Ensiling of fresh crops is a necessary and common procedure to provide nutrient-rich fodder for animals throughout the year, especially where the growing period is short. The increasing use of energy crops and residues needed as raw materials to fulfil the requirements of bio-based transport fuels can also benefit from preservation of the raw material. Energy and yield savings in the conversion process of the raw material can be obtained through efficient storage methods that avoid the expense of drying, while still preserving the valuable carbohydrates. Furthermore, ensiling may function as a beneficial pretreatment for lignocellulosic materials before further processing to methane or ethanol. Ensiling is normally carried out in anaerobic conditions in which organic acids, especially lactic and acetic acids produced by endogenous microflora, decrease the pH, preserve the substrate against growth of fungi, bacteria or yeasts, and thus prevent carbohydrate losses. During the fermentation, acids are formed from the easily available non-structural carbohydrates, as well as from structural carbohydrates that become available during prolonged storage as a result of hydrolysis by acids and endogenous enzymes [1, 2].
Acids, especially formic acid in the Nordic countries, are commonly added to initiate the preservation . Requirements to increase the nutritional value of animal feed and to preserve the carbohydrates during the storage have led to increased research on, for example, urea and acidic additives [4, 5]. Acidic pretreatments have been found to enhance the production of biogas or bioethanol by decreasing the crystallinity of the cellulosic material , by increasing the accessible surface area of plant substrates, and by altering lignin structure . Dilute sulfuric or hydrochloric acids, as well as organic acids, are commonly used for pretreatment of various raw materials prior to enzymatic hydrolysis , but usually in significantly more severe conditions than those applicable for ensiling. Sulfuric acid treatment prior to anaerobic ensiling was found to enhance the conversion of reed canary grass and switch grass to ethanol , indicating that lower levels of acid amendments to ensilage can have the desired effects.
Alkaline conditions may also be used for storing crops. Urea treatments, especially on late-harvested crops, were found to preserve the carbohydrate and nutrient contents during storage  and the added urea improved slightly the nutritive value of nitrogen-poor crops, such as whole-crop maize, as feed for ruminants . Alkaline preservation has been found to improve also the enzymatic conversion [9, 11–13]. Treatments with sodium hydroxide or urea in various conditions have been observed to cause swelling, to decrease the degree of polymerisation and crystallinity of cellulose, and expectedly, to degrade linkages between lignin and carbohydrates .
Decomposition of cell walls during ensiling could improve the availability of nutrients and carbohydrates for the methanogens and thus enhance methane production . Ensiled corn stover and grasses are commonly used raw materials in present methane production plants. Due to the increased formation of lactic and acetic acids in ensiling, higher methane yields have been obtained [15, 16]. Formic acid, used as an additive in ensiling, preserved the grass substrate well and enhanced methane yields , whereas biological additives, such as lactic acid bacteria or hydrolytic enzymes, had inconsistent effects on methane yields, mainly because of suboptimal ensiling methods [16–18]. However, addition of cellulolytic and hemicellulolytic enzymes to ensiling increased the amount of water-soluble carbohydrates and formation of lactic acid and was suggested as an effective pretreatment for the fibrous kenaf . Minimal carbohydrate losses during storage are essential for economic production of ethanol from energy crops. Ensiling of lignocellulosic materials has been found to be a promising and cost-effective storage and pretreatment method for ethanol production [20, 21], but the chemical modifications affecting the hydrolysability of ensiled crops have not been thoroughly examined. There is little published data on the use of urea as a pretreatment and storage agent of fresh crops prior to enzymatic conversion to sugars and further to fuels. Although ensiling is known to improve methane yields of many commonly used crops, the efficiency depends on plant species. In this work, three crops, maize, hemp and faba bean, producing high yields in boreal conditions , were ensiled without additives, as well as preserved with the addition of formic acid and urea. These three crops differ from each other in their chemical characteristics, hemp being rich in cellulose, lignin and pectin, maize in hemicelluloses, particularly xylans, and faba bean in starch and nitrogen . While maize and faba bean are common raw materials used for ensiling for feed , ensiling of hemp is a novel approach, supported by its established status as a high yielding energy crop. In this work, detailed analysis of the chemical composition of the high yield crops after various storage methods and conditions was carried out, and the potential for methane production and enzymatic conversion of these raw materials was evaluated.