The aim of this work was to evaluate the effect of diet, number of births and stage of lactation on the percentage of milk fat, concentration of conjugated linoleic acid (CLA-c9t11), trans-vaccenic acid (TVA- C18:1t11) and other milk unsaturated fatty acids (FA) of cows grazing in four production systems. This work was carried out from May to December 2012 in ten farms in Colombia, representing tropical dairy production system (LT) and dual purpose production system (DP) with and without intensive silvopastoral system of leucaena (SSPi). Three individual samples of milk from ve cows were taken in each farm (day 62 ± 22, day 147 ± 22 and day 227 ± 22 of lactation), at the same time, supplement and forage intake was estimated and FA were determined in these. Each production system was independently analyzed. The milk fat content was affected by the stage of lactation (p<0.05) in LTSSPi and DPSSPi, obtaining the lower value in the rst stage of lactation. CLA-c9t11 in the milk ranged between 1.02 and 2.22%, TVA between 4.40 and 6.50% and the atherogenic index between 1.69 and 2.89. The high share of fat supplements (51 to 84%) on total fat consumed and the composition of fat supplements affected the variations of the milk fatty acid pro le in LT, LTSSPi and DP systems. Births and stage of lactation had a lower effect than dieting. 

Keywords: conjugated linoleic acid, trans-vaccenic acid, tropical dairy system, dual purpose, leucaena.