Most farm — based anaerobic digesters in Europe use co — digestion of animal manures with organic wastes, such as crop residues or energy crops. In Germany, for example, a high per­centage (over 90%) of agro-biogas plants use co-digestion. Organic materials of over 30 byproducts and wastes are used, but energy crops and crop residues are favored relative to off — farm wastes to avoid the cost of transportation to the farm. Most crops can be co-digested with manure, with maize and grasses as most important examples. The advantages of maize and

grasses are the relatively high biogas yield per hectare for maize and the low energy input required for grasses. Both maize and grasses can be readily stored to provide a year-long input for digesters (Chynoweth et al. 1993; Gunaseelan 1997; Lewandowski et al. 2003; Lehtomaki et al. 2008) . The disadvantage may be the very long digestion residence times required. Ensilage of whole crop maize, for example, is one potentially important process to conserve organic material and provide for pretreatment of feedstock (Driehuis et al. 1999).

The results of some of these studies show that up to 3000-5000m) of methane can be generated from a hectare of cultivated energy crops (Lehtomaki et al. 2008). Most crops have methane potentials in the range of 0.25 to 0.4 L CH4/g VSadded. In general, the methane poten­tial increased with crop maturity. Anaerobic digestion of dairy manure with crop materials (sugar beet tops, grass silage, and oat straw) in completely stirred tank reactors (CSTRs) was feasible with up to 40% VS for the feedstock (60% for manure; Chynoweth et al. 1993; Lewandowski et al. 2003; Bouallagur et al. 2005). This is in agreement with Lehtomaki et al. (2008), who obtained specific methane yields between 0.21 and 0.27 L CH4/g VSadded for straw, sugar beet tops, and grass as feedstocks with cow manure (30% and 70% based on VS, respectively) in a co-digestion approach.