On-farm analysis of exhaled breath compounds as indicators for a postpartum health score in dairy cattle: a case study

The objective of this case study was to investigate if respiratory ratios derived from non-invasive exhaled breath measurements could be used as a tool to detect dairy cows at risk for impaired postpartum health. Measurements of exhaled breath from individual cows took place during visits to a concentrate feeder from 2 wk prepartum until 6 wk postpartum (Veterinary Metabolism System, Relitech). Per measurement, concentrations of oxygen (O2), carbon dioxide (CO2), and methane (CH4) (vol%) were measured. Subsequently, daily average respiratory exchange ratio (RER; CO2/O2), CH4 exchange ratio (MER; CH4/O2) and CH4-to-CO2 ratio (MCR; CH4/CO2) were calculated per cow. Cows were scored clinically twice weekly from 2 wk prepartum until 6 wk postpartum on 19 clinical signs. Blood β-hydroxybutyric acid was determined twice weekly, and additional blood samples were taken in week 1 and week 5 after calving to determine clinical-chemical parameters. A total deficit score (TDS) was assigned to each cow based on blood values and clinical scores. Per health score (e.g., decrease in body condition score (BCS) after calving, β-hydroxybutyric acid values, and TDS values), cows were divided into two categories (high or low). Differences in exhaled breath composition between these categories were evaluated using mixed models. The RER was lower in cows with a high reduction in BCS during the first 6 wk after calving, which can result in ketosis and fatty liver caused by the increased body fat mobilization. For TDS Locomotion, TDS Metabolic, TDS Liver, and TDS Macro-minerals, MER was lower in cows with a higher TDS compared with cows with a lower TDS, which may be related to decreased feed intake in cows at risk for disease leading to a decreased methane production. Prepartum MER was lower in cows with a high decrease in BCS, high TDS Total, or high TDS inflammation compared with cows with low values in these categories. The MCR was inconsistent for different disease categories. Based on the results of this experiment, respiratory ratios derived from non-invasive exhaled breath measurements seem to be promising indicators to detect cows at risk for disease around calving, but further validation of breath measurements is required. Variables such as individual daily milk production, dry matter intake, and time lag between breath measurement and feeding time should be taken into account in future research to improve the interpretation of results.