In the guideline for the determination of methane (CH4) emission from animal manure (IPCC) the amount of CH4 emitted is generally alculated according to an equation combining the amount of organicmatter (OM) or volatile solids excreted, the ultimate CH4 potential (B0) of excreta and a system-specificmethane conversion factor (MCF, %) that reflects the portion of B0 that is really converted into CH4. The objective of the present study was to investigate the effect of the modification of dietary crude proteinand fibre levels on B0 of pig slurry and on subsequent MCF according to different strategies of slurry management. Five experimental diets differing mainly in their crude protein and fibre content were compared. Two types of measurement of CH4 emission were performed. The first was the measurement of B0 of slurry using biomethanogene potential (BMP) test. The second consisted in a storage simulation, which was performed on different kinds of effluents: fresh slurry (FSl), stored slurry (SSl), and faeces mixed with water (FaW). The type of diet and the type of effluent affected (P < 0.001) CH4 production after 30, 50 and 100 days. Moreover, the interaction between type of effluent and type of diet was significant for CH4 emission and for MCF. CH4 production was the highest for BMP, the average production of CH4 during storage from FaW, FSl and SSl samples representing 77%, 58% and 64% of the B0 value. The dynamic of CH4 production during BMP tests was rather similar for all dietary treatments whereas it differed for storage simulation studies with significant effects of dietary CP and fibre contents. The results from this study indicate that the type of diet has a significant but rather limited effect on B0 value of effluent. The effect of diet is much more marked on MCF, with lower values for high protein diets, and higher values for high fibre diets. MCF is also affected by manure management, the values measured on separated faeces from urine being much higher than for slurry.