Meta-analysis of calorimeter data to establish relationships between methane and carbon dioxide emissions or oxygen consumption for dairy cattle

Recent developments suggest the use of other gases such as carbon dioxide (CO(2)) to estimate methane (CH(4)) emissions from livestock, yet little information is available on the relationship between these two gases for a wide range of animals. A large respiration calorimeter dataset with dairy cattle (n = 987 from 30 experiments) was used to investigate relationships between CH(4) and CO(2) production and oxygen (O(2)) consumption and to assess whether the predictive power of these relationships could be improved by taking into account some dietary variables, including forage proportion, fibre and metabolisable energy concentrations. The animals were of various physiological states (young n = 60, dry cows n = 116 and lactating cows n = 811) and breeds (Holstein-Friesian cows n = 876, Jersey × Holstein-Friesian n = 47, Norwegian n = 50 and Norwegian × Holstein-Friesian n = 14). The animals were offered forage as a sole diet or a mixture of forage and concentrate (forage proportion ranging from 10 to 100%, dry matter basis). Data were analysed using a series of mixed models. There was a strong positive linear relationship between CH(4) and CO(2), and observations within an experiment were very predictable (adjusted R(2) = 0.93). There was no effect of breed on the relationship between CH(4) and CO(2). Using O(2) instead of CO(2) to predict CH(4) production also provided a very good fit to the observed empirical data, but the relationship was weaker (adjusted R(2) = 0.86). The inclusion of dietary variables to the observed CO(2) emissions, in particular forage proportion and fibre concentration, provided a marginal improvement to the prediction of CH(4). The observed variability in the CH(4):CO(2) ratio could only marginally be explained by animal physiological state (lactating vs. dry cows and young cattle) and dietary variables, and thus most likely reflected individual animal differences. The CH(4):CO(2) ratio can therefore be particularly useful to identify low CH(4) producing cows. These findings indicate that CO(2) production data can be used to accurately predict CH(4) emissions to generate large scale data for management and genetic evaluations for the dairy industry.