Effect of three species of herbage (Medicago sativa, Lolium multiflorum, Avena sativa) on in vitro ruminal production of conjugated linoleic and vaccenic acids

Little information is available about the effect of different forage species on the rumen biohydrogenation process. The aim of the present work is to compare the in vitro production of CLA and C18:1 isomers after incubation of three different herbage species in rumen liquor from sheep. Pasture herbage samples of lucerne (Medicago sativa; MS), ryegrass (Lolium multiflorum; LM) and oats (Avena sativa; AS) were submitted to in vitro fermentation with sheep rumen inoculum. Samples were collected at 2, 4, 6 and 8 hours of fermentation. The fatty acid profile of MS was characterised by 11.62 (g/100 g of lipid extract) of linoleic acid (LA) and 27.08 (g/100 g of lipid extract) of alpha-linolenic acid (LNA), whereas LA in the other two herbages was 6.60 (g/100 g of lipid extract) and 6.95 (g/100 g of lipid extract) in AS and LM, respectively; LNA was 52.20 (g/100 g of lipid extract) and 54.49 (g/100 g of lipid extract) in AS and LM, respectively. The crude fat content of botanical species was respectively 11.90 (g/100g DM) for AS, and 15.77 (g/100g DM) for LM and 26.17 (g/100g DM) for MS. Rumenic acid (PA, cis-9, trans-11 CLA) was the predominant CLA isomer and the maximum yield was attained with AS after 6 hours of fermentation (0.81 g/100 g of lipid extract); RA concentration remained quite low with the other two herbages. The concentration of the other isomer (trans-10, cis-12 CLA) was always very low, the maximum yield (0.09 g/100 g of lipid extract) was reached after 6 hours with AS. The maximum yield of vaccenic acid (VA, trans-11 C18: 1) was reached after 8 hours with MS (2.64 g/100 g of lipid extract). This herbage also produced the highest amount of trans-10 C18: 1 at 6 and 8 hours (0.17 g/100 g of lipid extract). AS appeared to have induced the highest amounts of RA relative to the other two forages. The differences in conjugated dienes and C18:1 isomers content during fermentation could be due not only to different amounts of LA or LNA in the herbage, but also to different releasing times of FA from the plant substrate.