The objective of this study was to determine the concentration of fatty acids (FA) especially conjugated linoleic and trans fatty acids in milk from Jersey cows fed green alfalfa (GA) or alfalfa hay (AH). Forty Jersey cows that had from two to five calvings, 60 to 90 lactation days and were not pregnant were used in a confined production system. In one treatment (GA n=20) cows were fed chopped fresh green alfalfa and commercial concentrate. In the other one (AH n=20), they were fed alfalfa hay and the same commercial concentrate. A milk sample (10 mL) was collected from each cow. Total lipids, methyl-esterified fatty acids (FA), and conjugated linoleic acids (CLA) were determined by chromatography. The amount of saturated, monounsaturated and polyunsaturated FA were similar between treatments (P>0.05). The content of FA omega-3 and omega-6, as well as the proportion of omega-6:omega-3 were not affected by the type of forage (P>0.05). However, the concentrations of vaccenic acid-trans (p < 0.01), rumenic acid (p < 0.01), total FA trans (p < 0.001), and total CLA (p < 0.01) were higher in cows fed GA. We conclude that consumption of green chopped alfalfa in Jersey cows increased production of 18:1 n-7 trans-11 trans-vaccenic acid and CLA in milk. Manuscript profile