Two lab-scale, mesophilic anaerobic digesters (namely reactors R-1 and R-2) were operated simultaneously for continuous production of biogas from fodder beet silage as mono-substrate. The digesters were operated by a control system, developed at HAW Hamburg, which allowed an automatic auto-feeding up to a given maximum set-point. Within a period of 400 days of investigation, the hydraulic retention time (HRT) for reactor R-1 was adjusted to 10-12.5 days, and for reactor R-2, only to 6 days. The organic loading rate (OLR) varied with the total solids (TS) content of the harvest charge, ranging between 3.5 and 5 kg VS m(-3) d(-1). Although both digesters were run under stable conditions, indicated by low volatile fatty acids (VFA) concentrations below 200 mg l(-1), the specific gas production rate (spec. GPR) declined over time to 0.55 and 0.40 l g VS-1 d(-1), for R-1 and R-2, respectively. Therefore, it seemed that some nutrients were deprived, particularly in reactor R-2, which had a short HRT of 6 days. After supplementation with phosphate and sulphate, the improvement of the Spec. GPR was 11% for reactor R-1, and 20% for reactor R-2. Simultaneously performed anaerobic batch digestion test series with the beet silage also confirmed the positive effect of these nutrients on the spec. GPR. Furthermore, quantitative microscopic counts exhibited that the addition of these nutrients had no significant effect on the performance of reactor R-1, but the number of the fluorescent methanogenic bacteria increased four-fold in the high throughput reactor R-2, after supplementation with phosphate. However, additional supplementation with sulphate had no further effect on the performance of R-2. Therefore, phosphate seems to be the limiting macro-nutrient for anaerobic digestion of fodder beet silage, especially under high loading conditions. (C) 2009 Elsevier Ltd. All rights reserved.