differing planes of nutrition to study the effects of diet composition on methane production.