## 3 Results and Discussion
### Biogas Yield  
The cumulative production of biogas for 35 days in all three variants is shown in Figure 4. The diagram indicates that the lowest average yield of biogas from the very first day of the measurement was that of WSC silage. The total (cumulative) biogas yield from the WSC silage was 0.47 m\({}^{3}\)/kgys. A higher average biogas yield was recorded throughout the measurement in the silage of mixed FM and WSC culture (50:50), with a total yield reaching 0.54 m\({}^{3}\)/kgys. The highest production of biogas was recorded in the FM silage variant already from the first measurement. The total biogas yield was 0.60 m\({}^{3}\)/kgys. For the 35 days of the experiment duration, 79.1% (WSC) and 85.4% (FM) of total biogas yield was produced in the first seven days. The measured values further indicated that the production of biogas gradually began to stagnate from Day 16 of the measurement, with a final stagnation recorded on Day 25. A similar trend was also observed in other experiments [24; 29] where the production of biogas was measured in laboratory conditions using one batch of silage (plant mass). Therefore, a so-called breaking point should be established precisely for practical use and, hence, a moment when a further silage batch has to be added into the fermenter. This is not a problem in the case of silage grown in the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Treatment**} & \multicolumn{2}{c}{**Butyric Acid (BA)**} & \multicolumn{2}{c}{**Ethanol (EE)**} & \multicolumn{2}{c}{**Ammonia (NH\({}_{3}\)-N)**} \\ \cline{2-6}  & **\% DM** & **\% DM** & **HSD** & **\% CP** & **HSD** \\ \hline FM & N.D. & 0.30 \(\pm\) 0.15 & B & 4.82 \(\pm\) 0.78 & A \\ WSC & N.D. & 0.83 \(\pm\) 0.19 & A & 2.25 \(\pm\) 0.18 & B \\ FM + WSC & N.D. & 0.64 \(\pm\) 0.03 & AB & 2.03 \(\pm\) 0.16 & B \\ \hline \hline \end{tabular} Legend to Table 6: Mean of measured values (\(n\) = 3) \(\pm\) standard deviation (SD). All parameters were recalculated to sample dry weight (DM); ammonia content was expressed as % of crude protein (CP). Different letters indicate significant differences (\(p\) < 0.05) among individual variants in the specific parameter. FM—fodder mallow, WSC—white sweet clover, N.D.—not detected.  
\end{table} 
Table 6: Parameters of silage variants FM, WSC and their mixture. MC regime as it exhibited the most continuous curve of biogas production, i.e., a curve with the smallest fluctuations. Differences between the yields of respective variants were statistically significant (Table 7). The results indicated that the biogas yield from the FM silage (0.60 m\({}^{3}\)/kg\({}_{\text{VS}}\)) was statistically significantly higher than the biogas yields from the silage of mixed culture (0.54 m\({}^{3}\)/kg\({}_{\text{VS}}\)) and WSC (0.47 m\({}^{3}\)/kg\({}_{\text{VS}}\)). The values do not entirely confirm the hypothesis that the mixed culture of FM + WSC is more suitable for the production of biogas than FM or WSC monocultures. Only in the case of WSC is it possible to theoretically consider its enhanced properties for the production of biogas if grown in the mixed culture. On the other hand, it is relatively interesting that the highest biogas yield was reached in the FM variant in which the highest ash content and a demonstrably lower starch content were recorded compared with the WSC and FM + WSC variants (Figure 4). In contrast, the WSC variant exhibited the highest content of starch and the lowest content of ash and the lowest biogas yield. Based on the generally known information about the biodegradability of ash and starch [43; 44; 45], it is possible to state that ash adversely affects the yield of biogas while starch affects it positively as it is a source of energy for the microorganisms responsible for biomass degradation during AD [1; 2; 3].