The aim of this study was to evaluate the effect of the addition of vegetable oils to representative diets of dairy cows under grazing, on the in vitro fatty acids, ruminal fermentation and methane production. The research was conducted in the laboratory NUTRILAB-GRICA, belonging to the Faculty of Agricultural Sciences, University of Antioquia, Medellin, Colombia, in August 2013. The technique of in vitro gas production was used. In four diets (two Cynodon plectostachyus monoculture and two intensive silvopastoral system with Leucaena leucocephala), the addition of three vegetable oils (sun ower, linseed and palm) at 2 and 4% of MS was assessed using a factorial design completely random 4x3x2. In all diets, supplementation with sun ower oil increased the content of linoleic acid (C18: 2 c9, 12), conjugated linoleic acid (CLA-c9t11 ) and transvaccénico acid (ATV, C18:1 t11) after fermentation (p<0.0001), decreased by 2,4 and 0.9% DM digestibility regarding palm oil (p<0.05) and linseed oil (p>0.05). No oil affected fermentation kinetics, pH, total of volatile fatty acid (VFA), VFA ratio or methane production. 

Keywords: conjugated linoleic acid, trans vaccenic acid, ruminal fermentation, dairy cattle under grazing, silvopastoral system.