## 3 Results  
Biomass of the analysed crops harvested in two swaths in both the second and third year of vegetation showed significant differences in terms of the content of the analysed components (Figure 1). The biggest differences were found for XP and XL (Figure 1a,b). For the two-cut strategy Silphium contained, on average, 50% more XP in comparison to Sida. The content of XP in Silphium harvested once was, on average, two times lower than that found for Silphium harvested on two dates. Only in the case of Sida was it found that the content of this component increased with the age of the plant. For the two-cut strategy, both analysed crops harvested in June contained, on average, twice the amount of XP as compared with the plants harvested in October. The same tendency was found for XL; however,  
Figure 1: Elemental chemical composition g (kg DM)\({}^{-1}\) of the analysed crop biomass with two Sida phenotypes—Sidal and Sida2—and Silphium. neither the age of the plant nor the species had a significant effect on the content of this component--the average content in Sida and Silphium was 2.1% and 1.8% DM, respectively. The XF content in the analysed plants varied--the content determined for Sida was on average 39% DM and for Silphium 28.4% DM (Figure 1c). The content of the XF in the analysed plants was greatly affected by the date of harvest. The XF content in the plants from the second harvest was, on average, 40% higher in the case of Sida and 22% higher for Silphium as compared with the content determined in the plants from the first harvest. The content of XX in Sida and Silphium was comparable and amounted to 47.5 and 52.8% DM, respectively. The XA content was found to be higher in Silphium than in Sida and was 8.0% and 5.4% of DM, respectively (Figure 1d,e). The method of establishing the plantation and the type of Sida phenotype showed no significant effect on the content of the analysed components. The number of Silphium harvests per season were shown to have the greatest effect on the content of XP, which, in the case of biomass harvested twice, was on average higher by 100% as compared with biomass harvested only once--the XP content was 8.8% and 4.3% DM, respectively (Figure 1a). The estimated SBYs from 1 kg DM of biomass samples obtained from individual plants harvested on different dates and different Sida phenotypes were comparable. In the case of Sida, it ranged from 505 to 514 L kgDM\({}^{-1}\), and for Silphium from 483 to 504 L kgDM\({}^{-1}\) (Table 2). None of the analysed factors showed a significant effect on the amount of estimated SBY. Similar results were obtained for the share of biomethane in biogas (Table 3), which, for all analysed crops, ranged from 51.0% to 52.5%. The calculated biomethane share in biogas was comparable to that found in biogas obtained from different crop substrates--on average 51% [35]. The calculated values of BY from 1 ha showed clear differences among the analysed crops. It was found that the method of establishing the plantation and the date of harvest had significant effects on the aforementioned factor. The experiment revealed that markedly higher BY can be obtained from Silphium (Table 4)--the average amount obtained in one year was 8598 m\({}^{3}\) ha\({}^{-1}\) and 4759 m\({}^{3}\) ha\({}^{-1}\) from Sida. For both crops, the amount of BY from biomass harvested in June was several times higher than that harvested in October--for Sida it was on average 4.0 times higher and for Silphium 3.2 times higher. Higher BY was obtained from Silphium harvested two times per year as compared with the