In vitro study of the effects of condensed tannins in sainfoin on the digestive process in the rumen at two vegetation cycles

One approach to reducing enteric production of greenhouse gases (GHG; i.e., nitrous oxide and methane) is to feed tanniniferous forage plants such as sainfoin. The objective of this study was to investigate, in vitro, effects of the content and structural characteristics of condensed tannins (CT) in the whole plant, leaves and stems of sainfoin (Onobrychis viciifolia) on the digestive process in the rumen. Sainfoin, studied during the first vegetation cycle at the end of flowering, and during the second vegetation cycle at the start of flowering, was incubated in serum bottles containing buffered rumen fluid for 3.5 and 24h. To assess effects of CT, incubations were with and without added polyethylene glycol (PEG), to neutralise CT effects. Biological activity and prodelphinidin (PD) content were higher (P<0.001) in leaves than in stems, but cis value was lower (P<0.01). At 3.5h of incubation, gas production was higher for stems than leaves (P<0.05), but at 24h only tended to be higher for leaves than stems (P=0.096), at the end of flowering. The presence of PEG at both short and long fermentation times increased gas production of leaves, stems and whole plant (P<0.001). At 3.5h, PEG addition resulted in higher CH₄ values for stems and leaves at two vegetation cycles (P<0.01). The CO₂:CH₄ ratio was lower (P<0.01) at 3.5h and higher at 24h (P<0.01) with added PEG. No effect of PEG on whole plant CH₄ production, or the CO₂:CH₄ ratio, at either 3.5 or 24h of incubation. At both incubation periods, in vitro true organic matter digestibility (IVTOMD) was higher for leaves than stems (P<0.001), and decreased with added PEG at 24h (P<0.001). For the whole plant, PEG had no effect on IVTOMD at either 3.5 or 24h of incubation. At 3.5h, PEG resulted in higher N disappearance (ND) at both vegetation cycles (P<0.01), whereas values were higher at the second one (P<0.001). Mean volatile fatty acid production was higher for leaves than stems (P<0.01) and increased with PEG (P<0.001) for both vegetation cycles at 3.5 and 24h of incubation. For both incubation periods, PEG increased (P<0.001) NH₃–N for leaves and stems in both vegetation cycles. For the whole plant, PEG resulted in higher NH₃–N content (P<0.001) at both 3.5 and 24h of incubation. Utilisation of sainfoin curbs CH₄ production without altering its organic matter digestibility and N value, but research on the nutritional impact of CT must not only be based on CT content in plant tissue, but include structure-activity considerations of CT.