In this study we aimed to investigate the effects of starch digestibility of processed corn on milk fat depression and milk fat to protein ratio in Holstein dairy cattle. Twenty-four dairy cattle (milk yield 44.8±3 kg and DIM 159.6±10 d) were randomly assigned to one of four experimental diets: (1) grounded corn (G); (2) pre-grinding superconditioned corn (PSC); (3) extruded corn (E); and (4) steam-flaked corn (SF). Concentrate was adjusted to have 32% starch. Statistical analysis was performed using the GLM procedure based on completely randomized design of SAS (P<0.05). Milk yield was increased (P<0.05) using PSC corn than other processed corn (42.98, 44.40, 37.63 and 42.85, in G, PSC, E and SF corn, respectively). FCM 3.5% and ECM in both of PSC and SF had high tendency (P>0.05) than G and E corn. Percentage of milk fat was significantly decreased (P<0.05) in PSC than others (3.04, 2.51, 3.22 and 3.16 in G, PSC, E and SF corn, respectively), but kg of milk fat, percentage and kg of milk protein and percentage of milk lactose were not affected (P>0.05) using processed corn in diets. Fat to protein ratio in PSC was shown 0.84 that is under 1.00 that is specified for inducing of milk fat depression syndrome, whereas fat to protein ratio in G, E and SF corn was 0.99, 1.06 and 1.14, respectively. Percentage of MUN in both of PSC and SF was lower (P<0.05) and percentage of SNF was higher than G and E diets. It is concluded that PSC corn due to more starch degradation in the rumen may induce milk fat depression in diets containing high levels of starch (more than 32%) and SF corn may recovery from milk fat depression induced with feeding of high starch digestibility corn in dairy cattle diet.

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