## 4 Discussion  
The method adopted in the present paper, based on the assumption that biogas production from organic substrates mainly depends on the content of elemental nutrients (crude protein, crude fat, crude fibre, N-free extracts), is commonly used in establishing investment assumptions and planning raw material needs for biogas plants [35]. There are numerous papers which confirm compliance of SBY estimated on the grounds of the content of various components with SBY determined using laboratory methods in digesters. [PERSON] et al. [10] developed a methane energy value model for different energy crops on the basis of the content of nutrients. The SMY of maize and cereals measured in the audiometer batch digesters were compared with the values estimated with the model. Similar studies were conducted by [PERSON] et al. [36] for grass and maize--when compared with laboratory analysis, the estimated biogas and biomethane yield showed differences of less than 10%. [PERSON] et al. [8] compared the SMY from maize, determined by the Hohenheim biogas yield test from SMY estimated using three different models. Two of the tested chemical composition models showed a satisfactory performance compared to the mean of the measured methane values. The SBY of the analysed crops were very similar; estimated SBY of Sida was on average 2.8% higher than that of Silphium. As shown by [PERSON] [7], the SBY of the individual substrates varied considerably dependent on their origin, content of organic substance, and substrate composition. Fats provide the highest biogas yield but require a long retention time due to the fact of their poor bioavailability. Carbohydrates and proteins show much faster conversion rates but lower SBY. In our study, the components for which a difference was found among the analysed crops were particularly contents of ash and protein. Mean contents of XA and XP in Silphium were 48% and 49% higher, respectively, than the content of these components in Sida. Ash is not a substrate for biogas production, and protein has a markedly lower digestibility coefficient as compared with the other components included in the model. The XA of the Sida and Silphium were high in comparison with other energy crops, e.g., maize [37]. The XA content in the examined biomass was higher in crops harvested in  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Crop**} & \multicolumn{4}{c}{**2017**} & \multicolumn{4}{c}{**2018**} \\ \cline{2-9}  & **2H (V)** & **2H (Q)** & **sum** & **1H & **2H (V)** & **2H (Q)** & **sum** & **1H** \\ \hline Sida1 planting &  
June, just as in the study of the chemical properties of perennial energy crops depending on harvest period, which showed that the biomass of the other semi-wood or straw crop species contained less ash on later harvest dates [38]. Due to the relatively high contents of carbohydrates (i.e., Sida 86.4% DM, Silphium 81.3% DM) compared to the other components tested, no significant differences in terms of estimated SBY were found. Similar to the present study, the experiment was conducted by [PERSON] [33], who examined the biogas production potential of _Sida hermaphrodita_ and _Silphium perfolitatum_ harvested twice in June and September, the estimated SBYs were smaller and amounted to 417.8 and 474.8 L kg\({}_{\text{DM}}\)\({}^{-1}\), respectively, while BY was 7323 m\({}^{3}\) ha\({}^{-1}\) and 7013 m\({}^{3}\) ha\({}^{-1}\), calculated on the dry matter yield. Carbohydrates are composed of two fractions, one of which is XF-containing structural carbohydrates, mainly celluloses, lignins, and hemicelluloses, which ferment poorly. The second fraction is XX composed mainly of starch and sugars which are easily fermentable. Laboratory studies conducted for maize show that among the components of the substrate, the highest influence on SMY was found for starch. Correlations calculated for this component were positive and the coefficient of correlation was 0.61--the other components were water soluble carbohydrates (\(r=0.41\)) [39].