The purpose of this experiment was to investigate the effect of the diets supplemented with lard or prilled fat (Carolac  ) on lactation performance, plasma constituents and ruminal characteristics of Holstein cows under a warm climate. In trial 1, 18 Holstein cows, 14 primiparas at 43 DIM and 4 multiparas at 55 DIM, were randomly assigned into six 3×3 Latin squares, in which three dietary treatments were isoproteinous but varying in energy contents and three 21 d periods. The treatments were

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... asal diet (Control), basal diet supplemented with 2.5% lard (LD), and basal diet supplemented with 2.5% commercial Prilled fat (PF). In trial 2, three rumen canulated pregnant nonlactating Holstein cows with 550 kg average body weight were allotted into a 3×3 Latin square design with diets same as in trial 1 were fed to the cows at the level of 1.5% body weight on dry matter (DM) basis. The results indicated that the DM intake did not differ among the treatments. Milk yield and 4% FCM yield were greater (p<0.05) in PF than in Control. LD and PF resulted in greater milk fat percentage. Protein, lactose and solid contents in milk were not different among the three dietary treatments. The concentration of nonesterified fatty acids (NEFA) in plasma was significantly greater in LD and PF than that in Control. However, the concentrations of triglycerides, urea nitrogen, and cholesterol in plasma were not significantly different among the three treatments. Although the ruminal molar percentage of isobutyrate in LD and PF was greater, no significant difference was observed in ruminal pH, NH 3 -N concentration and VFA production among the three treatments. Diet supplemented with fat can improve milk yield and milk fat percentage without resulting in disadvantages of ruminal characteristics in cows at early lactation and under warm climate.