Significance of phenolic compounds in tropical forages for the ruminal bypass of polyunsaturated fatty acids and the appearance of biohydrogenation intermediates as examined in vitro

The purpose of the present study was to assess the influence of phenol-rich tropical ruminant feeds on the extent of ruminal biohydrogenation (BH) of polyunsaturated fatty acids (PUFA). Samples of 27 tropical forages (mainly tree and shrub leaves), characterised by different phenolic profiles, were incubated in vitro (n = 4 replicates) with buffered rumen fluid for 24 h using the Hohenheim gas test method. Linseed oil was added as a rich source of PUFA. In the plants, total extractable phenols (TEP), non-tannin phenols, condensed tannins, and fatty acids were determined. After terminating incubation, the fatty acid profile present in fermentation fluid (total syringe content) was analysed by gas chromatography. The relationship between TEP and the disappearance of a-linolenic acid from the incubation fluid was negative (R-2 = 0.48, P < 0.001), indicating that TEP reduced the ruminal BH of this PUFA. Similarly, TEP were negatively related with the disappearances of linoleic acid (R-2 = 0.52, P < 0.001) and oleic acid (R-2 = 0.58, P < 0.001). The appearance of rumenic acid, an important conjugated linoleic acid isomer, was positively correlated with TEP (R-2 = 0.30, P < 0.01), while the opposite result was seen with stearic acid (R-2 = 0.22, P < 0.05). Leaves of avocado (Persea americana) were particularly interesting, because they changed the BH pattern at a moderate TEP content of 73 g/kg DM. It is concluded that, in the tropical feedstuffs investigated, TEP have an impact on ruminal fatty acid BH and are associated with an increased bypass of PUFA and the generation of conjugated linoleic acid.