## 3 Results and Discussion
### Methane Yield
All stands were treated identically; in economic terms, this represents the same costs of sowing, fertilization, chemical protection and harvest. Converted per hectare, the lowest methane yield was recorded in the FM variant, and the highest methane yield was recorded in the FM + WSC variant. However, differences between the respective variants were not significant and were likely to have been caused by differences in TS values (Table A1). The FM variant exhibited the demonstrably lowest TS value (8.07 t/ha) compared with the other variants. The methane yields of all three silage variants (FM, WSC and FM + WSC; Figure 5) after 21 days of the fermentation process (0.28, 0.25 and 0.29 m\({}^{3}\)/kg\({}_{\rm SVS}\); Table 8) were fully comparable with results reached after 21 days of the fermentation process in the methane production from the silage of maize. [PERSON] et al. [29] declared a methane yield ranging from 0.27 to 0.30 m\({}^{3}\)/kg\({}_{\rm VS}\), [PERSON] et al. [52] 0.20-0.32 m\({}^{3}\)/kg\({}_{\rm VS}\) and [PERSON] et al. [53] 0.33-0.35 m\({}^{3}\)/kg\({}_{\rm VS}\). As mentioned, the measured values (Figure 6) did not show any significant differences among the individual variants. It is interesting, however, if these values are compared with the sole crop of maize. Such a comparison is provided by [PERSON] et al. [29], who tested maize silages prepared by the conventional method and by the shredlage technology in their work. After 21 days of anaerobic digestion (AD), the conventional silage and the shredlage exhibited a methane production of 0.27 m\({}^{3}\)/kg\({}_{\rm VS}\) and 0.30 m\({}^{3}\)/kg\({}_{\rm VS}\), respectively. Similar results (from 0.30 to 0.36 m\({}^{3}\)/kg\({}_{\rm VS}\)) were also reported by [PERSON] et al. [54]. From this aspect, the used alternative materials were apparently an interesting source of biomass for biogas production at an identical biogas yield; a potential exists for growing biomass with a lower negative influence on the soil compared with the maize monoculture. On the other hand, it should be pointed out that maize has a higher production of biomass and, hence, a higher converted biogas production per hectare, which usually ranges from 4500 to 6000 m\({}^{3}\) CH\({}_{4}\)/ha [55; 56], which is, on average, a value twice as  
Figure 5: Methane yield during 35 days of the experiment (average values for every day of measurement (\(n=3\)) for one measurement, \(\pm\) SD). FM—fodder mallow; WSC—white sweet clover. high as ours (Figure 6); therefore, it cannot be 100% replaced but, rather, a crop usable in AD can be added. Methane yield can be considered both as a quantitative and a qualitative parameter [56]. It depends, primarily, on the composition of plant biomass entering the fermenter in the form of silage [57]. The fact that the methane yield was higher in the mixed culture silage than in the two other silage variants suggests that the yield increase occurred thanks to synergy during the AcoD process. According to [PERSON] et al. [58], AcoD increases the process stability, optimizes the balance of nutrients and provides synergistic effects for microorganisms. However, this effect was not deeply studied in our experiment. Improvement and optimization of biogas production processes require further detailed research [59]. In principle, the AcoD process can be affected by several key parameters such as the properties of input substrate and the fermentation process itself (temperature in the fermenter, composition of microbial community, etc.) [60].