## 1 Introduction  
Global increases in energy demand and related increased greenhouse gas emission [1], as well as the need to ensure energy security and to further diversify energy sources [2], necessitates taking actions aimed at developing technology for energy production based on the use of biomass. National actions plans drawn up by EU member states envisage that by 2020, the share of energy produced from renewable energy sources should exceed 20% of total energy consumption in the EU. According to the Renewable Energy Directive, the use of biomass for energy production in the EU will provide for half of the renewable target [3]. In comparison with other forms of bioenergy, biogas production is one of the most environmentally friendly technologies [4]. It is estimated that by 2020 in Europe, biogas plant production potential will reach 770 PJ per year [3]. At the end of 2014, there were approximately more than 17,000 active biogas plants in Europe--the largest number were in Germany and Italy [5]. In Poland, only 95 agricultural biogas plants with a total electrical capacity of 101.3 MW operated in 2017 [2]. The structure of Polish agricultural biogas plants is dominated by installations with capacities in the range of 0.5 to 1.5 MW, which together constitute 56% of all biogas plants in Poland [6]. Biogas production potential from agricultural raw materials constitutes the highest share in total estimated biogas potential in Poland [6]. Development of agricultural biogas plants in Poland would result in diversification of the gas supply, the actually available potential of which is 1.7 billion m\({}^{3}\) annually, i.e., more than 10% of national consumption [2]. The key success factor of the future role of biogas will be the availability of biomass. All types of biomass can be used as substrates for biogas production as long as they contain carbohydrates, proteins, fats, cellulose, and hemicelluloses as main components [7]. Biogas plants increasingly rely on energy substrates cultivated for this particular reason. For evaluation of energy crops as substrates for biogas production, an easily identifiable criteria are needed [8]. A main factor is the biogas yield per hectare (BY), which is made up of the total dry matter yield (DMY) per area and specific biogas yield (SBY) of this dry mass. The prediction of BY is a very important tool in biogas plant management and bioenergy policies. The content of crude protein, crude fat, crude fiber, and N-free extracts determines the degradability and, thus, the biomethane yield that can be produced through anaerobic digestion [9; 10]. The SBY can be estimated under laboratory conditions based on fermentation studies using a continuous method or batch tests, e.g., the Hohenheim biogas yield test [11]. Laboratory tests are very expensive and time-consuming, they are a suboptimal instrument for evaluating short-term breeding progress [12]. Some batch tests allow determination of the specific biomethane yield potential with an acceptable repeatability [8]. Another method is theoretical estimation of biogas yield calculated from the elemental composition using different stoichiometric [9; 13] and empirical models developed for individual crops or estimates of SBY by multiple regression of various constituents [10; 14; 15]. The highest number of empirical models was developed for maize which is considered to be a crop with the highest biogas production potential [10; 14; 15]. For a practical farmer who plans to establish a biogas plant, the ultimately decisive question is which crop or which variety provides the highest methane yield per hectare, as this has a decisive influence on the profitability of his biogas plant [8; 16]. In comparison with laboratory methods, estimating biogas on the basis of the chemical composition of the biomass allows for obtention of results at a lower cost.