A new Canadian-led study, testing the math used to estimate the greenhouse gas (GHG) output from belching, farting dairy cows, shows the current models either overshoot or undershoot the mark.
The equations used to predict cows’ methane emissions are inaccurate and need improvement soon to help dairy farmers adjust their animals’ GHG releases, according to the study co-authored by scientists from the University of Guelph, the University of Manitoba and Wageningen University in the Netherlands.
New models are needed soon, as delegates meet in Mexico this month to discuss a successor to the Kyoto Accord on climate change and “scientists and policy-makers continue to debate the causes of climate change, including the level of emissions from livestock and fossil fuels,” the researchers said in a Guelph release Thursday.
The study, published this month in the journal Global Change Biology, looked at data from previous work in Canada, the Netherlands and the U.K. to see how well widely-used equations have predicted methane production.
Then the team compiled a dataset of “actual observations” on methane emissions of dairy cattle, based mostly from respiration chamber experiments, in which methane produced in the gut of the cow is accurately determined.
These observations were then used to evaluate the “predictive power” of equations to estimate methane production.
And nine of the equations used in whole-farm greenhouse gas models either over- or underestimate cows’ methane emissions, the new study found.
The whole-farm models used to estimate the effect of on-farm management changes such as manure and crop management, breeding and basic nutrition don’t account for effects of dietary changes, said the study’s lead author, Guelph Ph.D. student Jennifer Ellis.
In fact, the study found, most of the equations now used don’t involve any dietary data at all, but estimate methane production based on feed intake or milk output.
Feed versus fat
For example, the authors said, the “widely used” IPCC (Intergovernmental Panel on Climate Change) equation, which predicts methane production based on energy intake of the cow, can’t distinguish the effect of a higher energy intake on methane due to a rise in feed intake level, from that due to a rise in dietary fat content at the same feed intake level.
“A higher feed intake will increase methane production. A rise in dietary fat content will decrease methane production,” said Ellis, now doing her post-doctorate study at Wageningen.
“There is a lot of concern right now about the impact of farming and human life in general on the environment. The prediction accuracy of these equations is small, and the equations are not suitable to quantify methane production of cows.”
“The predictive power of methane equations will have to be markedly improved if such whole farm models are used for sound decisions by governments to reduce environmental impact of dairying,” said Jan Dijkstra, a senior researcher at Wageningen and adjunct professor at Guelph, in Wageningen’s separate release.
Such low prediction accuracy and poor prediction of variation in observed values may introduce “substantial error” into inventories of GHG emissions and lead to incorrect mitigation recommendations, the authors said.
The new study’s researchers are now working on more “detailed and accurate” models to predict methane production, based instead on the fermentation going on in cows’ gastrointestinal tracts.
Livestock worldwide are now estimated by the United Nations’ Food and Agriculture Organization (FAO) to be responsible for about 18 per cent of all GHG emissions. From dairy farms, the FAO estimates, about 52 per cent of emissions are in the form of methane.
Given that methane is 25 times more potent than carbon dioxide as a GHG, accurate estimates of total GHG emissions in whole-farm models largely depend on the accuracy of the prediction of methane emitted per cow, the new study’s authors note.