The increase of the organic loading rate from 4.45 to 7.78 g-VS/l-d in a one stage, pilot biogas plant was investigated. The experiment was conducted using a co-digestion of cow manure and crop silage mix. The test was divided in three subsequent phases with different feeding ratios. The benefits of optimizing the proportion of crops and loading rate in co-digestion were shown by the fact that during feeding with 70% VS of crop in the feedstock, up to 109% higher specific methane yield was obtained than during the start up phase (only manure). it was also found that further increasing the proportion of crop silage up (to 80%) led to a process breakdown with a decrease of methane proportion down to 48%. On the basis of the present results, it could be possible to obtain an electricity production equal to 15 kwh per 1 t/d. At the end of the monitoring biogas yield was equal to 237 and 249 I-CH(4)/kg-VS in the first two phases, and to 61.6 I-CH(4)/kg-VS in the third one that led to a process breakdown. The result of this Study show, that most agricultural biogas plant have a great potential for a significant capacity increase and technology improvement. (C) 2009 Elsevier Ltd. All rights reserved.