## 4 Discussion
Similar results were obtained by [PERSON] et al. [14] who demonstrated that SBY from corn was positively correlated with the contents of starch, XL and XX--correlation coefficients were 0.51, 0.66, and 0.43, respectively [14]. Comparison of the XF and XX contents in the crops under this analysis showed that among the analysed crops, the greatest efficacy of biogas production was found for Silphium harvested two times per year and contained, on average, 38% less XF and 11% more XX as compared with Sida. For Silphium harvested once a year, the content of particular carbohydrate fractions was comparable to that determined in Sida. The study of the effects of harvesting age of Napier grass showed that the different grass harvesting ages promoted different characteristics of biomass feedstock. Longer harvesting age grass contained higher concentrations of the difficult to degrade fraction such as lignin [40]. The most recent studies [14; 41; 42; 43] on the influence of the chemical composition of energy crops on biogas and methane yield show that the content of XF fractions is suggested as model variables. In their studies on 41 different crops in batch test, [PERSON] et al. [41] found that 80% of the sample variation of biogas yield can be explained through lignin which, together with hemicellulose, was suggested as model variable. In numerous cases, batch tests revealed significant negative effects of acid detergent lignin on the biogas yield or methane yield [14]. [PERSON] et al. [42] conducted a meta-analysis with the use of data from 678 observations of biomass components and SMY from 13 potential biogas crops. They conducted validation of seven published MY estimation models with lignin content as the main variable and established their own models. The studies [14; 41; 42; 43] confirmed that the prediction of SMY of crop biomass on lignocellulosic components has become a promising tool in biogas plant management and bioenergy policies, and the use of biomass source information can help optimize the precision of SMY prediction. Batch anaerobic digestion tests of SBY and SMY performed for 405 silages from 43 different crop species [43] showed that lignin content of crop biomass was found to be the most significant explanatory variable for SMY. Methane production decreases with increasing lignin content and fibre fractions, while the methane content and methane production rates were mainly affected by the content of XX and neutral detergent fibre, respectively. The energy potential of Sida and Silphium was predominantly determined by yield of the crops, which for Silphium was, on average, 87% larger than the yield of Sida. The analysed crops showed comparable vulnerability to drought which was recorded in 2018. Unfavourable meteorological conditions most likely resulted in lower yield of the analysed crops--in 2018, it was on average twice as lower than in 2017. A significant effect of dry biomass yield on the amount of obtained biomethane was demonstrated by [PERSON] et al. [16], who, on the basis of studies on several varieties of corn, betroot, sunflower, and sorghum, found a significant positive correlation (\(R^{2}=0.98\)) between MY per hectare and dry matter yield. The number of Silphium harvests had a significant effect on BY; in the case of the two-cut strategy, the effect was greater than in the case of the one-cut strategy for both plantation establishment methods. Greater differences were identified for the plantation set-up by planting; the BY was higher on average by 51%, as compared to 29% in the case of the plantation set-up by sowing. Similar results were obtained by [PERSON] et al. [31] for Sida. Higher BY was identified for biomass harvested two times per year (in June and October) than that harvested only once (in October)--3778 and 2379 m\({}^{3}\) ha\({}^{-1}\), respectively [31].