Nine primiparous Holstein cows 495±34.5 k:g of average body weight (BW) and 70±5 days in milk: were assigned to a 3 x3 Latin square design to determine the effects of whole (WFS) or physically broken flaxseed (GFS), used instead of extruded soybeans (ES S), on milk: fatty acid composition. Three iso-nitrogenous and iso-energetic diets (crude protein: 180 glkg DM and metabolizable energy: 12.6 MJ!kg DM) containing ES S, WFS or GFS used as 110, 90 and 90 g per kg DM, respectively, were provided. The diets were offered ad libitum as total mixed rations (forage to concentrate as 45:55) twice a day. Each experimental period consisted of 21 d of adaptation to the diets and 7 d for daily milk collection to determine milk fatty acid profiles. Data were statistically analyzed using GLM procedure of SAS. Milk fat concentration did not alter among the animals evaluated in this study (GFS = 32, WFS = 33 and ESS = 32 glkg milk). There was a significant difference (P<0.05) in both mono- and poly-unsaturated fatty acid concentrations (GFS = 11.9 and 2.8, WFS = 10.9 and 2.2, ESS = 9.9 and 2.1 glkg milk. respectively) among the cows fed the experimental diets. Inclusion of physically broken flaxseed in the diet caused a significant (P<0.05) increase in milk: Cl8:2trans and decrease in Cl8:2cis concentrations (GFS = 1.3 and 1.1 vs. WFS = 0.9 and 1.0 and ESS = 0.5 and 1.3 glkg milk, respectively). One of the finding of the study was a significant increase (P<0.05) ofC18:3 fatty acid proportion in the milk fat of cows fed GFS compared with those fed WFS or ESS (C18:3trans: 0.13 vs. 0.11 and 0.09, C18:3cis: 0.63 vs. 0.37 and0.27 glkg milk, respectively). It was concluded that increasing the availability of healthy fatty acids by feeding broken :Oaxseed in the lactating dairy cow diets would result in milk: with higher nutritive value.

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