The effect of a well-defined trace element solution and the elements nickel, cobalt and molybdenum on anaerobic digestion of a synthetic model substrate for maize silage was studied in batch reactor experiments at 35 degrees C. The defined substrate (dS) consisted of xylan and starch as the main carbon source, urea as nitrogen source and phosphorus from a 0.1M potassium phosphate buffer. Batch reactors were operated for 30 days with 1.5% organic dry matter (ODM) of inoculum sludge from a mono-maize biogas plant and 1% ODM of the defined substrate. Results showed an increase of methane yield of up to 30% upon addition of the trace element solution. With an addition of nickel at 10.6 microM, a final yield of 407 l kg(-1) ODM was reached and an enhanced methane production by 25% at day 25 of operation was observed. Total elimination of nickel from the trace element solution highly decreased methane formation and process stability. Cobalt in a concentration range of 0.4 up to 2.0 microM increased the methane production by 10% approximately. Interestingly, addition of molybdenum did not significantly effect methane production.