## 3 Results and Discussion
### Volatile Compounds in the Calves' faeces  
The comprehensive faecal VC profile of calves in the CON and MP groups is given in Table S8 (Supplementary File S8). However, due to the large amount of data, only the most prevalent VCs are summarised here. The following baseline measurements (0) of VCs in faeces in the CON0 group were found in significant concentrations (Table 4), with the following in decreasing amounts: nonanoic acid, 2-nonanone, butanoic acid, acetoin, octanoic acid, and indole. In the MP0 group, the predominant VCs were butanoic acid, indole, nonanoic acid, 2-propyl-1-pentanol, and octanoic acid/2-nonanone (Table 4). The concentration of each VC differed significantly between the groups (\(p<0.05\)), with the exception of \(n\)-decanoic acid. After 14 days of feeding, \(p\)-cresol, butanoic acid, and indole were the predominant compounds in both groups. The concentration of \(p\)-cresol was significantly higher (by 20.8%) in the MP14 group compared with the CON14 group (\(p=0.005\)). Compared with the beginning of the experiment, there was a 3.4-fold increase in butanoic acid in the CON14 group, while this VC increased only by 12.7% in the MP14 group (\(p\) = 0.003). There was not a significant difference between the MP14 and CON14 groups in the concentration of indole. However, there were significant differences between the MP14 and CON14 groups in the concentration of all the other VCs (\(p\) < 0.05), with the exceptions of hexanoic acid, 2-nonanone, and nonanoic acid (Table 4). Moreover, after 14 days of feeding, acetic acid in the faeces decreased by 39.6% and 67.2% in the CON14 and MP14 groups, respectively, whereas acetoin, 2-heptanone, and 2-propyl-1-pentanol were not detected in either group after 14 days. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Volatile**} & \multicolumn{4}{c}{**Calf Groups**} & \multicolumn{4}{c}{\(p\)} \\ \cline{2-9}  & **CON0** & **CON14** & **MP0** & **MP14** & **CON0** \(\times\) & **MP0** \(\times\) & **MP14** & **CON0** \(\times\) & **MP0** & **CON14** \(\times\) \\ \hline Acetic acid & 3.71 \(\pm\) 0.31 & 2.24 \(\pm\) 0.18 & 1.950 \(\pm\) 0.11 & 0.640 \(\pm\) 0.050 & 0.003 & 0.001 & 0.004 & 0.002 \\ Acetoin & 7.32 \(\pm\) 0.62 & nd & 1.40 \(\pm\) 0.25 & nd & 0.002 & 0.011 & - & 0.001 \\ Propanoic acid & nd & 2.62 \(\pm\) 0.18 & nd & 2.20 \(\pm\) 0.14 & 0.002 & 0.001 & - & 0.003 \\ Butanoic acid & 9.53 \(\pm\) 0.59 & 32.39 \(\pm\) 2.84 & 22.13 \(\pm\) 1.93 & 24.94 \(\pm\) 2.10 & 0.003 & 0.001 & 0.004 & 0.003 \\ Butanoic acid, & 2.36 \(\pm\) 0.24 & 2.33 \(\pm\) 0.18 & 0.59 \(\pm\) 0.04 & 2.21 \(\pm\) 0.22 & 0.564 & 0.004 & 0.004 & 0.032 \\ Butylc acid & & & & & & & & \\ \(<\)2-methyl-\(>\) & 4.24 \(\pm\) 0.38 & 4.27 \(\pm\) 0.29 & 1.09 \(\pm\) 0.09 & 2.65 \(\pm\) 0.17 & 0.603 & 0.001 & 0.003 & 0.002 \\ \(>\)-Heptanone & 6.77 \(\pm\) 0.34 & nd & 2.54 \(\pm\) 0.24 & nd & 0.001 & 0.003 & - & 0.0001 \\ Pentanoic acid & 0.22 \(\pm\) 0.02 & 5.87 \(\pm\) 0.41 & 0.711 \(\pm\) 0.010 & 2.40 \(\pm\) 0.15 & 0.002 & 0.001 & 0.009 & 0.002 \\ Hexanoic acid & 2.51 \(\pm\) 0.18 & 1.46 \(\pm\) 0.11 & 3.89 \(\pm\) 0.28 & 1.62 \(\pm\) 0.11 & 0.001 & 0.002 & 0.002 & 0.321 \\ Phosphomic acid & 12.17 \(\pm\) 1.23 & 1.13 \(\pm\) 0.56 & 10.39 \(\pm\) 1.23 & 0.921 \(\pm\) 0.110 & 0.001 & 0.005 & 0.001 & 0.515 \\ Indice & 6.87 \(\pm\) 0.65 & 11.90 \(\pm\) 0.92 & 12.24 \(\pm\) 0.87 & 12.16 \(\pm\) 1.18 & 0.001 & 0.085 & 0.001 & 0.231 \\ \(\sim\)Decanoic & 3.10 \(\pm\) 0.32 & 0.663 \(\pm\) 0.05 & 2.98 \(\pm\) 0.28 & 0.639 \(\pm\) 0.040 & 0.037 & 0.003 & 0.127 & 0.053 \\ \hline \hline \end{tabular}  
* The data are presented as the mean \(\pm\) standard error (\(n\) = 10/group). Baseline measurements; 14, after 14 days of feeding. CON, calves fed with milk replacer only; MP, calves fed with milk replacer and supplemented with fermented milk permeate; nd, not detected.  
\end{table}