Perspectives on peak liquid manure temperature with implications for methane emissions

Methane emissions from liquid manure storage are currently estimated with a methane conversion factor (MCF) based on manure temperature inputs or air temperatures as a substitute in the 2019 IPCC Tier 2 method. However, differences between peak manure temperature and peak air temperature (T_diff ) in warm seasons are likely to occur and result in poor estimates of MCF and methane emissions. To address this concern, this study aims to investigate the relationship between the T_diff and ratio of manure surface area to manure volume (R_s:v ) using a mechanistic model and by analyzing farm-scale measurement studies across Canada. Positive correlations between T_diff and R_s:v were found using a modeling approach and from farm-scale results (r = 0.55, p = 0.06). T_diff ranged from -2.2 to 2.6°C in farm-scale results mainly collected from eastern Canada. We suggest that manure volume and surface area, in addition to removal frequency, could be used to estimate T_diff and be part of the criteria for improving manure temperature estimates, which could lead to improved estimates of MCF.