Extensive ruminant farming systems often face fluctuating pasture availability, resulting in two periods with very different forage quality. Tannins are natural bioactive compounds able to modify ruminants' digestive metabolism thanks to their bioactive properties. To evaluate a differential effect of tannins in different feeding situations, according to the availability of pasture along the year, an in vitro rumen incubation trial was performed. Buffered sheep rumen content was incubated in vitro with two different substrates, vetch pasture (VP) and vetch hay (VH), in the presence of two different tannin extracts, from quebracho (QUE) and a mixture of quebracho and chestnut tannins (MIX). Each of the tannin extract was tested at 0, 15 and 30 g of tannin/kg DM. The gas production was determined at 3.5 and 24 h of fermentation. After 24 h in anaerobic conditions, volatile fatty acids (VFA), ammonia and fatty acids (FA) were analysed. Interesting significative two-way interactions with different tannin extracts were observed in some fermentation parameters. Adding MIX to VH treatment resulted in a higher depression (P < 0.050) of ammonia production and iso-VFA proportions, compared to QUE supplementation, while no differences were observed in VP treatment. QUE was more effective than MIX (P < 0.050) in reducing acetate/propionate ratio when supplemented to VH treatment. However, the MIX treatment showed a higher increase (P < 0.050) of CO2/CH4 ratio after 24 h of incubation when supplemented at 30 g/kg, compared to QUE treatment. MIX and QUE exerted similar effects on rumen biohydrogenation. Tannin extracts decreased the rumenic/linoleic acid ratio and iso-FA proportions (P < 0.050) only when added to the VH treatment. These results demonstrate that tannin extracts are more effective in modulating rumen metabolism when associated with a hay