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Methane is the most abundant organic chemical in the earth's atmosphere, and its abundance is increasing with time and has reached levels not seen in recent geological history. Methane is produced both naturally and anthropogenically. One of teh sources of anthropogenic methane is manure from domesticated animals. A methodology has previously been developed to estimate the amount of methane generated from this source. This was done by estimating the methane conversion factor (MCF) typically achieved by various waste management systems. The present study was conducted to evaluate these MCF assumptions using dairy manure as the representative livestock manure. The MCFs for the most dominant of disposal methods, rangeland/pasture disposal, were much lower than the earlier estimates. Other waste management systems, such as solid storage and liquid slurry storage had much higher MCFs, at 20 and 30°C. However, these waste management methods are more prevalent in parts of the world where the average annual temperature is closer to 10°C. At that temperature, the MCF is negligible in all waste management systems. This study showed that the previously reported estimates of MCF for some waste management systems were higher than was actually the case. Consequently earlier estimates of the amount of methane generated globally from manures were higher than those found in this study.