This study was conducted to determine the effects between alfalfa particle size (PS) with two unsaturated fat source on dairy cows. Eight mid-lactation cows (DIM = 132 ± 12), with average milk yield (37 ± 6) were selected. A replicated 4 × 4 Latin square design with 2 × 2 factorial arangment of treatments was used to determining the single and contrast effects of treatments on production and chewing activity. Rations was based on different alfalfa PS and oil sources including 1) TMR made of 2 mm alfalfa and soy oil, 2) TMR made of 5 mm alfalfa and soy oil, 3) TMR made of 2 mm alfalfa and yellow grease and 4) TMR made of 5 mm alfalfa and yellow grease. Treatments including soy oil consumed more feed (P 0.05) than those had included yellow grease because of its palatability and lower hipophagic effects of soy oil. Crude protein digestibility was higher for alfalfa with short PS that probably (P 0.05). Digestion of NDF and ADF was effected by fat sources as treatments of soy oil had lower digestion for fiber (P 0.05). Particle size of alfalfa hay did not affect milk production but fat production tended to be higher for short PS (P 0.15) and fat : protein (P 0.15) tended to be significantly for lower PS. Eating and ruminating time did not differ between treatments but total chewing time and for treatment with long particle size was higher (P 0.05). Ratio of chewing time to DMI, neutral detergent fiber, acid detergent fiber and physically effective NDF was significantly higher for lower PS (P 0.05). Two hours after feeding rumen fluid pH did not have diference between treatments but in 4 th hours after feeding yellow grease had a significantly higher pH in rumen (P 0.05). Result of this study showed that yellow grease did not have adverse effects on performance and can be replaced with soy oil in dairy cows ration.
