Measured versus modeled methane emissions from separated liquid dairy manure show large model underestimates

Liquid manure in storage is a significant and poorly quantified source of methane (CH4). A micrometeorological technique was used to measure CH4 emissions from stored liquid manure on a dairy farm with approximately 146 milking cows and a screw-press solid-liquid separator. Higher than expected CH4 emissions were observed from the liquid manure fraction in the concrete storage tank likely due the high biodegradability of the liquid manure fraction when a long retention time coincided with high temperature. Almost all of the annual CH4 emissions occurred when both manure volume and temperature were high during the five-month period from July to November. In Year 1, 85% of annual emissions occurred from July to September, with 53% occurring in August. In Year 2, 93% of annual emissions occurred from July to November, with 30% occurring in October and November (combined). Removing manure in early fall reduced CH4 emissions substantially: we observed 21% lower annual emissions when manure was removed in early fall (33 Mg CH4 in Year 1: September removal) compared to removal in late fall (42 Mg CH4 in Year 2: end of November). Comparisons between measured and modeled CH4 emissions showed that both the IPCC methane conversion factor (0.17) for cool climates (10 °C or less), and the USEPA model, underestimated annual emissions by up to 60%. We calculated CH4 conversion factors ranging from 0.58 to 0.89 using the measured VS loading rate both with and without VS in the sludge that remained after emptying. Our results suggest that farmers can substantially reduce CH4 emissions by removing manure in late summer and early fall.