Summary The aim of this study was to investigate the effect of diets containing roasted soybean (R), extruded soybean (E) or their combination (RE) on dairy cow performance and milk fatty acid (FA) profile. Nine multiparous lactating Holstein cows (680 ± 25 kg BW; 90 ± 10 DIM; means ± SD) were randomly assigned to a triple 3 × 3 Latin square design. Dry matter intake (DMI), apparent nutrient digestibility, milk yield, composition and FA profile and efficiency of nitrogen (N) utilization for milk production were measured. Cows fed R and RE had higher 4% fat corrected milk (FCM) yield and DMI compared to those fed the E (P<0.05). The efficiency of 4% FCM production per unit of DMI was not influenced by experimental diets (P>0.05). Experimental diets had no effect on milk fat and protein concentrations and nutrient digestibility (P>0.05). Milk fat concentrations of vaccenic acid (VA; C18:1 trans-11) and conjugated linoleic acid (CLA; C18:2 cis-9, trans-11) were higher for cows fed with RE compared to other groups (P<0.05). We concluded that cows fed with RE had similar milk production and fat content compared to those fed with R and a similar beneficial milk FA profile compared to those fed with E. Thus, feeding a diet with RE improved the quality and quantity of dairy cow milk. Key words: Animal performance, Extruded soybean, Lactating cow, Milk fatty acid profile, Roasted soybean
