Effect of dried or green herbage on vaccenic acid and conjugated linoleic acid production during in vitro rumen fermentation

Samples of two diets with the same forage/concentrate ratio (60/40 DM) and characterized by the presence of dried herbage (Diet D) or green herbage (Diet G), obtained from the same natural pasture and Lucerne (Medicago sativa), were incubated in vitro with rumen fluid with the aim of monitoring the concentration of cis and trans C18:1 fatty acids and of rumenic acid (RA) with fermentation time. More vaccenic acid (VA) and less RA were produced with Diet G than with Diet D. In particular, the biohydrogenation of unsatured fatty acid (UFA) showed a lower rate with Diet D than with Diet G. Moreover, for Diet G, the highest content of VA was reached as a simultaneous contribution of the direct hydrogenation of RA and linonenic acid (LNA), as reaction intermediate. The presence of green or dried herbage in the diets modified the profile of C18:1 isomers produced during rumen fermentations. In particular cis12, trans12 and cis13 isomers percentages were enhanced by the presence of fresh herbage in the diets while cis11 C18:1 increased with hay.