Exhaled aldehydes as promising compounds to describe the energy balance of lactating dairy cows on a fresh herbage-based diet

The energy balance (EB) of dairy cows is typically assessed through calculated metrics of energy intake and expenditure, which require elaborate individual measurements. Increased concentrations of biomarkers, such as BHB and nonesterified fatty acids (NEFA), are associated with a catabolic status and require individual measurements through invasive blood sampling for reliable and precise assessment. Exhaled volatile organic compounds (VOC) could be a promising low-invasive alternative, but their suitability for this purpose has been scarcely studied. Therefore, the aim of this study was to identify exhaled VOC correlated with the EB of dairy cows and compare their association with that between EB and serum BHB and NEFA concentrations. For this purpose, we used data from a feeding trial including 34 early-lactating dairy cows that were fed freshly cut herbage and concentrates according to their requirements for 6 weeks. Exhaled breath and blood were sampled on 2 consecutive days (same time of the day) in lactation wk 5, 7, and 10, and EB was calculated per lactation week. Exhaled breath was subjected to untargeted VOC analysis, and serum was analyzed for BHB and NEFA concentrations. To identify exhaled VOC associated with EB, we selected 19 cows that exhibited calculated negative EB (NEB) in lactation wk 5 (average days in milk 34.6 ± 13.4) and positive EB (PEB) in lactation wk 10 (average days in milk 76.6 ± 13.4). Using this subset of cows, we then performed partial least squares discriminant analysis (VOC data) and Wilcoxon signed rank tests (VOC data and serum BHB and NEFA concentrations) to investigate the differences between cows that initially exhibited an NEB followed by a PEB. Three exhaled VOC appeared to be discriminatory and differed between NEB and PEB of cows: the fatty aldehydes octanal, nonanal, and decanal. Serum BHB and NEFA concentrations were higher in cows exhibiting NEB than in cows with PEB. Using the data of all cows across all lactation weeks, we performed repeated measures correlations between EB, discriminatory VOC, and serum BHB and NEFA. The EB correlated positively with relative concentrations of octanal, nonanal, decanal, serum BHB, and serum NEFA. Relative concentrations of exhaled decanal, but not octanal and nonanal, correlated with serum BHB and NEFA concentrations, respectively. These findings suggest an association between exhaled fatty aldehydes and the energy status of cows that is as strong as that of serum BHB and NEFA. The physiological role and potential of these metabolites as new biomarkers of NEB in dairy cows and the causality of the association should be further investigated.