The objective of this study was to investigate if the level of corn silage influences the impact of supplemental fat (Yellow grease) on performance of dairy cows. Eight Holestein cows (4 cows multiparous, 4 cows primiparous) averaging 110 ± 30 d in milk, 34 ± 3.0 Kg/d of milk and 590 ± 125 kg were used in a replicated 4×4 Latin Square Design with 21-d periods. Treatments were 1) 40% of diet DM as alfalfa hay (control, alfalfa-based diet); 2) 24% alfalfa hay and 16% corn silage diet); 3) 24% corn silage and 16% alfalfa hay; 4) 20% alfalfa hay and 20% corn silage. All treatments contained 2% yellow grease and forage to concentrate ratio of 60 : 40. DMI was increased with increasing corn silage level. Treatments 3 or 4 increased DMI (P 0.05), NDF intake (NDFI), forage NDF intake (FNDFI), and NE L intake (P 0.01). Treatments 3 had higher NDFI, FNDFI, and NE L I than treatment 1 (P 0.01). Treatments had significantly effect on milk production (P 0.04). Treatment 3 had greater (P 0.05) milk production, milk protein content and yield, and lactose percentage and production than treatment 1. Milk fat content and production did not differ across treatments. Treatment 1 (P 0.05) decreased significantly digestibilities of ADF, CP, DM, and OM. Treatment 2 decreased (P 0.001) digestibility of ADF. Cows fed corn silage diets had numerically higher eating and chewing time than those fed treatment 1. Ruminating and chewing time increased with increasing corn silage level. Treatments had no effect on ruminal pH. Ruminal pH was numerically higher for cows fed treatment 3. It seems that forage type in diets containing yellow grease could alter performance of lactation Holestein cows and corn silage up to 20 % DM of diet ( 50 % of forage portion) can be replaced by alfalfa hay without any adverse effects on dairy cows
