Rehydrated and ensiled ground corn (REC) has high ruminal starch digestibility, but particle size (PS) and dietary starch concentration (S) can affect starch digestion. An 84% vitreousness hybrid was ground to geometric mean particle sizes (GMPS) of 1,591 (Fine) and 2,185 (Coarse) µm for rehydration (60% DM) and ensiling in 200 L buckets for at least 205 d. The rate of grinding (ton/h) was 3.9 for Fine and 11.7 for Coarse (P < 0.01). Ensiling increased (P < 0.01) ruminal in situ DM degradation (63.7 vs 34.1%), regardless of particle size. Sixteen Holsteins (152 ± 96 DIM) in 4 × 4 Latin squares (21-d periods) were individually fed a 2 × 2 factorial combination of low or high starch with Fine or Coarse. Variation in the concentration of starch in the diet (29.2 vs 23.5% of DM) was achieved by partial replacement of REC for citrus pulp. Milk yield (31.0 kg/d) and solids secretion did not differ (P ≥ 0.19). Fine REC tended to reduce DMI and to increase feed efficiency only with the high starch diet (P ≤ 0.10 for the interaction of S and PS). Total-tract starch digestibility tended (P = 0.06) to be reduced by Coarse (96.4 vs 97.2% of intake) and the daily secretion of urinary allantoin tended to be increased (P = 0.09). The molar proportions of acetate and butyrate in ruminal fluid were reduced and propionate and isoacids were increased by high starch (P < 0.01). Coarse reduced plasma d-lactate concentration when fed in the high starch diet (P < 0.08 for the interaction of S and PS). High starch reduced the proportion of daily intake from 1900 to 0700 h (P = 0.02) and induced preferential intake of feed particles <8 mm (P = 0.06) and greater refusal of particles >19 mm (P = 0.01) in the morning. The preferential intake of <8 mm particles tended (P = 0.06) to be increased by Fine in the morning. Coarse grinding of REC ensiled for more than 200 d had no effect on lactation performance and intake when starch in the diet was 23.5% of DM. When dietary starch was increased to 29.2%, fine grinding induced short-term intake depression, gain in feed efficiency, and increased plasma d-lactate concentration, suggestive of ruminal acidosis. Funded by FAPEMIG/CNPq.