Three experiments were conducted to study the effect of the administration of young leaves of Quercus pyrenaica (OL) on in vivo ruminal fermentation in relation to oak tannin toxicosis in cattle. In Experiment 1, six bulls were fed up to 5 kg fresh matter (FM) of OL per animal and day for 14 days. In Experiment 2, another six bulls were first subjected to severe feed restriction for 8 days and then fed a higher amount of OL (approximately 10 kg FM daily) for 3 days. In Experiment 3, three bulls received the same amount of OL as in Experiment 1 for 6 days, but adding a severe feed restriction as in Experiment 2. In situ DM disappearance of grass hay and OL, and pH and ammonia and volatile fatty acid concentrations were recorded throughout the three assays. Daily administration of up to 5 kg OL did not considerably affect ruminal fermentation, unless it was preceded by a severe feed restriction period. Administration of 10 kg OL preceded by undernutrition triggered a critical reduction in rumen fermentation activity concomitantly with an acute intoxication. Interestingly, some results differ from those observed previously in vitro, which highlights the importance of validating in vitro data with in vivo measurements, given the complexity of extrapolation in ruminants.
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